https://www.dropbox.com/s/8hbfb3wmc4eqcn1/Dpak_Heaters.jpg?raw=1

I can use 6 fets, 3 pairs, to heat my oven block.

Cascode pairs left, center, right with individual DACs to tune the
thermal gradients down a bit.

The fets can go on a PCB with thermal vias to the opposite side, where
the aluminum block is.

--

Father Brown's figure remained quite dark and still;
but in that instant he had lost his head. His head was
always most valuable when he had lost it.

On 2021/08/23 7:47 p.m., jlarkin@highlandsniptechnology.com wrote:
> https://www.dropbox.com/s/8hbfb3wmc4eqcn1/Dpak_Heaters.jpg?raw=1
>
> I can use 6 fets, 3 pairs, to heat my oven block.
>
> Cascode pairs left, center, right with individual DACs to tune the
> thermal gradients down a bit.
>
> The fets can go on a PCB with thermal vias to the opposite side, where
> the aluminum block is.
>
>
>

What have you got against resistors?

Or is it the handy metal tab as a more point heat source?

There are plenty of metal cased resistors after all and they don't tend
to short out if something goes wrong...heck there is nichrome wire for
that matter.

John :-#)#

On Mon, 23 Aug 2021 19:56:20 -0700, John Robertson <spam@flippers.com>
wrote:

>
>On 2021/08/23 7:47 p.m., jlarkin@highlandsniptechnology.com wrote:
>> https://www.dropbox.com/s/8hbfb3wmc4eqcn1/Dpak_Heaters.jpg?raw=1
>>
>> I can use 6 fets, 3 pairs, to heat my oven block.
>>
>> Cascode pairs left, center, right with individual DACs to tune the
>> thermal gradients down a bit.
>>
>> The fets can go on a PCB with thermal vias to the opposite side, where
>> the aluminum block is.
>>
>>
>>
>
>What have you got against resistors?

Power goes as I^2, which wrecks the control loop. Fet power is linear
on current.

The fets conduct heat through the board to the block nicely too.

Fets pick-and-place, no hardware, no tapped holes, no wires.

The fets are cheap!

>
>Or is it the handy metal tab as a more point heat source?

I'll use copper pours to spread the dpak heat contact area. Gap-pads
on the back.

>
>There are plenty of metal cased resistors after all and they don't tend
>to short out if something goes wrong...heck there is nichrome wire for
>that matter.

Too much hand labor.

Actually, we've had the metal case resistors short, cycling at higher
power.

--

Father Brown's figure remained quite dark and still;
but in that instant he had lost his head. His head was
always most valuable when he had lost it.

On a sunny day (Mon, 23 Aug 2021 19:47:46 -0700) it happened
jlarkin@highlandsniptechnology.com wrote in
<hum8igh943hr5u76at18sui2oqg80ajv3r@4ax.com>:

>https://www.dropbox.com/s/8hbfb3wmc4eqcn1/Dpak_Heaters.jpg?raw=1
>
>I can use 6 fets, 3 pairs, to heat my oven block.
>
>Cascode pairs left, center, right with individual DACs to tune the
>thermal gradients down a bit.
>
>The fets can go on a PCB with thermal vias to the opposite side, where
>the aluminum block is.

Did you copy that from my tir-pic experiment ;-)?
http://panteltje.com/panteltje/tri_pic/
TLC274 driving IRLZ34 on hotplate..

On Mon, 23 Aug 2021 19:47:46 -0700, jlarkin@highlandsniptechnology.com
wrote:

>https://www.dropbox.com/s/8hbfb3wmc4eqcn1/Dpak_Heaters.jpg?raw=1
>
>I can use 6 fets, 3 pairs, to heat my oven block.
>
>Cascode pairs left, center, right with individual DACs to tune the
>thermal gradients down a bit.
>
>The fets can go on a PCB with thermal vias to the opposite side, where
>the aluminum block is.

So long as you're happy with <Tsmax, and can avoid it
in transient conditions. The plated-through board won't
like anything over 130C for long, either.

