While working on a schematic for a small development board, I was curious how much work it would be to support an external voltage source (ie. bench-top power supply) in addition to the ICSP header and USB port.

For two sources this is pretty straightforward - I would normally run VBUS from the USB port to the emitter on a BC807 PNP transistor with the base driven by the ICSP power pin (self-power dominant configuration).

I've never worked with more than two sources, so this is a bit of a puzzler to me. Ideally the user would be able to (mistakenly) wire VDD to multiple sources without damaging the board.

(Be kind, first post!)


1 Answer 1


If voltage drop is not an issue you can just tie all the power inputs together with diodes: all the cathodes are connected together, and that's where you power your circuit from, and you hook one power supply per each anode. Issues? Voltage drop, power dissipation. That can't work for big boards where much current is needed. You can use Schottky diodes to achieve a lower drop but that pretty much is it.

If there's a some sort of precedence between the supplies you can sort of solve the problem still using only diodes. Here's a sketch:


simulate this circuit – Schematic created using CircuitLab

If PS1,2,3 are rated the same voltage then PS3 is the highest priority one, i.e. when it's on that's what's powering your circuit. If PS2 kicks in D2 is off since the voltage drop across it is 0V, but if PS3 fades away then your circuit is powered through D2. Note that when you are using PS1 you have two diodes drop to take in account.

  • \$\begingroup\$ I was hoping for a solution with less than .5V drop. Even with a pair of Schottky diodes, that's still at least .7V or higher at 500mA. Damn. \$\endgroup\$ Jun 23, 2014 at 23:10
  • \$\begingroup\$ @sstallion There's no need to have PS1 go through two diodes, the cathode of D1 can connect directly to Pout just like D2 does. \$\endgroup\$
    – tcrosley
    Jun 24, 2014 at 8:20
  • \$\begingroup\$ yeah that's my first solution... I did not draw the schematic for it since I thought it was pretty simple to understand. Maybe he needs priority? \$\endgroup\$ Jun 24, 2014 at 8:21
  • \$\begingroup\$ @tcrosley unfortunately there is a priority for one voltage source (VUSB). The others can contend amongst themselves. I'm thinking I'm attempting to design too much and I'm going to back down to just two supported sources - it was a good learning exercise though! \$\endgroup\$ Jun 25, 2014 at 14:38
  • \$\begingroup\$ well then you can use your two source circuit, but connect all the secondary sources to the lowest priority input with schottky diode. that should be less than .5V drop... \$\endgroup\$ Jun 25, 2014 at 14:40

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