Hi

I need to fool a heat pump into controlling the output temperature.

Normally the heat pump is controlled with an external NTC resistor (from 1kohm to 15kohm).

I will control it with USB from a PC by connecting a controlled source.

I have this one in my lab, but it only goes to 4kohm:

https://www.orbitadigital.com/en/cctv/accessoires/tools/34781-uni-trend-ut714-temperature-process-calibrator-lcd-display-up-to-2000.html

(900 USD)

Then I found a Fluke 753 that goes up to 10kohm, nice.

But, a whopping 8000 USD:

https://dk.rs-online.com/web/p/multifunktions-kalibratorer/2532268?cm_mmc=DK-PPC-DS3A-_-google-_-3_DK_DK_Test-+og+m%C3%A5leudstyr_Multifunktions+kalibratorer_BMM-_-Fluke+-+2532268+-+FLUKE+753-_-fluke+753&matchtype=p&kwd-27811768253&gclid=Cj0KCQjwoK2mBhDzARIsADGbjeovo4mcqBUBz15CsBcKfSEaHcXyftA6G7oUimzSK_dOQ-EzuqyA2J4aApF7EALw_wcB&gclsrc=aw.ds

I could spin my own, just for this function. It's for a client, so cannot be a project that takes time, since just 10 hours spend, then I could buy the Fluke 753 instead.

I could use an electronic load, but they normally have large capacitance, and I don't know how the heat pump implementation is made with respect to how it samples the resistance (standard pullup, slope converter or something different)

Any one has a hit for a low cost resistance simulator?

Thanks

Klaus

On Thu, 3 Aug 2023 14:29:44 -0700 (PDT), Klaus Kragelund
<klaus.kragelund@gmail.com> wrote:

>Hi
>
>I need to fool a heat pump into controlling the output temperature.
>
>Normally the heat pump is controlled with an external NTC resistor (from 1kohm to 15kohm).
>
>I will control it with USB from a PC by connecting a controlled source.
>
>I have this one in my lab, but it only goes to 4kohm:
>
>https://www.orbitadigital.com/en/cctv/accessoires/tools/34781-uni-trend-ut714-temperature-process-calibrator-lcd-display-up-to-2000.html
>
>(900 USD)
>
>Then I found a Fluke 753 that goes up to 10kohm, nice.
>
>But, a whopping 8000 USD:
>
>https://dk.rs-online.com/web/p/multifunktions-kalibratorer/2532268?cm_mmc=DK-PPC-DS3A-_-google-_-3_DK_DK_Test-+og+m%C3%A5leudstyr_Multifunktions+kalibratorer_BMM-_-Fluke+-+2532268+-+FLUKE+753-_-fluke+753&matchtype=p&kwd-27811768253&gclid=Cj0KCQjwoK2mBhDzARIsADGbjeovo4mcqBUBz15CsBcKfSEaHcXyftA6G7oUimzSK_dOQ-EzuqyA2J4aApF7EALw_wcB&gclsrc=aw.ds
>
>I could spin my own, just for this function. It's for a client, so cannot be a project that takes time, since just 10 hours spend, then I could buy the Fluke 753 instead.
>
>I could use an electronic load, but they normally have large capacitance, and I don't know how the heat pump implementation is made with respect to how it samples the resistance (standard pullup, slope converter or something different)
>
>Any one has a hit for a low cost resistance simulator?
>
>Thanks
>
>Klaus

Wow. I woke up at 3AM with a new concept for an electronic
load box. We have orders for 16 and no design, actually no
specs! It would have to be high power and needs to behave
well with PWM drive. It would look like a solenoid or a
torque motor or a servo valve to a controller.

I have a different simple circuit for a fake resistor, using
an opamp and an MDAC, which I could share. But you may as
well use a DPOT.

10 hours isn't a lot of time to do a USB programmed
resistor. Can you just turn a resistor on and off, with a
LabJack or something?

On Thursday, 3 August 2023 at 23:58:34 UTC+2, John Larkin wrote:
> On Thu, 3 Aug 2023 14:29:44 -0700 (PDT), Klaus Kragelund
> <klaus.k...@gmail.com> wrote:
>
> >Hi
> >
> >I need to fool a heat pump into controlling the output temperature.
> >
> >Normally the heat pump is controlled with an external NTC resistor (from 1kohm to 15kohm).
> >
> >I will control it with USB from a PC by connecting a controlled source.
> >
> >I have this one in my lab, but it only goes to 4kohm:
> >
> >https://www.orbitadigital.com/en/cctv/accessoires/tools/34781-uni-trend-ut714-temperature-process-calibrator-lcd-display-up-to-2000.html
> >
> >(900 USD)
> >
> >Then I found a Fluke 753 that goes up to 10kohm, nice.
> >
> >But, a whopping 8000 USD:
> >
> >https://dk.rs-online.com/web/p/multifunktions-kalibratorer/2532268?cm_mmc=DK-PPC-DS3A-_-google-_-3_DK_DK_Test-+og+m%C3%A5leudstyr_Multifunktions+kalibratorer_BMM-_-Fluke+-+2532268+-+FLUKE+753-_-fluke+753&matchtype=p&kwd-27811768253&gclid=Cj0KCQjwoK2mBhDzARIsADGbjeovo4mcqBUBz15CsBcKfSEaHcXyftA6G7oUimzSK_dOQ-EzuqyA2J4aApF7EALw_wcB&gclsrc=aw.ds
> >
> >I could spin my own, just for this function. It's for a client, so cannot be a project that takes time, since just 10 hours spend, then I could buy the Fluke 753 instead.
> >
> >I could use an electronic load, but they normally have large capacitance, and I don't know how the heat pump implementation is made with respect to how it samples the resistance (standard pullup, slope converter or something different)
> >
> >Any one has a hit for a low cost resistance simulator?
> >
> >Thanks
> >
> >Klaus
> Wow. I woke up at 3AM with a new concept for an electronic
> load box. We have orders for 16 and no design, actually no
> specs! It would have to be high power and needs to behave
> well with PWM drive. It would look like a solenoid or a
> torque motor or a servo valve to a controller.
>
> I have a different simple circuit for a fake resistor, using
> an opamp and an MDAC, which I could share. But you may as
> well use a DPOT.

