Forums

Transitor as heater

Started by George Herold February 13, 2014
On Thu, 13 Feb 2014 19:07:41 -0800 (PST),
bloggs.fredbloggs.fred@gmail.com wrote:

>On Thursday, February 13, 2014 10:26:01 AM UTC-5, George Herold wrote: ><> > What the heck is a "transitor" ? :-)
SITOR is an error correcting teletype system. <http://en.wikipedia.org/wiki/SITOR> A tranSITOR transmits SITOR. -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
George Herold wrote:
> Transistor as heater/ temp sensor. > So in looking for a small heater one idea is to use a transistor as heater. > And to take the idea one step further to use the same transistor (sequentially) > As first a heater then a temp sensor. > So the first question is then how to use it as a heater. I was thinking of controlling the current > At some fixed voltage.. something like this, > > https://www.dropbox.com/s/plcgg2rgnh0byt7/Tran-heat.JPG > > But other ideas would be welcome. One semi-crazy idea, I'm using the transistor as a temp sensor with the c-b shorted. (diode connected transistor.) Could I just push a bunch more current through it for a heater. > > The second problem I see with the heater idea is how to do the switching from heater to temp sensor. Do I use relays or analog switches? > (I'm off to look into anaolg switches.) > > Thanks again, > George H.
The Vbe tells you the temperature. To get a more accurate reading, switch its current by (say) an order of magnitude and measure the amplitude of the Vbe change. Be nasty, make the low current low enough so that the heating can be neglected (make that fixed), and make the high current hi enough to get the heating you desire (adjustable, natch). One part does both and there is NO thermal hysteresis.
Tim Wescott wrote:
> On Thu, 13 Feb 2014 07:26:01 -0800, George Herold wrote: > >> Transistor as heater/ temp sensor. >> So in looking for a small heater one idea is to use a transistor as >> heater. >> And to take the idea one step further to use the same transistor >> (sequentially) >> As first a heater then a temp sensor. >> So the first question is then how to use it as a heater. I was thinking >> of controlling the current At some fixed voltage.. something like this, >> >> https://www.dropbox.com/s/plcgg2rgnh0byt7/Tran-heat.JPG >> >> But other ideas would be welcome. One semi-crazy idea, I'm using the >> transistor as a temp sensor with the c-b shorted. (diode connected >> transistor.) Could I just push a bunch more current through it for a >> heater. >> >> The second problem I see with the heater idea is how to do the switching >> from heater to temp sensor. Do I use relays or analog switches? >> (I'm off to look into anaolg switches.) >> >> Thanks again, >> George H. > > 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. > > Since transistors are cheaper than relays or analog switches, why not use > one for heating and another for temperature sensing? >
That would be a dual like the MMDT2N2222. Rather close spacing.
On 14/02/14 13:45, George Herold wrote:
> On Thursday, February 13, 2014 7:43:58 PM UTC-5, Clifford Heath wrote: >> On 14/02/14 02:51, Tim Wescott wrote: >> >>> 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. >> >> But you have time - measure the temperature curve after a heating pulse >> and correlate that with a thermal model. Using a thermal model is the >> only way of calculating how much of your heat has reached the target at >> a given point in time, so you have to do it anyway. >> >> Clifford Heath. > 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.
Bear in mind that there'll be more than one curve. You need to model it as a multi-stage series-R parallel-C circuit. A single stage would get you close, and a second may well go within your measurement error.
On Thu, 13 Feb 2014 08:37:02 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

>On Thursday, February 13, 2014 10:51:06 AM UTC-5, Tim Wescott wrote: >> On Thu, 13 Feb 2014 07:26:01 -0800, George Herold wrote: >>=20 > >>=20 >>=20 >> Using it as a temperature sensor would be complicated by the fact =
that,=20
>> due to its recent service as a heater, it would be warmer than its=20 >> surroundings. > >Yeah well I'm going to have to pulse the heat and then wait for it to=20 >diffuse (or whatever heat does) into the sample. So I've got a pulse =
and then a wait time for the temperature to settle down. No matter what = I use I need good thermal contact between the sample and my heater/ temp = sensor, gizmo =20
>(sometimes called the addendum those that do heat capacity =
measurements.)=20
>>=20 >>=20 >>=20 >> Since transistors are cheaper than relays or analog switches, why not =
use=20
>> one for heating and another for temperature sensing? > >Yeah I could do that too. Cheap is not really the issue here. I want =
low mass/ easy to mount/ good themal contact. (yeah I know, pick two of = the three :^)
> >George H. >>=20 >>=20
If you are really interested in low mass and good thermal contact being sufficiently primary, you could solder quad transistor dice the thermal interface and wirebond to a PCB. The mounting is hell but it excels on the other parameters. ?-)
On Thursday, February 13, 2014 11:30:06 PM UTC-5, Robert Baer wrote:
> George Herold wrote: >=20 > > Transistor as heater/ temp sensor. >=20 > > So in looking for a small heater one idea is to use a transistor as hea=
ter.
