Isolated Circuits but with Equipotentialized Grounds - Page 1
Forum section: electronics / projects

author: TiagoSilva
author_rank: Newbie
timestamp: November 27, 2019, 12:21:45 am
content: Hi guys!I have been challenged for a few days from which I have not yet found a satisfactory solution.It would be very helpful if anyone could help.Check out the circuit below. There are two adjustable power supplies and a single circuit that monitors any short circuit at the output of these power supplies (voltage below 1.2V).I would like to keep each of the power supplies isolated so that I can use them independently, in series or in parallel.However, I need the circuit ground that monitors short circuit to be on the same level as the ground of the power supplies, that is, equipotentialized (I don't know if this word makes sense in English, my native language is PT-BR); so that it can detect whether or not the output voltage is above 1.2V correctly.Well, does anyone have any idea how I could keep them isolated but with equipotentialized ground?Thanks in advance!

author: Ian.M
author_rank: Super Contributor
timestamp: November 27, 2019, 12:40:24 am
content: You cant. Also that circuit wont work even with a common ground as LM324 OPAMPs re not comparators, they have a push-pull output stage not an open collector one so you can't wire-OR them.Therefore you will either need isolated sensing circuits, one for each supply, probably optocoupled to the single /short_circuit detected output, or if the difference between their grounds is within a certain range (i.e the max output of one supply), you could use differential amplifiers to sense the floating output voltage of each, and shift them down to a common ground. However then you'll need a separate isolated auxiliary supply to power the sensing circuit.

author: TiagoSilva
author_rank: Newbie
timestamp: November 27, 2019, 01:17:22 am
content: First, thanks for the reply!Your warning about the LM324 was not suitable for application as a voltage comparator was very relevant to me, led me to excellent articles on the subject. I learned new things, thanks for that.I confess that the idea of having two sensing circuits, one for each power supply, had crossed my head... but I was trying to escape this solution.About the possibility of using differential amplifiers, this was new, I had not thought of that.Well, the first solution seems simpler anyway, thanks a lot for your help, Ian!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 27, 2019, 06:06:06 am
content: Put a single comparator and reference on each channel, locally. (You can even get combined reference/comparator ICs!) Then use optoisolators to transmit the signals to a common location.If you know something about the voltage between grounds, you can potentially use a level shifting circuit, but it's damn hard to argue with a $0.10 opto.Tim

author: TiagoSilva
author_rank: Newbie
timestamp: November 27, 2019, 10:37:48 am
content: True, using the opto is much more interesting in this case.I'm going this way, thanks Tim!

author: TiagoSilva
author_rank: Newbie
timestamp: December 05, 2019, 12:43:10 am
content: What's up guys!I finished my circuit and everything works fine.I replaced the LM324 with the LM339, inserted the opto-coupler and an auxiliary supply completely isolated from the others adjustable supplies.Thank you.