PSU in series, what is the diode good for - Page 1
Forum section: electronics / beginners

author: pstimpel
author_rank: Contributor
timestamp: September 22, 2020, 03:50:09 pm
content: Hi,some examples of how to put two psu in series show a diode between plus and minus of each single psu.What is this diode exactly good for?Is there any harm in keeping the diodes there, even if this single supply was not put in series or parallel with other psu?ThanksPeter

author: Alaezae
author_rank: Contributor
timestamp: September 22, 2020, 04:06:30 pm
content: I would imagine some sort of voltage limiting. if the diode is reverse biased between the two rails then at the breakdown voltage of the diode current will be able to flow from the positive rail to the negative rail. That can protect the other power supply.Do you have a picture of the schematic?I’m also a beginner so sorry if I said something that’s incorrect

author: pstimpel
author_rank: Contributor
timestamp: September 22, 2020, 04:17:35 pm
content: The example is taken from MEAN WELL homepage, but I have seen it on other places as well. My limited understanding tells me such a diode does not harm anything if I put it on all my psu, just in case I will put them in series and forget about that diode. Is this true?Quote from: Alaezae on September 22, 2020, 04:06:30 pmGreat, so let's learn something together.

author: Benta
author_rank: Super Contributor
timestamp: September 22, 2020, 07:52:24 pm
content: They are for protection if your PSUs don't power up (or down) simultaneously. Imagine one powering up first. This would send current into the output of the other, which is undesirable.

author: pstimpel
author_rank: Contributor
timestamp: September 22, 2020, 08:05:29 pm
content: ok, got that. Thanks.So, what could possibly go wrong if I add a diode in a permanent way?

author: Benta
author_rank: Super Contributor
timestamp: September 22, 2020, 08:08:41 pm
content: Nothing except slimming your wallet

author: Alaezae
author_rank: Contributor
timestamp: September 22, 2020, 08:52:30 pm
content: I think that’s fine the way it is (once again, a beginner here) but I don’t understand why it needs one on both power supplies. If it was supposed to be split rail so that the ground was the point in between both power supplies, the two diodes would make sense. I think it would make more sense to have one diode across the whole thing, not across both of the PSUs. I don’t know, that’s just a thought.

author: MarkF
author_rank: Super Contributor
timestamp: September 22, 2020, 09:11:26 pm
content: I do not believe it has anything to do with PSU's in series.You find reverse bias diodes on single supplies also.They are there for reverse polarity protection.However without a fuse somewhere, they are only as good as the current capabilities of the diode.You will also find a diode around the input and output of simple 3-terminal regulators for over voltage protection.

author: TimFox
author_rank: Super Contributor
timestamp: September 22, 2020, 09:27:53 pm
content: Going back to the drawing from MeanWell posted above with two discrete power supplies. For sake of discussion, assume that the lower one (PS2) is set to 5 V, and the upper one (PS1) is set to 24 V, providing 29 V to the load. If there arises a dead short across the load, then the upper 24 V supply tries to force the connection point from +5V down to -24V, which seriously reverse-biases the lower supply. The diode across the lower supply will then conduct and limit the reverse voltage across the lower supply to 1 diode drop.

author: David Hess
author_rank: Super Contributor
timestamp: September 22, 2020, 11:15:18 pm
content: The diodes are added so that in the event that one of the supplies limits its output current, like if there was a short across both supplies or during startup when charging capacitors, the supply will not see reverse polarity, likely damaging it. Instead the diodes prevent the output of each supply from seeing more than 0.6 volts in reverse.