PicoPSU: is waste heat proportional to current used?

[Trash80toHP_Mini]

I've got a high output model/power brick on hand and a 160W PicoPSU on the way. Using the 160W unit inside a Duo case to power:

RPi Compute Module 4
RPi Pico KBD Matrix to USB Converter
Duo TrackBall (ADB)
ATmega32u4 based TrackBall to USB Converter
10.1" 1920x1200 LCD/Controller

Assumption would be that thermal profile would be acceptable in this use (wouldn't melt the) case.

 

[aladds]

Do you have a data sheet for the power supply (or supplies, if you want to put both the brick and PicoPSU inside)? You need to calculate the efficiency and then you can work out the heat output over time.

You would then do a "power in" in watts minus "power out" in watts and assume all of that "useless" power is dissipated as heat.

If I take the original Duo supply as an example, the input power is () 1 Amp at 100v (assuming that the biggest current draw is at the lowest accepted voltage) and the output is (full load) 1.5Amps at 24V.

That's 100W in and 36W out, dissipating 64W into the case, which is 64% efficiency. (this is probably quite far from what it actually generates)

Assuming you're generating multiple voltages you would need to do these calculations separately and add them up.

From there you need to know how much heat the Duo brick can dissipate, which I would model as a sealed box, something like: https://www.powerstream.com/temperature-rise-in-an-electronics-enclosure.htm

Putting in some assumed values for the Duo brick (I measured one but I've not got one to bits to measure the wall thickness):
Assuming it's made of ABS, the walls are 2mm thick and the size is 115x65x30mm in a 20C room the supply would raise to about 50 degrees C, with a case temperature of 30 C. ABS is generally considered safe at this temperature, and whilst it would feel warm it wouldn't set anything on fire.

That said, do please check my maths before burning your house down.

 

[Trash80toHP_Mini]

The OEM brick is a great example, thanks! PicoPSU 12V input jack replaces the Duo's power input jack, no brick inside.

Only the PicoPSU is enclosed within the Duo case. Don't have power requirements of all components or the peripherals in hand as yet.

Duo is worst case scenario and a handy starting point for this project even if it doesn't work out. PowerBook100 with 1400c KBD hack is final target. Quite a bit more available cubic, but the Duo with docking door flipped up exposes all CM4 I/O ports for access within, which is very cool. We'll see about fitting the other stuff in case and lid?

Investigating 8.9" 1920x21200 panel that matches height and needs a bit more width than the PB100 bezel. So 18W from Type C input.
CM4 I/O Board docs say to budget 9W @12V for the CM4.

5V/12V requirement put me on the path of using PicoPSU for powering CM4 and peripherals. Very light duty work for both rails I think, so not very much waste heat?


edit: PicoPSU is a switching PSU, so more efficient than the straight AC->DC power brick example?

 

[aladds]

Aha! Well in that case you're in a much better scenario. If we take the highest output of the existing supply, which is 36W, your replacement guts will be within spec if they consume less/the same as this. (technically the LCD backlight, since it outputs light, does not fully contribute to the heat output inside the case, but we can probably assume this is close enough to that of the new display)

Add up the CM4 power consumption, the display and the losses in the PicoPSU. Stay within 36W and you should be fine

Edit: Oh, and the PicoPSU is probably more efficient, but the original Duo brick is also a switching supply, just an older one. You can tell if you don't have a switching supply because it has a huge transformer inside it instead of a compact one!

 

[Trash80toHP_Mini]

May not even need the PicoPSU? Offer accepted on LCD/Controller with a 12V input, same as CM4!

Might a very simple Capacitor/Resistor board do the trick? If I go with the PicoPSU all I'd need would be current limiting resistors on two of the 12V lines.

Looking good for a setup running off the brick or 12V battery I think?





More pics/details here:

PowerBook 100/RPi-CM4-Lite

PowerBook 100/2300c LCD/Logic board upgrade has been in process for well over a decade. This revision has been on the back burner since the Compute Module 4 first came out. The I/O board makes it perfect for this upgrade of my beloved BabyPB, very first laptop, bought new and maxed out in the...

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