RL

On Tue, 24 Aug 2021 09:34:45 -0400, legg <legg@nospam.magma.ca> wrote:

>On Mon, 23 Aug 2021 19:47:46 -0700, jlarkin@highlandsniptechnology.com
>wrote:
>
>>https://www.dropbox.com/s/8hbfb3wmc4eqcn1/Dpak_Heaters.jpg?raw=1
>>
>>I can use 6 fets, 3 pairs, to heat my oven block.
>>
>>Cascode pairs left, center, right with individual DACs to tune the
>>thermal gradients down a bit.
>>
>>The fets can go on a PCB with thermal vias to the opposite side, where
>>the aluminum block is.
>
>So long as you're happy with <Tsmax, and can avoid it
>in transient conditions. The plated-through board won't
>like anything over 130C for long, either.
>
>RL

I want to heat the oven block to about 35C, which might need maybe 10
watts. I figure that each fet might have to dissipate a couple of
watts, and each dpak tab will rise to about 40C. We might push the
power up a bit at startup, to reduce warmup time, but that's just
code.

Fets make great heaters.

--

Father Brown's figure remained quite dark and still;
but in that instant he had lost his head. His head was
always most valuable when he had lost it.

On Mon, 23 Aug 2021 19:47:46 -0700, jlarkin@highlandsniptechnology.com
wrote:

>https://www.dropbox.com/s/8hbfb3wmc4eqcn1/Dpak_Heaters.jpg?raw=1
>
>I can use 6 fets, 3 pairs, to heat my oven block.
>
>Cascode pairs left, center, right with individual DACs to tune the
>thermal gradients down a bit.
>
>The fets can go on a PCB with thermal vias to the opposite side, where
>the aluminum block is.

Here's a small tweak:

https://www.dropbox.com/s/gc449mrbtg4jeej/Dpak_Heaters_2.jpg?raw=1

Of course, we could just use an opamp per fet and dump the cascode
idea. That would increase the cost of the project about 60 PPM.

https://www.dropbox.com/s/gc449mrbtg4jeej/Dpak_Heaters_2.jpg?raw=1

--

Father Brown's figure remained quite dark and still;
but in that instant he had lost his head. His head was
always most valuable when he had lost it.

On Tue, 24 Aug 2021 07:34:34 -0700, jlarkin@highlandsniptechnology.com
wrote:

>On Tue, 24 Aug 2021 09:34:45 -0400, legg <legg@nospam.magma.ca> wrote:
>
>>On Mon, 23 Aug 2021 19:47:46 -0700, jlarkin@highlandsniptechnology.com
>>wrote:
>>
>>>https://www.dropbox.com/s/8hbfb3wmc4eqcn1/Dpak_Heaters.jpg?raw=1
>>>
>>>I can use 6 fets, 3 pairs, to heat my oven block.
>>>
>>>Cascode pairs left, center, right with individual DACs to tune the
>>>thermal gradients down a bit.
>>>
>>>The fets can go on a PCB with thermal vias to the opposite side, where
>>>the aluminum block is.
>>
>>So long as you're happy with <Tsmax, and can avoid it
>>in transient conditions. The plated-through board won't
>>like anything over 130C for long, either.
>>
>>RL
>
>I want to heat the oven block to about 35C, which might need maybe 10
>watts. I figure that each fet might have to dissipate a couple of
>watts, and each dpak tab will rise to about 40C. We might push the
>power up a bit at startup, to reduce warmup time, but that's just
>code.
>
>Fets make great heaters.

That's 1/2 of a temperature regulator, even in a lab environment.

RL

jlarkin@highlandsniptechnology.com wrote:
> On Mon, 23 Aug 2021 19:56:20 -0700, John Robertson <spam@flippers.com>
> wrote:
>
>>
>> On 2021/08/23 7:47 p.m., jlarkin@highlandsniptechnology.com wrote:
>>> https://www.dropbox.com/s/8hbfb3wmc4eqcn1/Dpak_Heaters.jpg?raw=1
>>>
>>> I can use 6 fets, 3 pairs, to heat my oven block.
>>>
>>> Cascode pairs left, center, right with individual DACs to tune the
>>> thermal gradients down a bit.
>>>
>>> The fets can go on a PCB with thermal vias to the opposite side, where
>>> the aluminum block is.
>>>
>>>
>>>
>>
>> What have you got against resistors?
>
> Power goes as I^2, which wrecks the control loop. Fet power is linear
> on current.
>
> The fets conduct heat through the board to the block nicely too.
>
> Fets pick-and-place, no hardware, no tapped holes, no wires.
>
> The fets are cheap!
>
>
>>
>> Or is it the handy metal tab as a more point heat source?
>
> I'll use copper pours to spread the dpak heat contact area. Gap-pads
> on the back.