Yes, I was thinking about a DPOT. Maybe just an eval board, with a USB to SPI converter, and then it's easy and fast

MDAC could also be an option

>
> 10 hours isn't a lot of time to do a USB programmed
> resistor. Can you just turn a resistor on and off, with a
> LabJack or something?

Now you say it, I have a Siglent scanning multimeter, so could arrange the relays in R2R fashion. That would be fast and sure to work, with no capacitance at all and floating. Nice hint :-)

Could just buy some cheap relay modules from Aliexpress...

fredag den 4. august 2023 kl. 00.08.17 UTC+2 skrev Klaus Kragelund:
> On Thursday, 3 August 2023 at 23:58:34 UTC+2, John Larkin wrote:
> > On Thu, 3 Aug 2023 14:29:44 -0700 (PDT), Klaus Kragelund
> > <klaus.k...@gmail.com> wrote:
> >
> > >Hi
> > >
> > >I need to fool a heat pump into controlling the output temperature.
> > >
> > >Normally the heat pump is controlled with an external NTC resistor (from 1kohm to 15kohm).
> > >
> > >I will control it with USB from a PC by connecting a controlled source..
> > >
> > >I have this one in my lab, but it only goes to 4kohm:
> > >
> > >https://www.orbitadigital.com/en/cctv/accessoires/tools/34781-uni-trend-ut714-temperature-process-calibrator-lcd-display-up-to-2000.html
> > >
> > >(900 USD)
> > >
> > >Then I found a Fluke 753 that goes up to 10kohm, nice.
> > >
> > >But, a whopping 8000 USD:
> > >
> > >https://dk.rs-online.com/web/p/multifunktions-kalibratorer/2532268?cm_mmc=DK-PPC-DS3A-_-google-_-3_DK_DK_Test-+og+m%C3%A5leudstyr_Multifunktions+kalibratorer_BMM-_-Fluke+-+2532268+-+FLUKE+753-_-fluke+753&matchtype=p&kwd-27811768253&gclid=Cj0KCQjwoK2mBhDzARIsADGbjeovo4mcqBUBz15CsBcKfSEaHcXyftA6G7oUimzSK_dOQ-EzuqyA2J4aApF7EALw_wcB&gclsrc=aw.ds
> > >
> > >I could spin my own, just for this function. It's for a client, so cannot be a project that takes time, since just 10 hours spend, then I could buy the Fluke 753 instead.
> > >
> > >I could use an electronic load, but they normally have large capacitance, and I don't know how the heat pump implementation is made with respect to how it samples the resistance (standard pullup, slope converter or something different)
> > >
> > >Any one has a hit for a low cost resistance simulator?
> > >
> > >Thanks
> > >
> > >Klaus
> > Wow. I woke up at 3AM with a new concept for an electronic
> > load box. We have orders for 16 and no design, actually no
> > specs! It would have to be high power and needs to behave
> > well with PWM drive. It would look like a solenoid or a
> > torque motor or a servo valve to a controller.
> >
> > I have a different simple circuit for a fake resistor, using
> > an opamp and an MDAC, which I could share. But you may as
> > well use a DPOT.
> Yes, I was thinking about a DPOT. Maybe just an eval board, with a USB to SPI converter, and then it's easy and fast
>
> MDAC could also be an option
> >
> > 10 hours isn't a lot of time to do a USB programmed
> > resistor. Can you just turn a resistor on and off, with a
> > LabJack or something?
> Now you say it, I have a Siglent scanning multimeter, so could arrange the relays in R2R fashion. That would be fast and sure to work, with no capacitance at all and floating. Nice hint :-)
>
> Could just buy some cheap relay modules from Aliexpress...

https://www.conradelektronik.dk/p/deditec-usb-relais-8-a-udgangsmodul-usb-antal-reludgange-8-196660

