Transitor as heater

On Mon, 17 Feb 2014 07:40:48 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

>On Friday, February 14, 2014 10:57:39 PM UTC-5, josephkk wrote:
>> On Thu, 13 Feb 2014 18:45:32 -0800 (PST), George Herold
>>=20
>> <gherold@teachspin.com> wrote:
>>=20
>>=20
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>> >On Thursday, February 13, 2014 7:43:58 PM UTC-5, Clifford Heath =
wrote:
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>> >> On 14/02/14 02:51, Tim Wescott wrote:
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>> >>=20
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>> >> > On Thu, 13 Feb 2014 07:26:01 -0800, George Herold wrote:
>> >> >> Transistor as heater/ temp sensor.
>> >> > Using it as a temperature sensor would be complicated by the fact=
 that,
>> >> > due to its recent service as a heater, it would be warmer than =
its
>> >> > surroundings.
>> >>=20
>>=20
>> >> But you have time - measure the temperature curve after a heating =
pulse=20
>> >> and correlate that with a thermal model. Using a thermal model is =
the=20
>> >> only way of calculating how much of your heat has reached the =
target at=20
>> >> a given point in time, so you have to do it anyway.
>> >>=20
>> >> Clifford Heath.=20
>> >Yeah,  Well I wasn't thinking as far ahead as curve fitting.  I was =
hoping to be just able to wait ~5 time constants or so.  I must admit I =
thought it might be fun to watch the heat leak out of the transistor. =20
>> >=20
>> >George H.=20
>>=20
>>=20
>> Ya know George, the longer i think about this the less i like trying =
to=20
>> get a specific power for a specific duration with a transistor, and =
the
>> more i like SMD resistors, perhaps mounted "upside down".
>
>Hmm OK why is that Joseph?  I think I'm going to have to run four wires =
down to do either ther resistor or transistor as heater.  (measure the =
voltage and current.)  I guess there is some beta current error.. I've =
ordered a bunch of pnp medium power transistors.  Beta's in the ~200 =
range but that drops at low temperature.  But the SOT89 packs look =
great...=20
>they weight less than 100 mg!  I guess I should look at some FETS...=20
>
>George H.
>>=20
>>=20
I find resistors a lot easier to drive to get a readily traceable heat
pulse. =20
Controlling both current and voltage separately is a more difficult
proposition. Transistors do like to switch.

>>=20
>> ?-)

On Mon, 17 Feb 2014 12:23:25 -0800, the renowned josephkk
<joseph_barrett@sbcglobal.net> wrote:

>>> 
>I find resistors a lot easier to drive to get a readily traceable heat
>pulse.  
>Controlling both current and voltage separately is a more difficult
>proposition. Transistors do like to switch.

Resistors have a nonlinear response of power vs. input voltage and
it's more complex to correct for that. 

A MOSFET transistor as a current sink from a regulated supply gives
you an approximately linear response (exactly linear if you use the
sense resistor as a heater too). 


Best regards, 
Spehro Pefhany
-- 
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

On Mon, 17 Feb 2014 15:28:04 -0500, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

>On Mon, 17 Feb 2014 12:23:25 -0800, the renowned josephkk
><joseph_barrett@sbcglobal.net> wrote:
>
>>>>=20
>>I find resistors a lot easier to drive to get a readily traceable heat
>>pulse. =20
>>Controlling both current and voltage separately is a more difficult
>>proposition. Transistors do like to switch.
>
>Resistors have a nonlinear response of power vs. input voltage and
>it's more complex to correct for that.=20
>
>A MOSFET transistor as a current sink from a regulated supply gives
>you an approximately linear response (exactly linear if you use the
>sense resistor as a heater too).=20
>
>
>Best regards,=20
>Spehro Pefhany

I came to the same conclusion for a transistor heater.  I just saw more
problems driving it that than the issues of resistor non-ideality.  We =
may
each be seeing things that the other missed.

?-)