Watch out that the slow conduction through the gap pad doesn't trash
your control bandwidth. With a big chunk of metal like that you might
be in the thermal mass limit, but maybe not--aluminum is surprisingly good.

We attach boards to cold plates using an exposed ENIG bottom-side pour
with many vias to the ground plane, with a little bit of good-quality
thermal paste and lots of small screws.

The thermistors go on the bottom, hanging five off the edge of the cold
plate, with one side soldered to the big pour and the other side
connected with a longish skinny trace.

The resulting delay is under 100 milliseconds (which is as well as we
can measure it). That really helps the control BW, which in turn helps
the thermal forcing rejection at low frequency.

No additional hand work, muss, fuss, or bother.

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 Fri, 27 Aug 2021 09:17:30 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>jlarkin@highlandsniptechnology.com wrote:
>> On Mon, 23 Aug 2021 19:56:20 -0700, John Robertson <spam@flippers.com>
>> wrote:
>>
>>>
>>> On 2021/08/23 7:47 p.m., jlarkin@highlandsniptechnology.com wrote:
>>>> https://www.dropbox.com/s/8hbfb3wmc4eqcn1/Dpak_Heaters.jpg?raw=1
>>>>
>>>> I can use 6 fets, 3 pairs, to heat my oven block.
>>>>
>>>> Cascode pairs left, center, right with individual DACs to tune the
>>>> thermal gradients down a bit.
>>>>
>>>> The fets can go on a PCB with thermal vias to the opposite side, where
>>>> the aluminum block is.
>>>>
>>>>
>>>>
>>>
>>> What have you got against resistors?
>>
>> Power goes as I^2, which wrecks the control loop. Fet power is linear
>> on current.
>>
>> The fets conduct heat through the board to the block nicely too.
>>
>> Fets pick-and-place, no hardware, no tapped holes, no wires.
>>
>> The fets are cheap!
>>
>>
>>>
>>> Or is it the handy metal tab as a more point heat source?
>>
>> I'll use copper pours to spread the dpak heat contact area. Gap-pads
>> on the back.
>
>Watch out that the slow conduction through the gap pad doesn't trash
>your control bandwidth. With a big chunk of metal like that you might
>be in the thermal mass limit, but maybe not--aluminum is surprisingly good.

I use a soft 3G material that's about 5 w/m-K compressed. Under 0.5
k/w for a square inch; the vias are worse. The mosfet-to-block time
constant will be milliseconds (thanks for the number!) and the giant
oven block tau will be maybe half an hour.

>
>We attach boards to cold plates using an exposed ENIG bottom-side pour
>with many vias to the ground plane, with a little bit of good-quality
>thermal paste and lots of small screws.

I may as well leave the solder mask. Each of the six fets will only
dissipate about 3 watts.

>
>The thermistors go on the bottom, hanging five off the edge of the cold
>plate, with one side soldered to the big pour and the other side
>connected with a longish skinny trace.

I'm planning on a small pcb on the bottom of my eom mounting plate. It
will have three thermistor Wheatstone bridges and gap-pads. We'll use
a 24-bit ADC. Gross overkill.

>
>The resulting delay is under 100 milliseconds (which is as well as we
>can measure it). That really helps the control BW, which in turn helps
>the thermal forcing rejection at low frequency.

We're machining a prototype oven block and The Brat is laying out the
controller board, which we can drive from a LabJack and Python, which
setup the customer can play with.

They want sub-mK stability over hours. A Mach-Zender modulator splits
the light, modulates the delays in the two paths, and combines, with
180 degree optical phase shift making darkness. There might be 20,000
or so wavelengths in each leg and we want cancellation to a small
fraction of a wave. Temperature probably matters.