then you don't have to wait

On Friday, 4 August 2023 at 00:58:10 UTC+2, Lasse Langwadt Christensen wrote:
> fredag den 4. august 2023 kl. 00.08.17 UTC+2 skrev Klaus Kragelund:
> > On Thursday, 3 August 2023 at 23:58:34 UTC+2, John Larkin wrote:
> > > On Thu, 3 Aug 2023 14:29:44 -0700 (PDT), Klaus Kragelund
> > > <klaus.k...@gmail.com> wrote:
> > >
> > > >Hi
> > > >
> > > >I need to fool a heat pump into controlling the output temperature.
> > > >
> > > >Normally the heat pump is controlled with an external NTC resistor (from 1kohm to 15kohm).
> > > >
> > > >I will control it with USB from a PC by connecting a controlled source.
> > > >
> > > >I have this one in my lab, but it only goes to 4kohm:
> > > >
> > > >https://www.orbitadigital.com/en/cctv/accessoires/tools/34781-uni-trend-ut714-temperature-process-calibrator-lcd-display-up-to-2000.html
> > > >
> > > >(900 USD)
> > > >
> > > >Then I found a Fluke 753 that goes up to 10kohm, nice.
> > > >
> > > >But, a whopping 8000 USD:
> > > >
> > > >https://dk.rs-online.com/web/p/multifunktions-kalibratorer/2532268?cm_mmc=DK-PPC-DS3A-_-google-_-3_DK_DK_Test-+og+m%C3%A5leudstyr_Multifunktions+kalibratorer_BMM-_-Fluke+-+2532268+-+FLUKE+753-_-fluke+753&matchtype=p&kwd-27811768253&gclid=Cj0KCQjwoK2mBhDzARIsADGbjeovo4mcqBUBz15CsBcKfSEaHcXyftA6G7oUimzSK_dOQ-EzuqyA2J4aApF7EALw_wcB&gclsrc=aw.ds
> > > >
> > > >I could spin my own, just for this function. It's for a client, so cannot be a project that takes time, since just 10 hours spend, then I could buy the Fluke 753 instead.
> > > >
> > > >I could use an electronic load, but they normally have large capacitance, and I don't know how the heat pump implementation is made with respect to how it samples the resistance (standard pullup, slope converter or something different)
> > > >
> > > >Any one has a hit for a low cost resistance simulator?
> > > >
> > > >Thanks
> > > >
> > > >Klaus
> > > Wow. I woke up at 3AM with a new concept for an electronic
> > > load box. We have orders for 16 and no design, actually no
> > > specs! It would have to be high power and needs to behave
> > > well with PWM drive. It would look like a solenoid or a
> > > torque motor or a servo valve to a controller.
> > >
> > > I have a different simple circuit for a fake resistor, using
> > > an opamp and an MDAC, which I could share. But you may as
> > > well use a DPOT.
> > Yes, I was thinking about a DPOT. Maybe just an eval board, with a USB to SPI converter, and then it's easy and fast
> >
> > MDAC could also be an option
> > >
> > > 10 hours isn't a lot of time to do a USB programmed
> > > resistor. Can you just turn a resistor on and off, with a
> > > LabJack or something?
> > Now you say it, I have a Siglent scanning multimeter, so could arrange the relays in R2R fashion. That would be fast and sure to work, with no capacitance at all and floating. Nice hint :-)
> >
> > Could just buy some cheap relay modules from Aliexpress...
> https://www.conradelektronik.dk/p/deditec-usb-relais-8-a-udgangsmodul-usb-antal-reludgange-8-196660
>
> then you don't have to wait

Very nice, thanks Lasse

I found this on a Danish site:

https://arduinotech.dk/shop/relae-8-kanal/

But it is individual channels, so takes up pins on the Raspberry Pi I am using. So will definitely go for your suggestion

During the search I also found a USB controlled 8 channel Digital potentiometer. Quite nice:

https://relaypros.com/Relay/Device/ZPOT8PROXR_USB

(only 5V max)

SPI interface evaluation module:

https://www.digikey.com/en/products/detail/digilent-inc/410-239/4090065

On Thursday, August 3, 2023 at 2:29:49 PM UTC-7, Klaus Kragelund wrote:
> Hi
>
> I need to fool a heat pump into controlling the output temperature.
>
> Normally the heat pump is controlled with an external NTC resistor (from 1kohm to 15kohm).
>
> I will control it with USB from a PC by connecting a controlled source.
>
> I have this one in my lab, but it only goes to 4kohm:
>
> https://www.orbitadigital.com/en/cctv/accessoires/tools/34781-uni-trend-ut714-temperature-process-calibrator-lcd-display-up-to-2000.html
>
> (900 USD)
>
> Then I found a Fluke 753 that goes up to 10kohm, nice.
>
> But, a whopping 8000 USD:
>
> https://dk.rs-online.com/web/p/multifunktions-kalibratorer/2532268?cm_mmc=DK-PPC-DS3A-_-google-_-3_DK_DK_Test-+og+m%C3%A5leudstyr_Multifunktions+kalibratorer_BMM-_-Fluke+-+2532268+-+FLUKE+753-_-fluke+753&matchtype=p&kwd-27811768253&gclid=Cj0KCQjwoK2mBhDzARIsADGbjeovo4mcqBUBz15CsBcKfSEaHcXyftA6G7oUimzSK_dOQ-EzuqyA2J4aApF7EALw_wcB&gclsrc=aw.ds
>
> I could spin my own, just for this function. It's for a client, so cannot be a project that takes time, since just 10 hours spend, then I could buy the Fluke 753 instead.
>
> I could use an electronic load, but they normally have large capacitance, and I don't know how the heat pump implementation is made with respect to how it samples the resistance (standard pullup, slope converter or something different)
>
> Any one has a hit for a low cost resistance simulator?

If you know it's not gonna take a lot of current, a resistor is just a current controlled by a voltage;
LM13700 or any equivalent OTA has that basic function. You have to identify one terminal
of the resistance that is not grounded nor driven (that's where the current source/sinks from)
and connect the differential input (with suitable attenuation) to both terminals.
And, there's the small matter of nonlinearity for large voltages on the differential inputs.

The upside: it's cheap. About a dollar.
The downsides: you gotta power it, and it has compliance limited by its power rails, and the bias
current has to be supplied to a negative-rail current mirror input. If the thermistor is balancing
a bridge, that bias current controls the resistance (actually trans-resistance, before you connected
the output to one voltage-sense input pin).

