ay sometimes too. All the advantages you cite are real. However, there are a fair number of applications in EO systems where higher order effects are super-important, so switching junk is death and destruction.
FB loops. Well that is in my back pocket, no small thanks to Spehro.
George H.
One of the most elegant (cheapest) thermostatically controlled heaters that I have seen was used by some Germans who were running a diesel engine on vegetable oil. To get the viscosity low enough, you need to heat the fuel lines, injection pump, etc. and to do this before starting, they used a bunch of (nine) 3-terminal 78TXX regulators in TO-220 packages, with the output grounded, and the tab bolted to the injection pump.
Someone should make a TO-220 device like that which is specified and guaranteed for use as a temperature-limited current-limited heater, in a few different temperature values. It would be nice to know that the electromigration calculations etc. had been done at the thermal limiting temperature to guarantee a reasonable life under those conditions, which I would not be sure of with a normal 7805. Also according to the above article, the 78XX regulators had some hysteresis in their temperature limiting, which is undesirable and could be replaced with some nice PID algorithm.
I know that keeping the heater chip temperature constant is not as good as having a separate external temperature sensor, but in a lot of cases it would be good enough and lower parts count.
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The critical thing is to keep the PWM edges confined to the immediate vicin ity of the switches. If you can turn switched current into something that h asn't got a lot of switching noise left on it, with LC filtering close to t he switches.
Worked for me.
Sloman A.W., Buggs P., Molloy J., and Stewart D. ?A microcontroller
-based driver to stabilise the temperature of an optical stage to 1mK in th e range 4C to 38C, using a Peltier heat pump and a thermistor sensor? ? Measurement Science and Technology, 7 1653-64 (1996).
You can get PTC thermistors in any size you like, and they're the usual choice for that sort of thing, afaik.
The only real issue with them is that the thermal cycling is tough on their attachments, whether solder (which cracks) or clamps (which fret).
Cheers
Phil Hobbs
It's amazing how poor the thermal conductivity is for plastic insulated TO-220 packages, compared to every other way to achieve electrical insulation. They may be convenient, but that's all.
I've used TO92 LM317s as 2-terminal current limiters; just ignore the ADJ pin. They current limit, then thermal limit, and keep re-trying. Sort of self-resetting fuses.
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