--

Father Brown's figure remained quite dark and still;
but in that instant he had lost his head. His head was
always most valuable when he had lost it.

jlarkin@highlandsniptechnology.com wrote:
> On Fri, 27 Aug 2021 09:17:30 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>> jlarkin@highlandsniptechnology.com wrote:
>>> On Mon, 23 Aug 2021 19:56:20 -0700, John Robertson <spam@flippers.com>
>>> wrote:
>>>
>>>>
>>>> On 2021/08/23 7:47 p.m., jlarkin@highlandsniptechnology.com wrote:
>>>>> https://www.dropbox.com/s/8hbfb3wmc4eqcn1/Dpak_Heaters.jpg?raw=1
>>>>>
>>>>> I can use 6 fets, 3 pairs, to heat my oven block.
>>>>>
>>>>> Cascode pairs left, center, right with individual DACs to tune the
>>>>> thermal gradients down a bit.
>>>>>
>>>>> The fets can go on a PCB with thermal vias to the opposite side, where
>>>>> the aluminum block is.
>>>>>
>>>>>
>>>>>
>>>>
>>>> What have you got against resistors?
>>>
>>> Power goes as I^2, which wrecks the control loop. Fet power is linear
>>> on current.
>>>
>>> The fets conduct heat through the board to the block nicely too.
>>>
>>> Fets pick-and-place, no hardware, no tapped holes, no wires.
>>>
>>> The fets are cheap!
>>>
>>>
>>>>
>>>> Or is it the handy metal tab as a more point heat source?
>>>
>>> I'll use copper pours to spread the dpak heat contact area. Gap-pads
>>> on the back.
>>
>> Watch out that the slow conduction through the gap pad doesn't trash
>> your control bandwidth. With a big chunk of metal like that you might
>> be in the thermal mass limit, but maybe not--aluminum is surprisingly good.
>
> I use a soft 3G material that's about 5 w/m-K compressed. Under 0.5
> k/w for a square inch; the vias are worse. The mosfet-to-block time
> constant will be milliseconds (thanks for the number!) and the giant
> oven block tau will be maybe half an hour.

Okay, so you maybe don't care much unless you want to use local feedback
at the heater. But thermal diffusion in polymers is very very slow--the
diffusivity kappa is

alpha
kappa = --------
rho*c_P

where as usual alpha is the thermal conductivity, rho is the density,
and c_P is the specific heat of the material.

In ordinary plastic (alpha ~ 0.1 W/m/K), high c_P, the diffusivity can
be 5000 times less than in aluminum. For a given bandwidth, the
allowable thickness goes like sqrt(kappa)--i.e. 1 mm polymer is as slow
as 70 mm of aluminum.

Your stuff has much higher thermal conductivity, but from a bandwidth
POV a 5-mm pad will be something like 50 mm of aluminum.

>
>>
>> We attach boards to cold plates using an exposed ENIG bottom-side pour
>> with many vias to the ground plane, with a little bit of good-quality
>> thermal paste and lots of small screws.
>
> I may as well leave the solder mask. Each of the six fets will only
> dissipate about 3 watts.

Probably not a huge item if it's only 500 microinch or so.

>
>>
>> The thermistors go on the bottom, hanging five off the edge of the cold
>> plate, with one side soldered to the big pour and the other side
>> connected with a longish skinny trace.
>
> I'm planning on a small pcb on the bottom of my eom mounting plate. It
> will have three thermistor Wheatstone bridges and gap-pads. We'll use
> a 24-bit ADC. Gross overkill.
>
>>
>> The resulting delay is under 100 milliseconds (which is as well as we
>> can measure it). That really helps the control BW, which in turn helps
>> the thermal forcing rejection at low frequency.
>
> We're machining a prototype oven block and The Brat is laying out the
> controller board, which we can drive from a LabJack and Python, which
> setup the customer can play with.
>
> They want sub-mK stability over hours. A Mach-Zender modulator splits
> the light, modulates the delays in the two paths, and combines, with
> 180 degree optical phase shift making darkness. There might be 20,000
> or so wavelengths in each leg and we want cancellation to a small
> fraction of a wave. Temperature probably matters.

Yup.

Bandwidth really matters, because you need lots of loop gain to get good
rejection at low frequency.

For a spec like that I'd definitely use a proportional-only local
feedback loop around each heater as well as the main loop. It doesn't
have to be that that fast--a 100-Hz update rate should be lots--but your
main loop will see these lovely fast temperature actuators, so it ought
to be able to go much faster.

You can control gradients better that way too.