On Friday, 4 August 2023 at 09:09:17 UTC+2, whit3rd wrote:
> On Thursday, August 3, 2023 at 2:29:49 PM UTC-7, Klaus Kragelund wrote:
> > Hi
> >
> > I need to fool a heat pump into controlling the output temperature.
> >
> > Normally the heat pump is controlled with an external NTC resistor (from 1kohm to 15kohm).
> >
> > I will control it with USB from a PC by connecting a controlled source.
> >
> > I have this one in my lab, but it only goes to 4kohm:
> >
> > https://www.orbitadigital.com/en/cctv/accessoires/tools/34781-uni-trend-ut714-temperature-process-calibrator-lcd-display-up-to-2000.html
> >
> > (900 USD)
> >
> > Then I found a Fluke 753 that goes up to 10kohm, nice.
> >
> > But, a whopping 8000 USD:
> >
> > https://dk.rs-online.com/web/p/multifunktions-kalibratorer/2532268?cm_mmc=DK-PPC-DS3A-_-google-_-3_DK_DK_Test-+og+m%C3%A5leudstyr_Multifunktions+kalibratorer_BMM-_-Fluke+-+2532268+-+FLUKE+753-_-fluke+753&matchtype=p&kwd-27811768253&gclid=Cj0KCQjwoK2mBhDzARIsADGbjeovo4mcqBUBz15CsBcKfSEaHcXyftA6G7oUimzSK_dOQ-EzuqyA2J4aApF7EALw_wcB&gclsrc=aw.ds
> >
> > I could spin my own, just for this function. It's for a client, so cannot be a project that takes time, since just 10 hours spend, then I could buy the Fluke 753 instead.
> >
> > I could use an electronic load, but they normally have large capacitance, and I don't know how the heat pump implementation is made with respect to how it samples the resistance (standard pullup, slope converter or something different)
> >
> > Any one has a hit for a low cost resistance simulator?
> If you know it's not gonna take a lot of current, a resistor is just a current controlled by a voltage;
> LM13700 or any equivalent OTA has that basic function. You have to identify one terminal
> of the resistance that is not grounded nor driven (that's where the current source/sinks from)
> and connect the differential input (with suitable attenuation) to both terminals.
> And, there's the small matter of nonlinearity for large voltages on the differential inputs.
>
> The upside: it's cheap. About a dollar.
> The downsides: you gotta power it, and it has compliance limited by its power rails, and the bias
> current has to be supplied to a negative-rail current mirror input. If the thermistor is balancing
> a bridge, that bias current controls the resistance (actually trans-resistance, before you connected
> the output to one voltage-sense input pin).

It depends on how the measurement is made. The NTC could be fed from a standard pullup scenario, then a current source would work. It could be multiplexed, it might not work.
It could be a slope converter type, would not work. Could be a bridge configuration, higher voltage, would probably not work. The resistor method with relay is simple and guaranteed to work at almost any voltage and drive method.

On 03/08/2023 22:29, Klaus Kragelund wrote:
> Hi
>
> I need to fool a heat pump into controlling the output temperature.
>
> Normally the heat pump is controlled with an external NTC resistor (from 1kohm to 15kohm).

<snip>

> Any one has a hit for a low cost resistance simulator?
>
> Thanks
>
> Klaus

Vactrol? (Though I doubt they're still made.)

--
Cheers
Clive

On 2023-08-04 10:30, Klaus Kragelund wrote:
> On Friday, 4 August 2023 at 09:09:17 UTC+2, whit3rd wrote:
>> On Thursday, August 3, 2023 at 2:29:49 PM UTC-7, Klaus Kragelund wrote:
>>> Hi
>>>
>>> I need to fool a heat pump into controlling the output temperature.
>>>
>>> Normally the heat pump is controlled with an external NTC resistor (from 1kohm to 15kohm).
>>>
>>> I will control it with USB from a PC by connecting a controlled source.
>>>
>>> I have this one in my lab, but it only goes to 4kohm:
>>>
>>> https://www.orbitadigital.com/en/cctv/accessoires/tools/34781-uni-trend-ut714-temperature-process-calibrator-lcd-display-up-to-2000.html
>>>
>>> (900 USD)
>>>
>>> Then I found a Fluke 753 that goes up to 10kohm, nice.
>>>
>>> But, a whopping 8000 USD:
>>>
>>> https://dk.rs-online.com/web/p/multifunktions-kalibratorer/2532268?cm_mmc=DK-PPC-DS3A-_-google-_-3_DK_DK_Test-+og+m%C3%A5leudstyr_Multifunktions+kalibratorer_BMM-_-Fluke+-+2532268+-+FLUKE+753-_-fluke+753&matchtype=p&kwd-27811768253&gclid=Cj0KCQjwoK2mBhDzARIsADGbjeovo4mcqBUBz15CsBcKfSEaHcXyftA6G7oUimzSK_dOQ-EzuqyA2J4aApF7EALw_wcB&gclsrc=aw.ds
>>>
>>> I could spin my own, just for this function. It's for a client, so cannot be a project that takes time, since just 10 hours spend, then I could buy the Fluke 753 instead.
>>>
>>> I could use an electronic load, but they normally have large capacitance, and I don't know how the heat pump implementation is made with respect to how it samples the resistance (standard pullup, slope converter or something different)
>>>
>>> Any one has a hit for a low cost resistance simulator?
>> If you know it's not gonna take a lot of current, a resistor is just a current controlled by a voltage;
>> LM13700 or any equivalent OTA has that basic function. You have to identify one terminal
>> of the resistance that is not grounded nor driven (that's where the current source/sinks from)
>> and connect the differential input (with suitable attenuation) to both terminals.
>> And, there's the small matter of nonlinearity for large voltages on the differential inputs.
>>
>> The upside: it's cheap. About a dollar.
>> The downsides: you gotta power it, and it has compliance limited by its power rails, and the bias
>> current has to be supplied to a negative-rail current mirror input. If the thermistor is balancing
>> a bridge, that bias current controls the resistance (actually trans-resistance, before you connected
>> the output to one voltage-sense input pin).
>
> It depends on how the measurement is made. The NTC could be fed from a standard pullup scenario, then a current source would work. It could be multiplexed, it might not work.
> It could be a slope converter type, would not work. Could be a bridge configuration, higher voltage, would probably not work. The resistor method with relay is simple and guaranteed to work at almost any voltage and drive method.
>

Of course you could just put two NTCs on a small Peltier (TEC), close a
feedback loop on one, and connect the other to the heat pump.

