Forums

Transitor as heater

Started by George Herold February 13, 2014
On Fri, 14 Feb 2014 01:26:01 +1000, George Herold <gherold@teachspin.com>  
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.
I mentioned it in another thread - the on semi thermal track range of transistors - a power transistor plus an additional diode on the same bit of silicon in a 5 pin package
On Thu, 13 Feb 2014 12:18:02 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 2/13/2014 12:08 PM, Jeff Liebermann wrote: >> On Thu, 13 Feb 2014 07:26:01 -0800 (PST), George Herold >> <gherold@teachspin.com> wrote: >> >>> Transistor as heater/ temp sensor. >> >> Use a separate heater and sensor. The problem is that you're trying >> to regulate the temperature of some attached device, such as a >> crystal, and NOT the temperature of the heater. If you simply >> attached a crystal to your proposed stabilized heater, the transistor >> junction temperature would be stabilized, but the temperature of the >> crystal would be susceptible to ambient changes. In effect, you would >> have a temperature gradient starting with the stabilized heater, and >> ending with room temperature, with the crystal somewhere in between. >> You could insulate the entire affair to minimize this, but I think >> that will burn up all the alleged cost savings of having everything in >> one package. > >You inevitably have that anyway, unless the crystal itself is the >sensor. And combining the actuator and sensor can give you a good two >orders of magnitude better bandwidth, which translates to 40 dB better >forcing rejection at all frequencies. You just put the sensor/heater >between the mounting hardware and the controlled volume, and insulate >around it.
Agreed. A single sensor/heater combination will work nicely in a thermally insulated (adiabatic) environment. Eventually, the internal temperature will be uniform throughout the assembly thus eliminating the temperature gradient problem that I mentioned. At that point, one can consider the crystal temperature to be the same as the heating transistor junction temperature. Life is great in idealized environments. However, if there is any temperature gradient between the crystal and the heater, the crystal will NOT be at the heater temperature. When I tried to construct an insulated oven enclosure in a marine HF shore station transceiver, I had problems. There just wasn't enough room for a doubly insulated ovenized oscillator. It was easy enough to tell that there was thermal problem; the aluminum case was fairly warm. Never mind that I created most of the problems myself. I used an AT cut crystal (all I had) instead of the more appropriate SC cut. The fiberglass insulation I used was too dense. Those and other mistakes were eventually fixed. Before I the problems, I tried to fix the OCXO by rearranging the crystal, heater, and sensor positions. I suspected that it would be a bad idea for the temp sensor to "see" the heater directly. The idea was to regulate the crystal temperature, not the heater temperature. So, I created a sandwich with the crystal in the middle. That change alone almost produced a usable OCXO. Eventually, fixing the other problems (i.e urethane foam not fiberglass, SC cut crystal, double insulated, radiation shield, paper under the aluminum can, aluminum foil sandwich wrap, etc) made it all work correctly. The down side of the sandwich construction is that the thermal lag is long which requires too much time to stabilize. I "fixed" that by cranking up the maximum power on the heater to the point below where it tries to melt solder or blow a fuse. It was seriously under damped as it would overshoot the target temperature, but it would stabilize much quicker than when slowly approaching the target temperature from a cold start. It also had the side benefit of somewhat accelerating crystal aging, but would also occasionally destroy defective crystals from the thermal shock.
>Multiple-point heater/sensors can be used to control gradients if necessary.
Yep, that can be made to work. I sorta tried it but failed to create a worthwhile improvement. The problem was that I had to add heaters and sensors on all six sides of the package to block the leakage on that side. If one side did not have a heater, it would leak heat from the crystal to the outside, thus recreating the temperature gradient. 5 of the sides were not much of a problem, but the 6th side had all the circuitry, connections, and mounting hardware. There was no room for a 6th heater. Another temperature gradient I found was between the quartz crystal element and the nickel/tin crystal can. I tried to buy UV fused silica or quartz crystal cases, which are IR transparent, but the prices were too much. So, I convinced the vendor to fill the crystal cans with helium, which has a higher thermal conductivity than air. There was no difference with all the other thermal errors that I introduced, but once these were reduced, the helium helped a little with reducing yet another gradient. (I would have used hydrogen, but didn't like the idea of soldering to a potential bomb). -- 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
On Thu, 13 Feb 2014 16:20:05 -0800, Jeff Liebermann <jeffl@cruzio.com>
wrote:

>Agreed. A single sensor/heater combination will work nicely in a >thermally insulated (adiabatic) environment.
(...)
>Life is great in idealized environments.
I forgot to mumble something about another OCXO thermal screwup. I was doing all my testing in a fixture, without the radio. When installed in the radio, there was enough moving air from the power amp cooling fan to cause small frequency excursions when the fan was running. While the cause is quite obvious at this point, it took five experienced engineers and managers a full day to see the obvious cause, and then only after it was pointed out by one of the production techs. Very embarrassing. Rather than trying to move the fan or the OCXO, I concentrated on improving the insulation and adding an air flow deflector, which was sufficient to meet the specs. -- 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
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.
On Thu, 13 Feb 2014 10:16:57 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