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, August 27, 2021 at 8:58:03 AM UTC-7, jla...@highlandsniptechnology.com wrote:
> On Fri, 27 Aug 2021 09:17:30 -0400, Phil Hobbs
> <pcdhSpamM...@electrooptical.net> wrote:
>
> >jla...@highlandsniptechnology.com wrote:
> >> On Mon, 23 Aug 2021 19:56:20 -0700, John Robertson <sp...@flippers.com>
> >> wrote:

> >>> What have you got against resistors?
> >>
> >> Power goes as I^2, which wrecks the control loop. Fet power is linear
> >> on current.
> >>
> >> The fets conduct heat through the board to the block nicely too.

> >Watch out that the slow conduction through the gap pad doesn't trash
> >your control bandwidth. With a big chunk of metal like that you might
> >be in the thermal mass limit, but maybe not--aluminum is surprisingly good.
....
> >We attach boards to cold plates using an exposed ENIG bottom-side pour
> >with many vias to the ground plane, with a little bit of good-quality
> >thermal paste and lots of small screws.
> I may as well leave the solder mask. Each of the six fets will only
> dissipate about 3 watts.

Why bother? You have heat pipes, just use a resistor on each,
to heat the main block; a gap pad and heat spreader under the target
device , with a thermometer or so, will presumably have no large heat
sources, so will remain isothermal.

Since you WANT the gap pad to attenuate heat gradients, it doesn't need
to be anything high-performance; greased paper would work fine.
Mostly, power FETs are poorly characterized for non-switching use,
so the resistor presumed nonlinearity is not a useful decision criterion.
Just PWM the things; why not? Millikelvin switch noise is unlikely.

On Fri, 27 Aug 2021 14:54:39 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

>On Friday, August 27, 2021 at 8:58:03 AM UTC-7, jla...@highlandsniptechnology.com wrote:
>> On Fri, 27 Aug 2021 09:17:30 -0400, Phil Hobbs
>> <pcdhSpamM...@electrooptical.net> wrote:
>>
>> >jla...@highlandsniptechnology.com wrote:
>> >> On Mon, 23 Aug 2021 19:56:20 -0700, John Robertson <sp...@flippers.com>
>> >> wrote:
>
>> >>> What have you got against resistors?
>> >>
>> >> Power goes as I^2, which wrecks the control loop. Fet power is linear
>> >> on current.
>> >>
>> >> The fets conduct heat through the board to the block nicely too.
>
>> >Watch out that the slow conduction through the gap pad doesn't trash
>> >your control bandwidth. With a big chunk of metal like that you might
>> >be in the thermal mass limit, but maybe not--aluminum is surprisingly good.
>...
>> >We attach boards to cold plates using an exposed ENIG bottom-side pour
>> >with many vias to the ground plane, with a little bit of good-quality
>> >thermal paste and lots of small screws.
>> I may as well leave the solder mask. Each of the six fets will only
>> dissipate about 3 watts.
>
>Why bother? You have heat pipes, just use a resistor on each,
>to heat the main block; a gap pad and heat spreader under the target
>device , with a thermometer or so, will presumably have no large heat
>sources, so will remain isothermal.
>
>Since you WANT the gap pad to attenuate heat gradients, it doesn't need
>to be anything high-performance; greased paper would work fine.
>Mostly, power FETs are poorly characterized for non-switching use,
>so the resistor presumed nonlinearity is not a useful decision criterion.

There will be fixed 24 volts across each dpak mosfet. And its current
is set by a closed-loop current controller, set by a 16-bit DAC.

That's linear as hell.

--

Father Brown's figure remained quite dark and still;
but in that instant he had lost his head. His head was
always most valuable when he had lost it.

