| Electronics > Beginners |
| Microcontroller, DAC and Mosfet to control TEC? |
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| spec:
--- Quote from: Ian.M on February 01, 2019, 06:49:24 am ---The problem with TECs and PWM is Joule heating, which is proportional to the square of the current, therefore 10% PWM at the TEC's max rated current has ten time the self-heating losses vs running it at a steady 10% of its max current. --- End quote --- Are you sure about this. My calculations show that the losses are identical over integer periods of the PWM frequency. Take a theoretical example: Pulse PWM period 1 sec Pulse width: 0.1 sec PWM signal 100W int load for 0.1 sec every sec (to give 10W average) Load= 10R Series resistance (representing loss) = 1R Average power required into load= 10W DC Drive With a DC drive the current would be a constant 1A = I210 =10W Thus energy into load = 10W* 1sec = 10W/sec Similarly the energy lost in the load is 1W/sec PWM Drive For duration of the pulse the current would be 3.16A, so power = 3.16A210 = 100W but only for 0.1sec, so the average power would be 100W/10 = 10W which is the same as for the DC signal The same principle applies to the losses so the power in the 1R resistor would be 10W for the duration of the pulse but the average power would be 10W/10 = 1W. From the above you can see that the power loss is exactly the same for a DC signal or PWM. Of course, PWM would incur switching losses, but that is another matter. |
| radar_macgyver:
--- Quote from: Ian.M on February 01, 2019, 06:49:24 am ---What TECs really don't like is 'bang-bang' control - the thermal cycling and resulting differential thermal expansion and contraction can mechanically fatigue the thermocouple elements and junctions till they go high resistance or even open circuit. --- End quote --- Could you provide a citation? I ask because I've used commercial TEC industrial coolers that use a standard 1/32 DIN PID controller to drive the TEC elements. I've seen the occasional failure and it would be great if going to a linear control helps. The choice of MOSFET used here might potentially present a problem - these are not designed for linear use. |
| spec:
UPDATE #1 2019_02_02 (corrected figures) --- Quote from: radar_macgyver on February 02, 2019, 05:41:06 pm ---The choice of MOSFET used here might potentially present a problem - these are not designed for linear use. --- End quote --- This has often been stated, but I can't see a problem using any MOSFET for linear applications. Could you describe what the problem might be? The worst case VGth of the IRF540 NMOSFET is 4V5 and with a 5V supply the OP did say that he could not get enough drain current, which you would expect. But the BCHE TES1-1704QT125 TEC has a resistance of 0R03 * 170 = 5R1 and a maximum current of 3A9, meaning that a minimum of 19V89 would be required to fully drive it. So a 24V supply rail would seem sensible. With a 24V supply rail there would be no problem turning the IRF540 fully on. http://www.huimao.com/about/show.php?lang=en&id=6 |
| Kleinstein:
PWM does not work well with TEC: The loss is proportional to I², while the useful cooling effect is proportional to the average current. So efficiency really gets bad with PWM. Even with PWM control one would likely need a series resistor to limit the current to the nominal current and maximum allowed current. A second point that with slow PWM or bang/bang control there is thermal stress to the TEC that can lead to failure. Even simple linear control with a analog controlled transistor give better results: for the same average current, the PWM case has all the loss is in the TEC and thus about half effectively on the cold side, while linear control has some of the loss in the pass transistor, which does not interfere with the cold side. For low voltage operation (like starting from a 5 V supply) the limitations on linear operation are not that severe, even for more switching type MOSFETs. Unless one uses modern low voltage (e.g. 20 V or maybe 30 V) parts they are probably OK with only 3 or 4 V at the MOSFET. For only 2 A one could use old IRF510 or BUZ10 parts that are specified for linear operation - though might need more than 5 V for the gate. There is still the option to use a BJT (like TIP120) With 1.4 V needed for base to emitter and 2 V for the TEC there is enough headroom from a 5 V source. Switched mode operation with an inductor and diode and optional capacitor is also not that complicated. |
| spec:
--- Quote from: Kleinstein on February 02, 2019, 06:12:04 pm ---PWM does not work well with TEC: The loss is proportional to I², while the useful cooling effect is proportional to the average current. --- End quote --- Not sure what you are saying here- see my reply #12. I suspect you are comparing bananas with carrots. The TEC is a resistor and so is the loss element. Unless you are now talking about switching losses all of a sudden. Taking your statement at face value, no PWR system would work. You can't get away from the consevation of energy law. |
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