Directly energize a bank of 12 relay coils from lab PSU without flyback diode? - Page 1
Forum section: electronics / beginners

author: incf
author_rank: Frequent Contributor
timestamp: April 11, 2025, 02:32:05 pm
content: Suppose I directly energize a bank of 12 relay coils in parallel (2A signal relays) from a digital bench power supply - and switch it on an off. There are no flyback diodes or any other circuit elements.Can this damage the PSU?I'm thinking it's unlikely to damage anything since the rise and fall times are slow.

author: MarkF
author_rank: Super Contributor
timestamp: April 11, 2025, 02:39:41 pm
content: I've measured the back EMF of just ONE similar relay coil with a scope.You will easily see a spike between 200V to 400V peak.Multiply by 12.Judge for yourself if it's a good idea....

author: TimFox
author_rank: Super Contributor
timestamp: April 11, 2025, 05:27:12 pm
content: If you try to test your hypothesis, the results of a back-EMF larger than you guess might be catastrophic.How much money do you save by omitting a flyback diode or 12?

author: PGPG
author_rank: Frequent Contributor
timestamp: April 11, 2025, 06:30:32 pm
content: Quote from: incf on April 11, 2025, 02:32:05 pmI'm not enough good in English to understand what is switched on and off - relays bank or power supply?If power supply - than probably you are right, but it can depend on how switching off is organised in supply.If relays bank - than switch that is used to switch them is exposed to overvoltages of up to several hundred volts.Quote from: MarkF on April 11, 2025, 02:39:41 pmSince when connecting voltage sources in parallel gives you sum of voltages?

author: incf
author_rank: Frequent Contributor
timestamp: April 11, 2025, 06:42:06 pm
content: It's a DP832 type linear power supply. I have no idea how what it's output stage looks like or fast it switches off.I'll probably rig up a snubber. But I am still curious if it is possible to concretely say anything about the proposed scenario other than "dunno, why risk it?".

author: floobydust
author_rank: Super Contributor
timestamp: April 11, 2025, 06:53:06 pm
content: DP832 has big fat 1,000uF output caps at the binding posts, for stability and extra bang as CC mode activates lol. I didn't see reverse-diodes.The relay coil back-EMF will not damage it. Probably lol.

author: David Hess
author_rank: Super Contributor
timestamp: April 11, 2025, 07:17:44 pm
content: Most bench power supplies have so much output capacitance, that they would absorb flyback spikes easily.

author: PGPG
author_rank: Frequent Contributor
timestamp: April 11, 2025, 07:36:17 pm
content: Quote from: David Hess on April 11, 2025, 07:17:44 pmWhen there is output capacitor than there is practically nothing to absorb. Flyback spike is generated by high dI/dt when switching off. With big capacitor in parallel to relay coil there will be very, very small dI/dt so no real problem.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 12, 2025, 07:29:11 am
content: Essentially all lab supplies have a diode at the output that prevents a larger negative voltage at the output. If one turn off the relays with the enable switch or the mains switch at the supply the kickback would continue to draw current for some time, even of the voltage drops. In most cases there is enough capacitance at the output would not even go negative from that.Even without the capacitance or a larger inductive laod (e.g. larger magnet), the supply internal diode would limit the negative voltage to some -0.7 V or so and the supply is OK with this. That supply internal diode is kind of the flyback diode. There are inside for the cause of using 2 supplies in series and are thus normally made to withstand the full rated current or a bit more.If one turn off the relay with an external switch / remove the wires, there would be some spark at that contact - worst case the isolation in the relay coil may not like it, but usually they are OK with this.

author: SteveThackery
author_rank: Super Contributor
timestamp: April 12, 2025, 10:47:59 am
content: For decades in the late 19th and early 20th century inductive loads were switched without the help of flywheel diodes. RC snubbers were used widely to good effect, as were voltage dependent resistors.This is a solved problem.