I have a 36V 8.8 Amp qualtek power supply that I want to bring for a friend. I'm not planning on checking bags so it will have to be carry on. Is this ok to bring on a plane? I can't seem to find any rules on power supplies on airplanes in the US.
The What Can I Bring page does not list this explicitly but it does list Phone Chargers which are similar in function but also Bread Machine, Laptops, Sewing Machine all of which require AC/DC units. Also power inverter. If you want to be 100% sure
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If by "power supply" you mean something you connect to a wall socket, and which can then supply DC power to some electronics, then certainly -- people routinely fly with those in their cabin baggage (they might call them laptop chargers instead, but a power supply is what it is).
If you're speaking about a battery bank that doesn't need to be receive grid power while it puts out DC, then different rules apply.
In general, electronic devices are allowed, provided they don't contain forbidden substances (anything highly flammable, radioactive, explosive, liquids etc.). Practically, most devices without large batteries in them are OK. Batteries are indeed dangerous, hence the limits and the requirements to keep devices with batteries in carry-on.
Anecdotally, I once travelled with as similarly looking device which weighted about 16kg and occupied most of my checked in bag.