On Saturday, August 28, 2021 at 12:54:18 PM UTC+10, jla...@highlandsniptechnology.com wrote:
> On Fri, 27 Aug 2021 14:54:39 -0700 (PDT), whit3rd <whi...@gmail.com>
> wrote:
> >On Friday, August 27, 2021 at 8:58:03 AM UTC-7, jla...@highlandsniptechnology.com wrote:
> >> On Fri, 27 Aug 2021 09:17:30 -0400, Phil Hobbs
> >> <pcdhSpamM...@electrooptical.net> wrote:
> >>
> >> >jla...@highlandsniptechnology.com wrote:
> >> >> On Mon, 23 Aug 2021 19:56:20 -0700, John Robertson <sp...@flippers.com>
> >> >> wrote:
> >
> >> >>> What have you got against resistors?
> >> >>
> >> >> Power goes as I^2, which wrecks the control loop. Fet power is linear
> >> >> on current.
> >> >>
> >> >> The fets conduct heat through the board to the block nicely too.
> >
> >> >Watch out that the slow conduction through the gap pad doesn't trash
> >> >your control bandwidth. With a big chunk of metal like that you might
> >> >be in the thermal mass limit, but maybe not--aluminum is surprisingly good.
> >...
> >> >We attach boards to cold plates using an exposed ENIG bottom-side pour
> >> >with many vias to the ground plane, with a little bit of good-quality
> >> >thermal paste and lots of small screws.
> >> I may as well leave the solder mask. Each of the six fets will only
> >> dissipate about 3 watts.
> >
> >Why bother? You have heat pipes, just use a resistor on each,
> >to heat the main block; a gap pad and heat spreader under the target
> >device , with a thermometer or so, will presumably have no large heat
> >sources, so will remain isothermal.
> >
> >Since you WANT the gap pad to attenuate heat gradients, it doesn't need
> >to be anything high-performance; greased paper would work fine.
> >Mostly, power FETs are poorly characterized for non-switching use,
> >so the resistor presumed nonlinearity is not a useful decision criterion..
> There will be fixed 24 volts across each dpak mosfet. And its current
> is set by a closed-loop current controller, set by a 16-bit DAC.
>
> That's linear as hell.

It isn't. It's 65,536 step staircase approximation to a linear solution. In practice, if there's enough noise in your temperature sensor, that will dither the heat delivered across enough steps to make it sensibly linear. If you look at my 1996 paper on the millidegree thermostat, you will find a short discussion of this point - our pulse width modulated current drive for the Peltier junction could have run out of resolution if we hadn't been careful.

--
Bill sloman, Sydney

24.08.21 17:24, jlarkin@highlandsniptechnology.com wrote:
>On Mon, 23 Aug 2021 19:47:46 -0700, jlarkin@highlandsniptechnology.com
>wrote:
>
>>https://www.dropbox.com/s/8hbfb3wmc4eqcn1/Dpak_Heaters.jpg?raw=1
>>
>>I can use 6 fets, 3 pairs, to heat my oven block.
>>
>>Cascode pairs left, center, right with individual DACs to tune the
>>thermal gradients down a bit.
>>
>>The fets can go on a PCB with thermal vias to the opposite side, where
>>the aluminum block is.
>
>Here's a small tweak:
>
>https://www.dropbox.com/s/gc449mrbtg4jeej/Dpak_Heaters_2.jpg?raw=1
>
>Of course, we could just use an opamp per fet and dump the cascode
>idea. That would increase the cost of the project about 60 PPM.
>
>https://www.dropbox.com/s/gc449mrbtg4jeej/Dpak_Heaters_2.jpg?raw=1
>
>

I am late into the discussion, but why not just a PWM control of a FET turned on fully

Seems you need a loop control of the temperature and that loop is sloooooow

You can buy this off the shelf instead of building it

https://www.google.com/search?q=temperature+controller

--
Klaus

On Sunday, August 29, 2021 at 5:42:05 PM UTC+10, Klaus Kragelund wrote:
> 24.08.21 17:24, jla...@highlandsniptechnology.com wrote:
> >On Mon, 23 Aug 2021 19:47:46 -0700, jla...@highlandsniptechnology.com
> >wrote:
> >
> >>https://www.dropbox.com/s/8hbfb3wmc4eqcn1/Dpak_Heaters.jpg?raw=1
> >>
> >>I can use 6 fets, 3 pairs, to heat my oven block.
> >>
> >>Cascode pairs left, center, right with individual DACs to tune the
> >>thermal gradients down a bit.
> >>
> >>The fets can go on a PCB with thermal vias to the opposite side, where
> >>the aluminum block is.
> >
> >Here's a small tweak:
> >
> >https://www.dropbox.com/s/gc449mrbtg4jeej/Dpak_Heaters_2.jpg?raw=1
> >
> >Of course, we could just use an opamp per fet and dump the cascode
> >idea. That would increase the cost of the project about 60 PPM.
> >
> >https://www.dropbox.com/s/gc449mrbtg4jeej/Dpak_Heaters_2.jpg?raw=1
> >
> I am late into the discussion, but why not just a PWM control of a FET turned on fully

He wants to pretend to be original. I published that back in 1996.