>=20 > > And to take the idea one step further to use the same transistor (seque=
ntially)
>=20 > > As first a heater then a temp sensor. >=20 > > So the first question is then how to use it as a heater. I was thinkin=
g of controlling the current
>=20 > > At some fixed voltage.. something like this, >=20 > > >=20 > > https://www.dropbox.com/s/plcgg2rgnh0byt7/Tran-heat.JPG >=20 > > >=20 > > But other ideas would be welcome. One semi-crazy idea, I'm using the t=
ransistor as a temp sensor with the c-b shorted. (diode connected transist= or.) Could I just push a bunch more current through it for a heater.
>=20 > > >=20 > > The second problem I see with the heater idea is how to do the switchin=
g from heater to temp sensor. Do I use relays or analog switches?
>=20 > > (I'm off to look into anaolg switches.) >=20 > > >=20 > > Thanks again, >=20 > > George H. > The Vbe tells you the temperature. > To get a more accurate reading, switch its current by (say) an order=
=20
> of magnitude and measure the amplitude of the Vbe change. > Be nasty, make the low current low enough so that the heating can be=
=20
> neglected (make that fixed), and make the high current hi enough to get=
=20
> the heating you desire (adjustable, natch). >=20
Hi Robert, Yeah I've done the current scaling of the Vbe change. =20 This turns out not to work so well for just Vbe, because of the non-idealit= y factor which is something near 2 for most diodes. However it works prett= y well for a diode connected transistor. (good to 1% or so, well 1% at 300= K is 3 degrees so it's not all that great.) =20
> One part does both and there is NO thermal hysteresis.
Arghh.., ya know I just realized that I'd been ignoring the self heating of= my temperature sensor. (Bill S. will be scolding me shortly.) .. OK I'm = going to go sharpen my pencil. =20 I might have to pulse the current through the sensor. =20 George H.
On Thursday, February 13, 2014 10:34:19 PM UTC-5, Jeff Liebermann wrote:
> On Thu, 13 Feb 2014 19:07:17 -0800 (PST), George Herold >=20 > <gherold@teachspin.com> wrote: >=20 >=20 >=20 > >On Thursday, February 13, 2014 8:04:02 PM UTC-5, Jeff Liebermann wrote: >=20 > >> On Thu, 13 Feb 2014 10:16:57 -0800 (PST), George Herold >=20 > >>=20 >=20 > >> <gherold@teachspin.com> wrote: >=20 > ><<<Big snip.. read upstream>>> > =20 >=20 > > Hi Jeff, I'm going to cut your response. > >(because google groups sucks, but is also easy.) > >And I'm much too lazy to try and trim the whole thing. >=20 > Sniff... I'm crushed but can take it. Besides, I hate reading my own > stuff more than once.
(Well I didn't mention to other reason which is that you tend to write *a l= ot* :^)
>=20 > >1.) I love the horror stories and drift is no problem.=20 > My main thing has always been repair and troubleshooting, which is why > I'm into such horror stories. Note that my internet domain is > LearnByDestroying.com. >=20 > >(It's all been thermal drift after all.) >=20 > Yep. Just remember that hot air rises to the top of the discussion. > > >2.) For the LED idea, I need to know the power accurately. > >How much of the IR is reflected? >=20 > I hate solving problems I know nothing about. I guess the best way to > find an unknown is to compare it with something that's known in the > same setup. In my days of running a print shop, we used a "gray card"
<snip rest of IR led stuff>
>=20 >=20
>=20 > >3.) yeah, subtraction: the first task is to measure the heat capacity of=
just the addendum.
>=20 > >(the heater and sensor.) so you can subtract that from later measurement=
s.
