Forums

Transitor as heater

Started by George Herold February 13, 2014
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 >>=20 >> >On Thursday, February 13, 2014 7:43:58 PM UTC-5, Clifford Heath =
wrote:
>>=20 >> >> On 14/02/14 02:51, Tim Wescott wrote: >>=20 >> >>=20 >>=20 >> >> > 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. ?-)