Sloman A.W., Buggs P., Molloy J., and Stewart D. “A microcontroller-based driver to stabilise the temperature of an optical stage to 1mK in the range 4C to 38C, using a Peltier heat pump and a thermistor sensor” Measurement Science and Technology, 7 1653-64 (1996)

> Seems you need a loop control of the temperature and that loop is sloooooow.

It doesn't have to be. Using a fast sensor and putting it close to the heat source can help.

> You can buy this off the shelf instead of building it
>
> https://www.google.com/search?q=temperature+controller

Getting the right sensor in the right place can be tricky with a bought-in controller. During the development of the 1996 gadget we did use a bought-in controller. It wasn't cheap and it was bulky. The auto-tuning was conservative. Setting the numbers using the Ziegler J and Nichols N 1942 Trans. ASME 64 759 procedure gave us much faster settling.

--
Bill Sloman, Sydney

On a sunny day (Sun, 29 Aug 2021 09:41:57 +0200) it happened Klaus Kragelund
<klauskvik@hotmail.com> wrote in <tscheppe.1giyd7yh3fc7k@nntp.aioe.org>:

>24.08.21 17:24, jlarkin@highlandsniptechnology.com wrote:
>>On Mon, 23 Aug 2021 19:47:46 -0700, jlarkin@highlandsniptechnology.com
>>wrote:
>>
>>>https://www.dropbox.com/s/8hbfb3wmc4eqcn1/Dpak_Heaters.jpg?raw=1
>>>
>>>I can use 6 fets, 3 pairs, to heat my oven block.
>>>
>>>Cascode pairs left, center, right with individual DACs to tune the
>>>thermal gradients down a bit.
>>>
>>>The fets can go on a PCB with thermal vias to the opposite side, where
>>>the aluminum block is.
>>
>>Here's a small tweak:
>>
>>https://www.dropbox.com/s/gc449mrbtg4jeej/Dpak_Heaters_2.jpg?raw=1
>>
>>Of course, we could just use an opamp per fet and dump the cascode
>>idea. That would increase the cost of the project about 60 PPM.
>>
>>https://www.dropbox.com/s/gc449mrbtg4jeej/Dpak_Heaters_2.jpg?raw=1
>>
>>
>
>I am late into the discussion, but why not just a PWM control of a FET turned on fully

Cannot speak for John, but in tri_pic I get PWM from a PIC micro and filter it
and then do the current control of the power FET to avoid high current spikes (disturbing the rest of the circuit).
Also saves a power resistor, now the FET itself heats the hotplate.
Slow? A few ms...
Outside temp variations are already much slower than that.
And it worked for about 8 years to within 2 parts in 500, data available
http://panteltje.com/panteltje/tri_pic/
scroll down for circuit and data.

Try:
cat tritium_decay_experiment_one_year_data_14_5_2013_to_14_5_2014.txt | awk 'BEGIN {FS=" "} {print $9}' | sort -n

Took an hour or so to figure out the control so no significant temperature overshoot on powerup.

On Sun, 29 Aug 2021 09:41:57 +0200, Klaus Kragelund
<klauskvik@hotmail.com> wrote:

>24.08.21 17:24, jlarkin@highlandsniptechnology.com wrote:
>>On Mon, 23 Aug 2021 19:47:46 -0700, jlarkin@highlandsniptechnology.com
>>wrote:
>>
>>>https://www.dropbox.com/s/8hbfb3wmc4eqcn1/Dpak_Heaters.jpg?raw=1
>>>
>>>I can use 6 fets, 3 pairs, to heat my oven block.
>>>
>>>Cascode pairs left, center, right with individual DACs to tune the
>>>thermal gradients down a bit.
>>>
>>>The fets can go on a PCB with thermal vias to the opposite side, where
>>>the aluminum block is.
>>
>>Here's a small tweak:
>>
>>https://www.dropbox.com/s/gc449mrbtg4jeej/Dpak_Heaters_2.jpg?raw=1
>>
>>Of course, we could just use an opamp per fet and dump the cascode
>>idea. That would increase the cost of the project about 60 PPM.
>>
>>https://www.dropbox.com/s/gc449mrbtg4jeej/Dpak_Heaters_2.jpg?raw=1
>>
>>
>
>I am late into the discussion, but why not just a PWM control of a FET turned on fully

What would get hot?

I don't want any PWM noise anyhow.

