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I want to power my 12V devices with USB PD. Looks like 12V is optional in the spec and is supported only by some devices (eg. UGREEN), and not by others (eg. An

by kbouck 2y ago

I want to power my 12V devices with USB PD. Looks like 12V is optional in the spec and is supported only by some devices (eg. UGREEN), and not by others (eg. Anker) Given a USB PD power supply which supports 15V but not 12V, and a usb-c/barrel-jack cable configured to negotiate for 15V, what would be the simplest (yet safe) circuit i could add via barrel jack to regulate the to voltage down to safe/consistent 12V? is a simple linear voltage regulator (LM7812) sufficient? would i need capacitors to smooth it out?

klysm 2y ago
A 3V drop over an LDO is usually reasonable with low enough currents. Some LDOs require capacitors to be stable, and it’s usually a good idea to have some capacitance on your power rails anyway.
15155 2y ago
The one important thing missing from your query here is: How much current do you need? If you need, say, >100A, the possible architecture looks very different than ~1A or less.
_Microft 2y ago
It might be cheaper to get a power supply that supports later PD standards? E.g. IKEA is selling some cheap here for either 8€ (Sjöss, 1 USB-C port (max. 30W, up to 3A)) or 15€ (Sjöss, 2 USB-C port (combined power output of 45W, up to 3A, also on a single port)). Both support PD 3.0 and PPS (that's the fanciest PD standard that implements requesting arbitrary voltages from the power supply) They also stock nice and cheap USB-C cables. These power supplies work fine with USB-PD trigger boards set to 12V.
a96 2y ago
Linear loss obeys Kirchoff's and Ohm's and power laws pretty simply. If you need to lose a certain voltage at a given (max) current, the power lost is P = U * I. For example, 3 V * 0.1 A = 0.3 W. If your LM7812 can run at that overhead (yes), take the voltages and currents (yes) and dissipate the 0.3 W as heat (almost certainly yes) then it'll work. Caps will depend on how fast the response needs to be. If your load jumps too much too fast and there's no reservoir, you'll get a voltage drop. If it feeds too much back, you'll get a voltage climb. Caps may help and are pretty cheap and IIRC 78xx chips are very stable, but the datasheet will have limits and recommendations. All the information will be in the data sheet and making some test circuits is easy for these, especially if you have some measuring equipment. Anything below 1A will probably be easy, anything above might be risky, as a rule of thumb. Very small or very large voltage differences will also be tricky or impossible.

by kbouck 2y ago

klysm 2y ago

A 3V drop over an LDO is usually reasonable with low enough currents. Some LDOs require capacitors to be stable, and it’s usually a good idea to have some capacitance on your power rails anyway.

15155 2y ago

The one important thing missing from your query here is: How much current do you need? If you need, say, >100A, the possible architecture looks very different than ~1A or less.

_Microft 2y ago

It might be cheaper to get a power supply that supports later PD standards? E.g. IKEA is selling some cheap here for either 8€ (Sjöss, 1 USB-C port (max. 30W, up to 3A)) or 15€ (Sjöss, 2 USB-C port (combined power output of 45W, up to 3A, also on a single port)). Both support PD 3.0 and PPS (that's the fanciest PD standard that implements requesting arbitrary voltages from the power supply) They also stock nice and cheap USB-C cables. These power supplies work fine with USB-PD trigger boards set to 12V.

a96 2y ago

Linear loss obeys Kirchoff's and Ohm's and power laws pretty simply. If you need to lose a certain voltage at a given (max) current, the power lost is P = U * I. For example, 3 V * 0.1 A = 0.3 W. If your LM7812 can run at that overhead (yes), take the voltages and currents (yes) and dissipate the 0.3 W as heat (almost certainly yes) then it'll work. Caps will depend on how fast the response needs to be. If your load jumps too much too fast and there's no reservoir, you'll get a voltage drop. If it feeds too much back, you'll get a voltage climb. Caps may help and are pretty cheap and IIRC 78xx chips are very stable, but the datasheet will have limits and recommendations. All the information will be in the data sheet and making some test circuits is easy for these, especially if you have some measuring equipment. Anything below 1A will probably be easy, anything above might be risky, as a rule of thumb. Very small or very large voltage differences will also be tricky or impossible.