I usually do temperature control loops by putting a copper pour next to
the TEC (with HS grease), with one side hanging out a bit, and soldering
one end of an 0603 NTC to it, right next to the TEC.

One of those on each side, with an aluminum spreader to make sure the
two sides track each other, would be easy to hack together in an
afternoon, including the FB loop.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On Friday, 4 August 2023 at 17:35:52 UTC+2, Phil Hobbs wrote:
> On 2023-08-04 10:30, Klaus Kragelund wrote:
> > On Friday, 4 August 2023 at 09:09:17 UTC+2, whit3rd wrote:
> >> On Thursday, August 3, 2023 at 2:29:49 PM UTC-7, Klaus Kragelund wrote:
> >>> Hi
> >>>
> >>> I need to fool a heat pump into controlling the output temperature.
> >>>
> >>> Normally the heat pump is controlled with an external NTC resistor (from 1kohm to 15kohm).
> >>>
> >>> I will control it with USB from a PC by connecting a controlled source.
> >>>
> >>> I have this one in my lab, but it only goes to 4kohm:
> >>>
> >>> https://www.orbitadigital.com/en/cctv/accessoires/tools/34781-uni-trend-ut714-temperature-process-calibrator-lcd-display-up-to-2000.html
> >>>
> >>> (900 USD)
> >>>
> >>> Then I found a Fluke 753 that goes up to 10kohm, nice.
> >>>
> >>> But, a whopping 8000 USD:
> >>>
> >>> https://dk.rs-online.com/web/p/multifunktions-kalibratorer/2532268?cm_mmc=DK-PPC-DS3A-_-google-_-3_DK_DK_Test-+og+m%C3%A5leudstyr_Multifunktions+kalibratorer_BMM-_-Fluke+-+2532268+-+FLUKE+753-_-fluke+753&matchtype=p&kwd-27811768253&gclid=Cj0KCQjwoK2mBhDzARIsADGbjeovo4mcqBUBz15CsBcKfSEaHcXyftA6G7oUimzSK_dOQ-EzuqyA2J4aApF7EALw_wcB&gclsrc=aw.ds
> >>>
> >>> I could spin my own, just for this function. It's for a client, so cannot be a project that takes time, since just 10 hours spend, then I could buy the Fluke 753 instead.
> >>>
> >>> I could use an electronic load, but they normally have large capacitance, and I don't know how the heat pump implementation is made with respect to how it samples the resistance (standard pullup, slope converter or something different)
> >>>
> >>> Any one has a hit for a low cost resistance simulator?
> >> If you know it's not gonna take a lot of current, a resistor is just a current controlled by a voltage;
> >> LM13700 or any equivalent OTA has that basic function. You have to identify one terminal
> >> of the resistance that is not grounded nor driven (that's where the current source/sinks from)
> >> and connect the differential input (with suitable attenuation) to both terminals.
> >> And, there's the small matter of nonlinearity for large voltages on the differential inputs.
> >>
> >> The upside: it's cheap. About a dollar.
> >> The downsides: you gotta power it, and it has compliance limited by its power rails, and the bias
> >> current has to be supplied to a negative-rail current mirror input. If the thermistor is balancing
> >> a bridge, that bias current controls the resistance (actually trans-resistance, before you connected
> >> the output to one voltage-sense input pin).
> >
> > It depends on how the measurement is made. The NTC could be fed from a standard pullup scenario, then a current source would work. It could be multiplexed, it might not work.
> > It could be a slope converter type, would not work. Could be a bridge configuration, higher voltage, would probably not work. The resistor method with relay is simple and guaranteed to work at almost any voltage and drive method.
> >
> Of course you could just put two NTCs on a small Peltier (TEC), close a
> feedback loop on one, and connect the other to the heat pump.
>
> I usually do temperature control loops by putting a copper pour next to
> the TEC (with HS grease), with one side hanging out a bit, and soldering
> one end of an 0603 NTC to it, right next to the TEC.
>
> One of those on each side, with an aluminum spreader to make sure the
> two sides track each other, would be easy to hack together in an
> afternoon, including the FB loop.
>
Yes, that also crossed my mind. Never tried that before, but could be a fun exercise.

Could do it mechanically, a servo motor on a dual gang potentiometer. One potentiometer connected to the regulation loop, the other to the heat pump.