> Yep. >=20 > >4.) servo. My fault I posted two threads=20 > >and I'm totaly confused about what I've said where. =20 > > > >The sample and addendum will sit inside another metal can. > >The outer can will servoed to be at the same temp as the sample. > >(the can will have a 'weak' thermal link to 77K, a heater and sensor.) >=20 > I don't understand what that will do, but I'll dig through your other > thread and see if I can decode what you're doing. >=20
I started to draw a picture.. but let me just try words. =20 I've got a sample hanging from a string in vacuum. =20 There are two sources of heat leak. Radiation, Thermal conduction along the wires and support string Self heating of the temp sensor. Three, three sources of heating. =20 So to take care of the first two the sample sits inside metal can. The tem= perature of the outer can is set to be the same as the temperature of the s= ample. Now it won't be perfect because the outer can won't be a perfect is= otherm. There will be some radition from those areas that are hotter or co= lder than the sample.. but that should be small. =20 That's the idea anyway. I haven't tried it yet and perhaps it will crash a= nd burn because of somethng I've forgotten. =20 George H.
>=20 >=20 >=20 > --=20 >=20 > Jeff Liebermann jeffl@cruzio.com >=20 > 150 Felker St #D http://www.LearnByDestroying.com >=20 > Santa Cruz CA 95060 http://802.11junk.com >=20 > Skype: JeffLiebermann AE6KS 831-336-2558
On 2/13/2014 11:43 PM, Clifford Heath wrote:
> On 14/02/14 13:45, George Herold wrote: >> On Thursday, February 13, 2014 7:43:58 PM UTC-5, Clifford Heath wrote: >>> On 14/02/14 02:51, Tim Wescott wrote: >>> >>>> 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. >>> >>> But you have time - measure the temperature curve after a heating pulse >>> and correlate that with a thermal model. Using a thermal model is the >>> only way of calculating how much of your heat has reached the target at >>> a given point in time, so you have to do it anyway. >>> >>> Clifford Heath. >> 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. > > Bear in mind that there'll be more than one curve. > You need to model it as a multi-stage series-R parallel-C circuit. > A single stage would get you close, and a second may well go within > your measurement error. >
Assuming the whole thing is small enough. The thermal mass approximation is horrible at short time scales. Cheers Phil -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Clifford Heath wrote:
> On 14/02/14 13:45, George Herold wrote: >> On Thursday, February 13, 2014 7:43:58 PM UTC-5, Clifford Heath wrote: >>> On 14/02/14 02:51, Tim Wescott wrote: >>> >>>> 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. >>> >>> But you have time - measure the temperature curve after a heating pulse >>> and correlate that with a thermal model. Using a thermal model is the >>> only way of calculating how much of your heat has reached the target at >>> a given point in time, so you have to do it anyway. >>> >>> Clifford Heath. >> 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. > > Bear in mind that there'll be more than one curve. > You need to model it as a multi-stage series-R parallel-C circuit. > A single stage would get you close, and a second may well go within > your measurement error. >
Like your pun; SexTronix used the current switching method (i think current ratio was 1:10) to measure temperature with reasonable accuracy over a fair temperature range.
Robert Baer wrote:
> Tim Wescott wrote: >> On Thu, 13 Feb 2014 07:26:01 -0800, George Herold wrote: >> >>> Transistor as heater/ temp sensor. >>> So in looking for a small heater one idea is to use a transistor as >>> heater. >>> And to take the idea one step further to use the same transistor >>> (sequentially) >>> As first a heater then a temp sensor. >>> So the first question is then how to use it as a heater. I was thinking >>> of controlling the current At some fixed voltage.. something like this, >>> >>> https://www.dropbox.com/s/plcgg2rgnh0byt7/Tran-heat.JPG >>> >>> But other ideas would be welcome. One semi-crazy idea, I'm using the >>> transistor as a temp sensor with the c-b shorted. (diode connected >>> transistor.) Could I just push a bunch more current through it for a >>> heater. >>> >>> The second problem I see with the heater idea is how to do the switching >>> from heater to temp sensor. Do I use relays or analog switches? >>> (I'm off to look into anaolg switches.) >>> >>> Thanks again, >>> George H. >> >> 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. >> >> Since transistors are cheaper than relays or analog switches, why not use >> one for heating and another for temperature sensing? >> > That would be a dual like the MMDT2N2222. Rather close spacing. >
...and one can measure the cooling from service as a heater to get system TC and make appropriate thermal correction.