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Old January 23rd, 2021, 04:04 PM   #1
ganutenator
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noise

I don't want to state the obvious to those in the know, but I wasn't in the know.
Apparently, the way dc switching power supplies work, they are the cause of most noise.
When I say noise, let me explain.
We were doing an install.
Customer complained that his crescent wrench sparked on the machine and that he got a jolt.
I measure the voltage in ac from the equipment to ground. It was aprox 20VAC.
We both walked back to the panel.
Everything was turned off except for the 24dc power supply.
I was like, bet the ground wire isn't landed.
Sure enough, the electricians had yet to land the ground wire.
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Old January 23rd, 2021, 07:57 PM   #2
Holmux
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Nothing beats "transformer + Bridge + Capacitor"

I know you have to look for them, but they are out there.

I even take it a step further, I always start with a transformer using a 2 fase input, giving me a fase and "Neutral" as the output.

I never pull a neutral from the plant into my cabinet.
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Old January 23rd, 2021, 10:55 PM   #3
James Mcquade
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as an insurance policy, i always connect x2 of the secondary to the control panel. may have nothing to do with your issue, but i have never had or heard of (until now) what you are describing.
what brand power supply was it?
james
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Old January 24th, 2021, 02:47 AM   #4
parky
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20 vac is not much & probably general pickup could be from anywhere, did you isolate the 24v power ?, did it make a difference?, I once had a panel that was not even wired to any external power or any of the I/O, in actual fact there were no wires at all, a colleague said he had a shock off the panel, it was still on a pallet so checked to earth and got a varying ac voltage of 10-40v but as soon as I put my hand on the panel it dropped to about 10, so it may not be anything to do with the PSU. The other thing is a multi meter is quite a high impedance, so draws little current.
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Old January 24th, 2021, 07:59 AM   #5
ganutenator
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Any solid state power supply causes noise. Back in the day they didn't. But the high frequency switching that they use now does.
The only thing turned on was the 24 vdc pwr supply.
It induced enough voltage on the wires to cause you to get shocked.
Turn it off and it goes away.
Connect the ground and it goes away.
So yes, probably best to go old school w/ the power supply.
Or shield the f*ck out of everything.
Just thought this find would be useful is all.
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Old January 24th, 2021, 10:42 AM   #6
BryanG
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I don't think it would be anything to do with induced voltages. It would be one of the filters that they use to stop noise going back to the supply that was putting volts on to the unearthed earth connection. You can't really blame the Power Supply if the system isn't earthed.

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Old January 24th, 2021, 12:31 PM   #7
Yacineg
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Quote:
Originally Posted by ganutenator View Post
I don't want to state the obvious to those in the know, but I wasn't in the know.
Apparently, the way dc switching power supplies work, they are the cause of most noise.
When I say noise, let me explain.
We were doing an install.
Customer complained that his crescent wrench sparked on the machine and that he got a jolt.
I measure the voltage in ac from the equipment to ground. It was aprox 20VAC.
We both walked back to the panel.
Everything was turned off except for the 24dc power supply.
I was like, bet the ground wire isn't landed.
Sure enough, the electricians had yet to land the ground wire.
Forgive me for a stupid question. It is a 24v DC power supply & he is reading 20V AC from equipment to ground. How is the reading in AC ?
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Old January 25th, 2021, 10:27 AM   #8
ganutenator
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Quote:
Originally Posted by Yacineg View Post
Forgive me for a stupid question. It is a 24v DC power supply & he is reading 20V AC from equipment to ground. How is the reading in AC ?
Apparently induced voltage turns into AC. Only a SWAG but probably due to the high frequency switching of the power supply.

Additionally, turning on DC braking on the drives does the same thing.

The noise will actually turn the inductive prox's into capacitive prox's.
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Old January 26th, 2021, 08:12 AM   #9
Yacineg
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Thank you.
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