fredag den 4. august 2023 kl. 18.01.36 UTC+2 skrev Klaus Kragelund:
> On Friday, 4 August 2023 at 17:35:52 UTC+2, Phil Hobbs wrote:
> > On 2023-08-04 10:30, Klaus Kragelund wrote:
> > > On Friday, 4 August 2023 at 09:09:17 UTC+2, whit3rd wrote:
> > >> On Thursday, August 3, 2023 at 2:29:49 PM UTC-7, Klaus Kragelund wrote:
> > >>> Hi
> > >>>
> > >>> I need to fool a heat pump into controlling the output temperature.
> > >>>
> > >>> Normally the heat pump is controlled with an external NTC resistor (from 1kohm to 15kohm).
> > >>>
> > >>> I will control it with USB from a PC by connecting a controlled source.
> > >>>
> > >>> I have this one in my lab, but it only goes to 4kohm:
> > >>>
> > >>> https://www.orbitadigital.com/en/cctv/accessoires/tools/34781-uni-trend-ut714-temperature-process-calibrator-lcd-display-up-to-2000.html
> > >>>
> > >>> (900 USD)
> > >>>
> > >>> Then I found a Fluke 753 that goes up to 10kohm, nice.
> > >>>
> > >>> But, a whopping 8000 USD:
> > >>>
> > >>> https://dk.rs-online.com/web/p/multifunktions-kalibratorer/2532268?cm_mmc=DK-PPC-DS3A-_-google-_-3_DK_DK_Test-+og+m%C3%A5leudstyr_Multifunktions+kalibratorer_BMM-_-Fluke+-+2532268+-+FLUKE+753-_-fluke+753&matchtype=p&kwd-27811768253&gclid=Cj0KCQjwoK2mBhDzARIsADGbjeovo4mcqBUBz15CsBcKfSEaHcXyftA6G7oUimzSK_dOQ-EzuqyA2J4aApF7EALw_wcB&gclsrc=aw.ds
> > >>>
> > >>> I could spin my own, just for this function. It's for a client, so cannot be a project that takes time, since just 10 hours spend, then I could buy the Fluke 753 instead.
> > >>>
> > >>> I could use an electronic load, but they normally have large capacitance, and I don't know how the heat pump implementation is made with respect to how it samples the resistance (standard pullup, slope converter or something different)
> > >>>
> > >>> Any one has a hit for a low cost resistance simulator?
> > >> If you know it's not gonna take a lot of current, a resistor is just a current controlled by a voltage;
> > >> LM13700 or any equivalent OTA has that basic function. You have to identify one terminal
> > >> of the resistance that is not grounded nor driven (that's where the current source/sinks from)
> > >> and connect the differential input (with suitable attenuation) to both terminals.
> > >> And, there's the small matter of nonlinearity for large voltages on the differential inputs.
> > >>
> > >> The upside: it's cheap. About a dollar.
> > >> The downsides: you gotta power it, and it has compliance limited by its power rails, and the bias
> > >> current has to be supplied to a negative-rail current mirror input. If the thermistor is balancing
> > >> a bridge, that bias current controls the resistance (actually trans-resistance, before you connected
> > >> the output to one voltage-sense input pin).
> > >
> > > It depends on how the measurement is made. The NTC could be fed from a standard pullup scenario, then a current source would work. It could be multiplexed, it might not work.
> > > It could be a slope converter type, would not work. Could be a bridge configuration, higher voltage, would probably not work. The resistor method with relay is simple and guaranteed to work at almost any voltage and drive method.
> > >
> > Of course you could just put two NTCs on a small Peltier (TEC), close a
> > feedback loop on one, and connect the other to the heat pump.
> >
> > I usually do temperature control loops by putting a copper pour next to
> > the TEC (with HS grease), with one side hanging out a bit, and soldering
> > one end of an 0603 NTC to it, right next to the TEC.
> >
> > One of those on each side, with an aluminum spreader to make sure the
> > two sides track each other, would be easy to hack together in an
> > afternoon, including the FB loop.
> >
> Yes, that also crossed my mind. Never tried that before, but could be a fun exercise.
>
> Could do it mechanically, a servo motor on a dual gang potentiometer. One potentiometer connected to the regulation loop, the other to the heat pump..

motorized potentiometers are available off the shelf

fredag den 4. august 2023 kl. 16.52.00 UTC+2 skrev Clive Arthur:
> On 03/08/2023 22:29, Klaus Kragelund wrote:
> > Hi
> >
> > I need to fool a heat pump into controlling the output temperature.
> >
> > Normally the heat pump is controlled with an external NTC resistor (from 1kohm to 15kohm).
> <snip>
> > Any one has a hit for a low cost resistance simulator?
> >
> > Thanks
> >
> > Klaus
> Vactrol? (Though I doubt they're still made.)

you can still get them, https://softgenie.dk/ic/1536-vtl5c-analog-opto-coubler.html

On Friday, 4 August 2023 at 18:12:22 UTC+2, Lasse Langwadt Christensen wrote:
> fredag den 4. august 2023 kl. 16.52.00 UTC+2 skrev Clive Arthur:
> > On 03/08/2023 22:29, Klaus Kragelund wrote:
> > > Hi
> > >
> > > I need to fool a heat pump into controlling the output temperature.
> > >
> > > Normally the heat pump is controlled with an external NTC resistor (from 1kohm to 15kohm).
> > <snip>
> > > Any one has a hit for a low cost resistance simulator?
> > >
> > > Thanks
> > >
> > > Klaus
> > Vactrol? (Though I doubt they're still made.)
> you can still get them, https://softgenie.dk/ic/1536-vtl5c-analog-opto-coubler.html

https://softgenie.dk/?controller=attachment&id_attachment=291

Not very linear though, for my application it needs to be linear, or I would need to measure it out

Could be easy to compensate though, just record what the heat pump reports the outside temperature is, and make a table lookup

On 04/08/2023 17:27, Klaus Kragelund wrote:
> On Friday, 4 August 2023 at 18:12:22 UTC+2, Lasse Langwadt Christensen wrote:
>> fredag den 4. august 2023 kl. 16.52.00 UTC+2 skrev Clive Arthur:
>>> On 03/08/2023 22:29, Klaus Kragelund wrote:
>>>> Hi
>>>>
>>>> I need to fool a heat pump into controlling the output temperature.
>>>>
>>>> Normally the heat pump is controlled with an external NTC resistor (from 1kohm to 15kohm).
>>> <snip>
>>>> Any one has a hit for a low cost resistance simulator?
>>>>
>>>> Thanks
>>>>
>>>> Klaus
>>> Vactrol? (Though I doubt they're still made.)
>> you can still get them, https://softgenie.dk/ic/1536-vtl5c-analog-opto-coubler.html
>
> https://softgenie.dk/?controller=attachment&id_attachment=291
>
> Not very linear though, for my application it needs to be linear, or I would need to measure it out
>
> Could be easy to compensate though, just record what the heat pump reports the outside temperature is, and make a table lookup

IIRC you used to be able to get two in a package - one LED with two LDRs
- and you could wrap one LDR around an op-amp to linearise the other one.

