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Advice for microprocessor (PI) controlled variable voltage power supply?

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Kleinstein:
Just on  / off control is really not very efficient, this gets especially important when cooling with a relatively large dT. The peltier internal losses are proportional to the RMS current, while the wanted heat transport is proportional to the average current.  The higher loss not only need more power but the extra heat (actually about half of it) also needs to be transported by the peltier element and ends up at the hot side.
Even just linear control is more efficient than simple on / off: the extra heat is not dissipated in the peltier element but at the external control transistor.

There is no real need to have a large capacitor for filtering the peltier current, the main part is usually from the inductor. A small capacitor can still help with possible EMI issues. There is no need to get very low ripply - the extra loss goes down with the square of the ripple amplitude. So 10% ripple can be acceptable.

The nominal current for the peltier elements is current to get the most cooling in case of perfect cooling of the hot side. More current gives less cooling. Real world with a limited heat sink at the hot side this optimum current shifts down.

If used in a temperature control, there is no need to have a feedback from the actual voltage or current. Just the PWM setting can be used as a control variable. For better regulation it may help to include the nonlinear current to power relationship of the peltier element.

h82fail:
Thanks everybody.  Interesting info about the peltiers.  I designed a simple dc/dc step down with the digital pot in LTspice (high side sw) that seems to work fine but picked some random sizing on the parts just to get it working.  I will do more research on proper sizing and also try a low side sw design since so many recommended that style before I post the asc file and make myself look like a idiot!  Any reason i should use a PWM vs digital pot controlled MOSFET?  8bit digital pot is only $1 and easy enough to control with the PI using SPI and that way I don't have to worry about filtering the output so much?

As far as the peltier setup, I'm just using what came on the thing.  I picked it up on craigslist for $9 because the control board was broken (one of the 3 MOSFETs on it looked like it exploded!).  The peltier works fine, I have a bench psu that only goes up to 8v and from 4 to 8v its about 2.5ohm when the hot/cold sides are near the same temp and the resistance goes up a bit closer to 3 after it warms.  The hot side heatsink on it probably actually is about 3.14159 times larger then the cold side and was barely warm at 8v without the fan running on it.  The wine cooler is fairly small and I will only be cooling it to between 65-70F/18.5-21C in a space that will likely be 75-80F/24-26.5C so I probably wont need more then 8volts unless maybe house a/c failed.  I also didn't want to run it at full power unnecessarily because of condensation since its going to be a humidor.

Thanks again, I wasn't expecting everyone to be so friendly to a newb.

Kleinstein:
With digi-pots one has to be careful with the voltage range. They usually need the analog voltages to be inside the supply range. So for the cheap parts this usually limits the voltage to some 5 V.  Many chips use adjusting the feedback path for setting the voltage, so this can be a problem.

PWM control can be relatively simple,  for low power it could be as little as connecting to a suitable low level MOSFET as a low side switch.

For the beginning it would be good if the supply voltage is low (possibly just 5 V in this case), so nothing bad would happen even if the µC stops working with the FET turned on. A lower voltage also reduces the needed size of the inductor.

Ian.M:
A Pi is a poor choice for realtime control.   You'd be side-stepping most of the issues if your digipot idea works out as the update rate is fairly non-critical, but its a PITA to get clean PWM out of a Pi  and even tougher if you need a rapid acting trigger input for over-current.protection.  Also it doesn't have any built in ADC so closing the loop for output voltage or with analog temperature sensors requires an external ADC.

You'd be better off with a small MCU - an 8 bit one would be entirely adequate.   Microchip have a whole bunch of 8 bit PICs that are well suited to  switched mode converter applications (albeit not at very high operating frequencies), including cycle by cycle current limiting, or if you are a MCU novice you could go down the Arduino road.  e.g. an Arduino Nano would do nicely.    If you still want the Pi for a GUI or to support a fancy web interface etc., it could command and monitor the MCU, which would hold its last settings if the Pi was off, crashed or otherwise indisposed.

 

Siwastaja:
I would just use an off-the-shelf DC/DC buck and use Raspberry PI IO to inject a small current to the feedback voltage divider - i.e., a resistor from the divider to the IO pin. This way, by configuring the pin as high, floating or low, you have already three different voltage settings. Choose the series resistor wisely, do a bit of math in Excel, and you have five levels by configuring pullup/pulldown resistors on/off. Another option is to have 2 or 3 IO pins with different resistors, and you have a parallel DAC.

Likely enough to do the job. You can slowly (no real-time capability needed) dither between two voltage levels and it will be good enough (order of magnitude better than on/off).

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