sandysound
Posts: 100
Joined: Thu Sep 14, 2017 6:05 pm

Triggering relay trips power supply.

Mon May 03, 2021 8:49 pm

Hi,

I am using a 4 ch opto isolated relay module bought cheap as chips from AliExpress. There's your problem I hear you all say.

Everything is powered from an old pc power supply I had laying around. I have measured the output and it is within spec.

I have cut a 5v and ground wire and spliced to USB for the PI.

The relay board is 3.3v and I have two of them, so each has independent 3.3v and ground cables from the power supply.
Then I have connected my gpio's to the relay triggers.

Everything powers on and is happy, until I try to trigger a relay. I hear the click, and then the power supply instantly shuts off in a failsafe mode, and all power is lost.
After resetting the supply everything is happy again until the next attempt.

I have only connected the Pi's GPIO's to the relay boards and not the ground, as I figured seeing as everything is coming from the same power supply then they all share the same ground anyway.

Should I be connecting the ground between the units?

Having never used a pc power supply in this way before, have I done something wrong? I haven't put a dummy load on it, could this be an issue?

Cheers.

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neilgl
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Re: Triggering relay trips power supply.

Mon May 03, 2021 10:52 pm

Have you checked that the old PC power supply has a 3V3 output?

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Imperf3kt
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Re: Triggering relay trips power supply.

Mon May 03, 2021 11:20 pm

How much current are you drawing, from what you describe it sounds like the PC power supply is being overloaded after you power whatever your relay is connected to.

Be aware that pc power supplies have multiple 5v and 12v rails, which can supply a limited amount of current each. If the unit has a maximum of say 10A at 12v, and it has two 12v rails, then it really can only deliver 5A@12v to two rails and will trip if you try to power something that requires say 7A@12v, despite being within spec.
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LTolledo
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Re: Triggering relay trips power supply.

Tue May 04, 2021 12:40 am

let's SEE how you actually wired your setup (that means... dont let us guess or imagine it!)
"Don't come to me with 'issues' for I don't know how to deal with those
Come to me with 'problems' and I'll help you find solutions"

Some people be like:
"Help me! Am drowning! But dont you dare touch me nor come near me!"

sandysound
Posts: 100
Joined: Thu Sep 14, 2017 6:05 pm

Re: Triggering relay trips power supply.

Thu May 06, 2021 4:22 pm

Ok, so, after triple double checking my wiring and measuring the current being asked of the power supply, everything was as it should be and well within spec.
I took the RPi out of the equation and powered up the relay boards by themselves. I took a jumper cable to trigger the relays and every time the power supply tripped.
So I tested the relay boards with another power supply and everything was fine!
So, long story short, I replaced the power supply and now everything works like it should.
Man, what a frustrating experience that was.
Everything about that power supply checked out, until under load.
Is there something I could look at within the unit to possibly repair the power supply? Or is it toast?

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neilgl
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Location: Near The National Museum of Computing

Re: Triggering relay trips power supply.

Thu May 06, 2021 4:54 pm

Good troubleshooting there mate !

drgeoff
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Re: Triggering relay trips power supply.

Fri May 07, 2021 7:47 am

sandysound wrote:
Thu May 06, 2021 4:22 pm
Everything about that power supply checked out, until under load.
Is there something I could look at within the unit to possibly repair the power supply?
Computer PSUs (and switch mode supplies with mains input) are complex and potentially dangerous beasts. Anyone who asks what you asked is not competent to repair one.
Quis custodiet ipsos custodes?

emma1997
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Re: Triggering relay trips power supply.

Fri May 21, 2021 5:40 pm

Imperf3kt wrote:
Mon May 03, 2021 11:20 pm
Be aware that pc power supplies have multiple 5v and 12v rails, which can supply a limited amount of current each. If the unit has a maximum of say 10A at 12v, and it has two 12v rails, then it really can only deliver 5A@12v to two rails and will trip if you try to power something that requires say 7A@12v, despite being within spec.
In reality 'mulitple outputs' on most PC supplies are generally the same wire, all connected together on the PCB. Small exception is the 'always on' 5v or rare special purpose rails on some high end units.

Many look down their nose at these and go on and on about noise etc but I love them and have modified dozens over the years for chargers, lab PSU, and whatnot. There's even a couple original 63w still in use here after almost half century.

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