--
Cheers
Clive

On Friday, August 4, 2023 at 7:30:35 AM UTC-7, Klaus Kragelund wrote:
> On Friday, 4 August 2023 at 09:09:17 UTC+2, whit3rd wrote:
> > On Thursday, August 3, 2023 at 2:29:49 PM UTC-7, Klaus Kragelund wrote:
> > > Hi
> > >
> > > I need to fool a heat pump into controlling the output temperature.
> > >
> > > Normally the heat pump is controlled with an external NTC resistor (from 1kohm to 15kohm).
> > >
> > > I will control it with USB from a PC by connecting a controlled source.
....

> > > Any one has a hit for a low cost resistance simulator?

> > If you know it's not gonna take a lot of current, a resistor is just a current controlled by a voltage;
> > LM13700 or any equivalent OTA has that basic function.

> > The upside: it's cheap. About a dollar.
> > The downsides: you gotta power it, and it has compliance limited by its power rails, and the bias
> > current has to be supplied to a negative-rail current mirror input. If the thermistor is balancing
> > a bridge, that bias current controls the resistance (actually trans-resistance, before you connected
> > the output to one voltage-sense input pin).

> It depends on how the measurement is made. The NTC could be fed from a standard pullup scenario, then a current source would work. It could be multiplexed, it might not work.

Why would multiplexing be a problem? There's a bit of imbalance if the current has no where to go, but
you could clamp it somewhere safe.

> It could be a slope converter type, would not work.

Would work fine, as far as I can tell. It might have a current-limit issue if the slope is very fast (high
current, low duty cycle doesn't self-heat the original thermistor), though. My proposal is NOT a
fixed current source, it uses the differential input terminals to make a resistance-like element, so
if the original thermistor made a R-C exponential decay, so will the OTA circuit,

>Could be a bridge configuration, higher voltage, would probably not work.

If the thermistor is 1k to 15k, and it doesn't self-heat horribly, it's NOT driven by 'higher voltage'. That's
only a problem if your power supply for the chip isn't ground-referenced at the appropriate level.

>The resistor method with relay is simple and guaranteed to work at almost any voltage and drive method.

On 04-08-2023 21:24, whit3rd wrote:
> On Friday, August 4, 2023 at 7:30:35 AM UTC-7, Klaus Kragelund wrote:
>> On Friday, 4 August 2023 at 09:09:17 UTC+2, whit3rd wrote:
>>> On Thursday, August 3, 2023 at 2:29:49 PM UTC-7, Klaus Kragelund wrote:
>>>> Hi
>>>>
>>>> I need to fool a heat pump into controlling the output temperature.
>>>>
>>>> Normally the heat pump is controlled with an external NTC resistor (from 1kohm to 15kohm).
>>>>
>>>> I will control it with USB from a PC by connecting a controlled source.
> ...
>
>>>> Any one has a hit for a low cost resistance simulator?
>
>>> If you know it's not gonna take a lot of current, a resistor is just a current controlled by a voltage;
>>> LM13700 or any equivalent OTA has that basic function.
>
>>> The upside: it's cheap. About a dollar.
>>> The downsides: you gotta power it, and it has compliance limited by its power rails, and the bias
>>> current has to be supplied to a negative-rail current mirror input. If the thermistor is balancing
>>> a bridge, that bias current controls the resistance (actually trans-resistance, before you connected
>>> the output to one voltage-sense input pin).
>
>> It depends on how the measurement is made. The NTC could be fed from a standard pullup scenario, then a current source would work. It could be multiplexed, it might not work.
>
> Why would multiplexing be a problem? There's a bit of imbalance if the current has no where to go, but
> you could clamp it somewhere safe.
>
>> It could be a slope converter type, would not work.
>
> Would work fine, as far as I can tell. It might have a current-limit issue if the slope is very fast (high
> current, low duty cycle doesn't self-heat the original thermistor), though. My proposal is NOT a
> fixed current source, it uses the differential input terminals to make a resistance-like element, so
> if the original thermistor made a R-C exponential decay, so will the OTA circuit,
>

For a slope converter you have a rising voltage and falling voltage, or
it could be a more advanced method where a fixed resistance is used for
reference, and only the time deviation is observed to calculate the
resistance. Such a converter could be fast, so a emulated resistor needs
to have be faster with no overshoots or other settle times.

Your right it could work, it might not work.

>> Could be a bridge configuration, higher voltage, would probably not work.
>
> If the thermistor is 1k to 15k, and it doesn't self-heat horribly, it's NOT driven by 'higher voltage'. That's
> only a problem if your power supply for the chip isn't ground-referenced at the appropriate level.

