On Saturday, October 15, 2022 at 11:49:55 AM UTC-4, Tauno Voipio wrote:
> On 6.10.22 9.24, John Robertson wrote:
> > Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
> > emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
> > resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
> > controller.
> >
> > Problem is of course, that R2 puts the ~20VDC to the TTL output gate on
> > a 74LS138.
> >
> > Trying to solve this without a driver transistor. The circuit is for a
> > 1ms ~20V strobe pulse repeated every ten ms.
> >
> > I thought of putting a 10ufd cap in series with R2, but don't like
> > electrolytics as they fail after a few thousand hours.
> >
> > Possible to use a 15V or so Zener Diode, but the Vbb is not regulated so
> > that won't work reliably.
> >
> > Anyone have a single component in mind that will essentially emulate
> > (isolate) an Open-Collector output for the 138?
> >
> > Thanks!
> >
> > John :-#)#
> >
> If you can handle LTspice, here's a solution.
>
> The original circuit overloads the LS138 with both voltage and current.
> The abs max voltage on LSTTL pin is 7V and maximum current for LS138
> output is 8mA. The circuit gives 20V and 20mA.
>
> There is no information on the lamp load, but the Darlingtons cannot
> handle more then 5A, which can be reached with 5mA on base.
>
> I made a LTspice model with a guessed 240mA lamp load. The value is
> not critical.

Rather than making us fire up LTspice, what is your approach based on? Are you adding a transistor, or just passives? Or something entirely different?

--

Rick C.

+-+ Get 1,000 miles of free Supercharging
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On 15.10.22 21.43, Ricky wrote:
> On Saturday, October 15, 2022 at 11:49:55 AM UTC-4, Tauno Voipio wrote:
>> On 6.10.22 9.24, John Robertson wrote:
>>> Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
>>> emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
>>> resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
>>> controller.
>>>
>>> Problem is of course, that R2 puts the ~20VDC to the TTL output gate on
>>> a 74LS138.
>>>
>>> Trying to solve this without a driver transistor. The circuit is for a
>>> 1ms ~20V strobe pulse repeated every ten ms.
>>>
>>> I thought of putting a 10ufd cap in series with R2, but don't like
>>> electrolytics as they fail after a few thousand hours.
>>>
>>> Possible to use a 15V or so Zener Diode, but the Vbb is not regulated so
>>> that won't work reliably.
>>>
>>> Anyone have a single component in mind that will essentially emulate
>>> (isolate) an Open-Collector output for the 138?
>>>
>>> Thanks!
>>>
>>> John :-#)#
>>>
>> If you can handle LTspice, here's a solution.
>>
>> The original circuit overloads the LS138 with both voltage and current.
>> The abs max voltage on LSTTL pin is 7V and maximum current for LS138
>> output is 8mA. The circuit gives 20V and 20mA.
>>
>> There is no information on the lamp load, but the Darlingtons cannot
>> handle more then 5A, which can be reached with 5mA on base.
>>
>> I made a LTspice model with a guessed 240mA lamp load. The value is
>> not critical.
>
> Rather than making us fire up LTspice, what is your approach based on? Are you adding a transistor, or just passives? Or something entirely different?

Changing the base resistors so that the current drive capability is not
exceeded and adding a common-base npn transistor to take care of the
voltage.

--

-TV

On Sunday, October 16, 2022 at 4:42:41 AM UTC-4, Tauno Voipio wrote:
> On 15.10.22 21.43, Ricky wrote:
> > On Saturday, October 15, 2022 at 11:49:55 AM UTC-4, Tauno Voipio wrote:
> >> On 6.10.22 9.24, John Robertson wrote:
> >>> Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
> >>> emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
> >>> resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
> >>> controller.
> >>>
> >>> Problem is of course, that R2 puts the ~20VDC to the TTL output gate on
> >>> a 74LS138.
> >>>
> >>> Trying to solve this without a driver transistor. The circuit is for a
> >>> 1ms ~20V strobe pulse repeated every ten ms.
> >>>
> >>> I thought of putting a 10ufd cap in series with R2, but don't like
> >>> electrolytics as they fail after a few thousand hours.
> >>>
> >>> Possible to use a 15V or so Zener Diode, but the Vbb is not regulated so
> >>> that won't work reliably.
> >>>
> >>> Anyone have a single component in mind that will essentially emulate
> >>> (isolate) an Open-Collector output for the 138?
> >>>
> >>> Thanks!
> >>>
> >>> John :-#)#
> >>>
> >> If you can handle LTspice, here's a solution.
> >>
> >> The original circuit overloads the LS138 with both voltage and current.
> >> The abs max voltage on LSTTL pin is 7V and maximum current for LS138
> >> output is 8mA. The circuit gives 20V and 20mA.
> >>
> >> There is no information on the lamp load, but the Darlingtons cannot
> >> handle more then 5A, which can be reached with 5mA on base.
> >>
> >> I made a LTspice model with a guessed 240mA lamp load. The value is
> >> not critical.
> >
> > Rather than making us fire up LTspice, what is your approach based on? Are you adding a transistor, or just passives? Or something entirely different?
> Changing the base resistors so that the current drive capability is not
> exceeded and adding a common-base npn transistor to take care of the
> voltage.

Thanks

--

Rick C.

++- Get 1,000 miles of free Supercharging
++- Tesla referral code - https://ts.la/richard11209