Electronics > Projects, Designs, and Technical Stuff
Advice for microprocessor (PI) controlled variable voltage power supply?
h82fail:
I have a project where I want to control the voltage on a peliter with a PI (max would be 12v @ 6A). Its been a while since I've designed a proper circuit and I really just want a easy solution so I can get to the programming part of my project.
My initial thoughts were to just purchase something like this: https://www.amazon.com/gp/product/B00C4QVTNU then to replace the pot with a digital pot like MCP4151-503E/P. Then test the output voltage during programming and hope it stays consistent over time or add a DAC to the Pi and use a voltage divider to keep track of the voltage. This for a appliance, a wine fridge being turned into a cooled humidor so I want it to be reliable. I would replace those Chenxing caps as I don't trust them long term. Maybe it would be better to just build something from scratch I just don't trust designing it myself.
Could i just use a N MOSFET with a proper filter to smooth the output (I know I would need a inductor/cap/diode just not sure how to size). I heard that driving it with PWM lowers the efficiency of a peltier quite a bit so I would want a decently smooth output. Or if anyone has a better solution, either something without a ton of parts or a better off the shelf variable step down. I have LTspice installed if anyone has any examples.
Thanks a bunch for any help!
profdc9:
If you are using the Peltier in a feedback loop to control temperature, would turning the Peltier on and off every few seconds be inefficient? How closely does the temperature need to be controlled? Is the inefficiency from having to charge and discharge the Peltier junction capacitance? Maybe a minimum on or off time of 5 seconds or more would prevent excessive energy from being expended in the junction.
NiHaoMike:
Just use a low side switched buck converter topology driven from the PWM signal via a MOSFET driver.
jbb:
--- Quote from: NiHaoMike on October 03, 2019, 01:57:23 am ---Just use a low side switched buck converter topology driven from the PWM signal via a MOSFET driver.
--- End quote ---
Yes, I think this is a good starting point. Gate drivers to convert a 3.3V PWM signal to a 12V gate drive signal are cheap and readily available. I recommend against messing about with discrete transistor designs.
As I understand it, Peltier elements aren’t much more efficient when driven with a steady variable DC current, instead of all on / all off control. This is because the Peltiers have quite a lot of resistance, so higher drive voltages = higher drive currents = much higher losses. Also the all on / all off cycles may cause fatigue.
If you have a look around, you can find training videos from the Peltier manufacturers for how to size things. It turns out that you probably don’t want to run the Peltier at more than half it’s delta-T rating or wattage rating.
Also, at least for cooling mode, hot side heatsinking to ambient is critical.
Siwastaja:
--- Quote from: profdc9 on October 03, 2019, 01:54:56 am ---If you are using the Peltier in a feedback loop to control temperature, would turning the Peltier on and off every few seconds be inefficient?
--- End quote ---
Yes. On/offing peltiers is not recommended for neither efficiency nor reliability.
First, the Peltier efficiency drops at higher currents (so-called diminishing returns). So basically if you have a heat transfer of, say, 20W at 20W input power (100% COP), you won't have 40W heat transfer at 40W input power, but maybe only 35W (88% COP). (Made-up example numbers.) This starts to drop quite quickly when getting near to the nameplate power, so if you have to on/off control it, derate the power.
This means, high current should be used only when high level of cooling is actually needed. 100%-0% control (very slow, or faster PWM, doesn't matter) always runs it at the lowest efficiency possible.
Secondly, despite being marketed as a "solid state device", Peltiers have limited on/off cycles due to thermal stress and may resonate with higher PWM frequencies.
Hence, voltage control is definitely recommended. It doesn't need to be super accurate, and a small amount of ripple won't kill it.
Direct PWM with a MOSFET cannot be recommended for the reasons mentioned, but open-loop buck, which basically is the same but with added output filter (inductor + capacitor), is well acceptable. (Remember the proper two-switch topology: a freewheeling diode is definitely needed once you add the inductor.) For sizing the inductor and the capacitor, look up for buck converter design aids. You'll find appnotes, spreadsheet calculators, etc.
Although, in that case, just using a bog standard DC/DC converter IC is likely easier, because it implements overcurrent protection (which, again, becomes important when you add capacitance to the output).
And yes, as a general rule of thumb, there is this physical law regarding Peltiers that the hot side cooling is always 3.14159 times more important and difficult than you thought it is, no matter how well you did your homework.
This is because all experience people have on cooling electronics assumes some dT=60 (so that, for example, the semiconductor junction sits at 100 degC at 40 degC ambient). But using Peltiers efficienctly typically requires optimizing it below dT=10 degC in order to have any meaningful cooling, so it's completely different ball game. If you are dissipating 100W, you need to use solutions you'd normally use for dissipating 1kW.
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