>
>Seems you need a loop control of the temperature and that loop is sloooooow

The oven block thermal tau might be a half hour. Gotta test that.

>
>You can buy this off the shelf instead of building it
>
>https://www.google.com/search?q=temperature+controller

The customer wants sub-millikelvin stability. I'll use a thermistor
Wheatstone bridge and a 24-bit ADC and a software PID loop and 16-bit
DACs to set the heater currents. Gross overkill.

--

Father Brown's figure remained quite dark and still;
but in that instant he had lost his head. His head was
always most valuable when he had lost it.

On Monday, August 30, 2021 at 12:42:13 AM UTC+10, jla...@highlandsniptechnology.com wrote:
> On Sun, 29 Aug 2021 09:41:57 +0200, Klaus Kragelund
> <klau...@hotmail.com> wrote:
>
> >24.08.21 17:24, jla...@highlandsniptechnology.com wrote:
> >>On Mon, 23 Aug 2021 19:47:46 -0700, jla...@highlandsniptechnology.com
> >>wrote:
> >>
> >>>https://www.dropbox.com/s/8hbfb3wmc4eqcn1/Dpak_Heaters.jpg?raw=1
> >>>
> >>>I can use 6 fets, 3 pairs, to heat my oven block.
> >>>
> >>>Cascode pairs left, center, right with individual DACs to tune the
> >>>thermal gradients down a bit.
> >>>
> >>>The fets can go on a PCB with thermal vias to the opposite side, where
> >>>the aluminum block is.
> >>
> >>Here's a small tweak:
> >>
> >>https://www.dropbox.com/s/gc449mrbtg4jeej/Dpak_Heaters_2.jpg?raw=1
> >>
> >>Of course, we could just use an opamp per fet and dump the cascode
> >>idea. That would increase the cost of the project about 60 PPM.
> >>
> >>https://www.dropbox.com/s/gc449mrbtg4jeej/Dpak_Heaters_2.jpg?raw=1
> >>
> >>
> >
> >I am late into the discussion, but why not just a PWM control of a FET turned on fully
> What would get hot?
>
> I don't want any PWM noise anyhow.

But you should be able minimise it enough by careful design and compact layout. We did back in 1993.
> >Seems you need a loop control of the temperature and that loop is sloooooow
> The oven block thermal tau might be a half hour. Gotta test that.

We ended up with 414+/-2sec - not quite seven minutes - but the thermal resistance through the Peltier junction was a problem, and our light path was through open air, rather than an optical fibre

> >You can buy this off the shelf instead of building it
> >
> >https://www.google.com/search?q=temperature+controller
> The customer wants sub-millikelvin stability. I'll use a thermistor
> Wheatstone bridge and a 24-bit ADC and a software PID loop and 16-bit
> DACs to set the heater currents. Gross overkill.

That's roughly what we used in 1993. We were worried about pulse-width modulation noise, but put lots of filtering right up against the point where it got generated, and it didn't mess up up the temperature sensing.

We got +/-1mK stability. The pulse-width modulation only had 10bit resolution, but that did turn out to be enough. I wrote it up and it got published in 1996.

Sloman A.W., Buggs P., Molloy J., and Stewart D. “A microcontroller-based driver to stabilise the temperature of an optical stage to 1mK in the range 4C to 38C, using a Peltier heat pump and a thermistor sensor” Measurement Science and Technology, 7 1653-64 (1996).

Google scholar says that it has 24 citations, but two of them are mine.

John Larkin should find it much easier to insulate his electro-optic modulator, and he's not using Peltier junctions where the exhaust side has to have a low thermal resistance to ambient, so he should be able to do well - perhaps not as well as Larsen did in 1968 (Larsen N T 1968 Rev. Sci. Instrum. 39 1–12) or Priel did in 1978 (Priel Z 1978 J. Phys. E: Sci. Instrum. 11 27–30) - but both relied on well-stirred, well insulated water-baths.

Priel stirred his water-bath hard enough that his stirring paddle was also his heater. Niether Priel nor Larsen used 20-bit bit A/D converters - they weren't around back then - and they probably wouldn't have been good enough to get the +/-50uK that Larsen managed. or the +/3.5uK that Priel got.

For the kind of fixed temperature application that John has to deal with they don't seem to offer much, but they aren't expensive.

--
Bill Sloman, Sydney