>Hi Guys, (Fred, Jeff, Phil) I'm just going to make one global response.
Good idea, since the thread has drifted off into several directions based on the respondents bad guesses as to what you're trying to accomplish, what problem are you trying to solve, and what you have to work with. Context is always helpful (but topic drift is often more interesting).
>So this gizmo is to measure the heat capacity of something. (How much >does the temperature rise when you add a joule of heat.) It will be >in a probe down a LN2 dewar. And in vacuum with a radiation shield >around it. The radiation shield will be servoed to stay at the same >temp as the sample inside*. So I'm not trying to stabilize the temperature.
Now you tell me. Please ignore my previous OCXO horror stories. This is quite different.
>I give it a pulse of heat and then measure how much the temperature rises. >I need to know the amount of heat in the pulse. And then the final >temperature of the sample.
I have an idea, but no clue if it can actually be made to work. Combine the thermal generator and the detector. Use an IR LED to heat the sample. Then, turn it off and use it as an IR photodetector to measure the temperature rise. You already have the LED at cryogenic temperatures, so thermal noise should not be a (major) problem.
>(There will be an initial temperature spike as the heater warms mostly >the temp sensor, but then the heat will diffuse into the sample.) >That needs to be as fast as I can make it.. which means small samples.
Not really. Since the LED will start measuring the temperature the instant its heating function is turned off, you can watch the temperature curve as the heat diffuses through the sample. From the curve, you can predict an end point and don't really need to wait until it gets there.
>But I then get pulled in the other direction because the temp sensor/ heater >and bits of metal that hold all that also have heat capacity. This >(addendum) heat capacity has to be measured and then subtracted from >the total to get the contribution from the sample. Which means that >large samples would be better.
Maybe. Test the fixture without a sample and subtract the graphs. I'm not sure if the results are useful or meaningful, but if you see a difference, it should be due totally to the effects of the sample.
>OK the more I think about it the less I like the transistor as both >heater and sensor. It's cute (which makes it appealing), but I'm >thinking it's easier, both to build and understand, if I keep the >functions separate.
Think again. If you combine the functions so that one device is switched between a thermal emitter and a thermal detector, you might get something that works.
>* Hmm OK I'm seeing some problems with having my temp sensor "off line" >for some of the time. It's going to confuse my servo. I also haven't >made the servoed radiatin shield yet so...
What servo? If you know how many coulombs you're feeding the LED thermal emitter, the LED efficiency at cryo temps, the radiation pattern, and the area covered, one should be able to calculate how much heat is being delivered to the target. No need for feedback. <http://link.springer.com/article/10.1007%2Fs10909-011-0415-4#page-1> Disclaimer: I know nothing about cryogenic techniques and devices, but I'm really good at guessing and don't really mind being totally wrong. -- 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
In article <cfafb55b-5a79-48d8-8e12-78ed90469664@googlegroups.com>, 
gherold@teachspin.com says...
> Yeah I've done that in the past. (put both the pass element and resistor on the thing to be heated.) This is all about small and low mass. > I've got a little clamp sketeched out. A couple of 2-56 screws (0.4g) and what looks to be about 1.5 grams of copper on the bottom part and maybe 1/2 gram of G-10 for the flexible top of the clamp. > > George H. > >
Sounds like you're making it harder than you need? Use a PTC and drive it with a constant current source that can be set via your desired to obtain the given heat you need. The feed back would be the applied voltage required to obtain the current verses the temperature you need. Depending on what you're doing, you could use the tab mounted styles if you are trying heat a specific spot. We've done such a thing making a make shift vacuum sensor. Jamie. Jamie
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. George H.
On Thursday, February 13, 2014 8:04:02 PM UTC-5, Jeff Liebermann wrote:
> On Thu, 13 Feb 2014 10:16:57 -0800 (PST), George Herold > > <gherold@teachspin.com> wrote:
<<<Big snip.. read upstream>>>
> >
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. 1.) I love the horror stories and drift is no problem. (It's all been thermal drift after all.) 2.) For the LED idea, I need to know the power accurately. How much of the IR is reflected? 3.) yeah, subtraction: the first task is to measure the heat capacity of just the addendum. (the heater and sensor.) so you can subtract that from later measurements. 4.) servo. My fault I posted two threads and I'm totaly confused about what I've said where. 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.) George H.
> > > -- > > 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
On Thursday, February 13, 2014 10:26:01 AM UTC-5, George Herold wrote:
<>
 What the heck is a "transitor" ? :-)
On Thu, 13 Feb 2014 19:07:17 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

>On Thursday, February 13, 2014 8:04:02 PM UTC-5, Jeff Liebermann wrote: >> On Thu, 13 Feb 2014 10:16:57 -0800 (PST), George Herold >> >> <gherold@teachspin.com> wrote: ><<<Big snip.. read upstream>>>
> 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.
Sniff... I'm crushed but can take it. Besides, I hate reading my own stuff more than once.
>1.) I love the horror stories and drift is no problem.
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.
>(It's all been thermal drift after all.)
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?
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" <http://en.wikipedia.org/wiki/Gray_card> which has a guaranteed 18% reflectivity. Whether that works in the IR region can be tested with an IR source and light meter at room temperature. If it works (or is close), cram the card into your LN2 bath, and you should see 82% absorption. You can measure the heat rise of the card with the LED running as an IR photo detector. You can also get gray cards with different reflectivity, which will give you a calibration range. The test sample should fit in between some of the gray card test points. Don't forget to mask off everything except the area of interest. Also, watch out for nearby object re-radiating absorbed IR from the mask material.
>3.) yeah, subtraction: the first task is to measure the heat capacity of just the addendum. >(the heater and sensor.) so you can subtract that from later measurements.
Yep.
>4.) servo. My fault I posted two threads >and I'm totaly confused about what I've said where. > >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.)
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. -- 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