The temperature circuits I have seen/have designed, are all driven from
voltages below 5V, so yes, only a slight risk it would not work.
>
>> The resistor method with relay is simple and guaranteed to work at almost any voltage and drive method.

lørdag den 5. august 2023 kl. 00.27.53 UTC+2 skrev Klaus Vestergaard Kragelund:
> On 04-08-2023 21:24, whit3rd wrote:
> > On Friday, August 4, 2023 at 7:30:35 AM UTC-7, Klaus Kragelund wrote:
> >> On Friday, 4 August 2023 at 09:09:17 UTC+2, whit3rd wrote:
> >>> On Thursday, August 3, 2023 at 2:29:49 PM UTC-7, Klaus Kragelund wrote:
> >>>> Hi
> >>>>
> >>>> I need to fool a heat pump into controlling the output temperature.
> >>>>
> >>>> Normally the heat pump is controlled with an external NTC resistor (from 1kohm to 15kohm).
> >>>>
> >>>> I will control it with USB from a PC by connecting a controlled source.
> > ...
> >
> >>>> Any one has a hit for a low cost resistance simulator?
> >
> >>> If you know it's not gonna take a lot of current, a resistor is just a current controlled by a voltage;
> >>> LM13700 or any equivalent OTA has that basic function.
> >
> >>> The upside: it's cheap. About a dollar.
> >>> The downsides: you gotta power it, and it has compliance limited by its power rails, and the bias
> >>> current has to be supplied to a negative-rail current mirror input. If the thermistor is balancing
> >>> a bridge, that bias current controls the resistance (actually trans-resistance, before you connected
> >>> the output to one voltage-sense input pin).
> >
> >> It depends on how the measurement is made. The NTC could be fed from a standard pullup scenario, then a current source would work. It could be multiplexed, it might not work.
> >
> > Why would multiplexing be a problem? There's a bit of imbalance if the current has no where to go, but
> > you could clamp it somewhere safe.
> >
> >> It could be a slope converter type, would not work.
> >
> > Would work fine, as far as I can tell. It might have a current-limit issue if the slope is very fast (high
> > current, low duty cycle doesn't self-heat the original thermistor), though. My proposal is NOT a
> > fixed current source, it uses the differential input terminals to make a resistance-like element, so
> > if the original thermistor made a R-C exponential decay, so will the OTA circuit,
> >
> For a slope converter you have a rising voltage and falling voltage, or
> it could be a more advanced method where a fixed resistance is used for
> reference, and only the time deviation is observed to calculate the
> resistance. Such a converter could be fast, so a emulated resistor needs
> to have be faster with no overshoots or other settle times.
>
> Your right it could work, it might not work.
> >> Could be a bridge configuration, higher voltage, would probably not work.
> >
> > If the thermistor is 1k to 15k, and it doesn't self-heat horribly, it's NOT driven by 'higher voltage'. That's
> > only a problem if your power supply for the chip isn't ground-referenced at the appropriate level.
> The temperature circuits I have seen/have designed, are all driven from
> voltages below 5V, so yes, only a slight risk it would not work.

is it a one-of ? it shouldn't take many minutes with a scope and a few resistors to figure out if it's just a pullup and you can just inject a voltage

On 05-08-2023 00:37, Lasse Langwadt Christensen wrote:
> lørdag den 5. august 2023 kl. 00.27.53 UTC+2 skrev Klaus Vestergaard Kragelund:
>> On 04-08-2023 21:24, whit3rd wrote:
>>> On Friday, August 4, 2023 at 7:30:35 AM UTC-7, Klaus Kragelund wrote:
>>>> On Friday, 4 August 2023 at 09:09:17 UTC+2, whit3rd wrote:
>>>>> On Thursday, August 3, 2023 at 2:29:49 PM UTC-7, Klaus Kragelund wrote:
>>>>>> Hi
>>>>>>
>>>>>> I need to fool a heat pump into controlling the output temperature.
>>>>>>
>>>>>> Normally the heat pump is controlled with an external NTC resistor (from 1kohm to 15kohm).
>>>>>>
>>>>>> I will control it with USB from a PC by connecting a controlled source.
>>> ...
>>>
>>>>>> Any one has a hit for a low cost resistance simulator?
>>>
>>>>> If you know it's not gonna take a lot of current, a resistor is just a current controlled by a voltage;
>>>>> LM13700 or any equivalent OTA has that basic function.
>>>
>>>>> The upside: it's cheap. About a dollar.
>>>>> The downsides: you gotta power it, and it has compliance limited by its power rails, and the bias
>>>>> current has to be supplied to a negative-rail current mirror input. If the thermistor is balancing
>>>>> a bridge, that bias current controls the resistance (actually trans-resistance, before you connected
>>>>> the output to one voltage-sense input pin).
>>>
>>>> It depends on how the measurement is made. The NTC could be fed from a standard pullup scenario, then a current source would work. It could be multiplexed, it might not work.
>>>
>>> Why would multiplexing be a problem? There's a bit of imbalance if the current has no where to go, but
>>> you could clamp it somewhere safe.
>>>
>>>> It could be a slope converter type, would not work.
>>>
>>> Would work fine, as far as I can tell. It might have a current-limit issue if the slope is very fast (high
>>> current, low duty cycle doesn't self-heat the original thermistor), though. My proposal is NOT a
>>> fixed current source, it uses the differential input terminals to make a resistance-like element, so
>>> if the original thermistor made a R-C exponential decay, so will the OTA circuit,
>>>
>> For a slope converter you have a rising voltage and falling voltage, or
>> it could be a more advanced method where a fixed resistance is used for
>> reference, and only the time deviation is observed to calculate the
>> resistance. Such a converter could be fast, so a emulated resistor needs
>> to have be faster with no overshoots or other settle times.
>>
>> Your right it could work, it might not work.
>>>> Could be a bridge configuration, higher voltage, would probably not work.
>>>
>>> If the thermistor is 1k to 15k, and it doesn't self-heat horribly, it's NOT driven by 'higher voltage'. That's
>>> only a problem if your power supply for the chip isn't ground-referenced at the appropriate level.
>> The temperature circuits I have seen/have designed, are all driven from
>> voltages below 5V, so yes, only a slight risk it would not work.
>
> is it a one-of ? it shouldn't take many minutes with a scope and a few resistors to figure out if it's just a pullup and you can just inject a voltage
>
>

They may move to another heat pump, so I need it to work nomatter how
the temperature measurement is done. But, your right, could be
interesting to see how they do it. One always learns from other designs :-)