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Author Topic: 46 VAC on the case  (Read 1309 times)

Frank DeWitt

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46 VAC on the case
« on: January 03, 2019, 02:15:19 pm »

I have three 3000 series Audio-Technica receivers. They receive there power from wall warts. Each one has 46 volts AC on the case until a audio cable is connected.

I have checked and the voltage is coming from the wall wart.  Unplug the wall wart and the voltage goes away.  Plug the wall wart into the wall and not into the receiver and I measure 46 VAC to ground from the low voltage plug from the wall wart.

Clearly there is no power there because connecting the sound system pulls it down to zero but the voltage can be felt with the receiver in one hand and the sound system connector in the other.

BTW I checked.  The sound system is grounded.  Pin 1 is grounded. and the shell of the line level input cables 1/4 in TRS are grounded to building and power ground.  There is clearly no issue with the sound system. all the fault is with all three wall warts.

Is this normal?  How do I stop it? 

Thanks.
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Daniel Levi

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Re: 46 VAC on the case
« Reply #1 on: January 03, 2019, 02:55:58 pm »

Probably normal if it's a switching supply with no earth connection.
In these PSU's there are/is a capacitor(s) between the primary and secondary side that are there to reduce RFI, most modern un-grounded equipment has the same problem, this will generally be ~half mains voltage but at very low amps so whilst it doesn't feel to good it's not actually dangerous.
Some equipment is worse than other though, although better quality equipment is actually more likely to exhibit this effect due to it being more likely to be RFI/EMC compliant.
I couldn't use my previous laptop connected to my grounded projector with it's genuine ungrounded supply plugged in as the   
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John Roberts {JR}

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Re: 46 VAC on the case
« Reply #2 on: January 03, 2019, 02:58:02 pm »

Is it a problem?

If it goes away from audio connections it is likely stray leakage voltage with very little current.

Relax...  2 pin wall warts "should" be double insulated (check for UL marking).

JR
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Mike Sokol

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Re: 46 VAC on the case
« Reply #3 on: January 03, 2019, 06:52:49 pm »

Is it a problem?

If it goes away from audio connections it is likely stray leakage voltage with very little current.

Relax...  2 pin wall warts "should" be double insulated (check for UL marking).

JR

JR is right. All wall warts without a ground pin will have a certain amount of leakage, creating a hot-skin/stray-voltage around 1/2 of line potential but with way less than 1ma of fault current. For example, your iPhone will typically have between 40 and 80 volts on its body while the charger is plugged in, depending on plug orientation in the receptacle. And itís not dangerous since the current is below perception levels. However, if the double insulation fails in the charger or wall-wart fails, then the wall wart can pass significant current into the chassis. So donít go cheap on your wall warts, and never use a damaged wall wart or charger. And donít talk on your phone while itís plugged into an outlet charger and youíre in the bathtub. While electrocutions from wall-wart failures are pretty rare, that's likely because most of the time your body isn't sitting in a tub of nicely grounded water.
« Last Edit: January 04, 2019, 07:37:03 am by Mike Sokol »
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Frank Koenig

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Re: 46 VAC on the case
« Reply #4 on: January 04, 2019, 03:41:49 pm »

All good guidance above. I'll add that lowering the input resistance of the meter from (typically) 10 M Ohms to something like 10 k Ohms will make the readings far less frightening -- unless there really is a leakage problem. It's surprising to me that more meters don't have a "low-Z" mode for making these kinds of AC line measurements. -F
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Frank DeWitt

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Re: 46 VAC on the case
« Reply #5 on: January 04, 2019, 04:55:55 pm »

All good guidance above. I'll add that lowering the input resistance of the meter from (typically) 10 M Ohms to something like 10 k Ohms will make the readings far less frightening -- unless there really is a leakage problem. It's surprising to me that more meters don't have a "low-Z" mode for making these kinds of AC line measurements. -F

I will go back and measure the current.  This one is a bit different. I found it because a tech complained of getting a shock

When I serviced Fire alarms I carried a 25 watt light bulb in a socket with pigtails so I could load the meter and get rid of the phantom reading.
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Frank Koenig

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Re: 46 VAC on the case
« Reply #6 on: January 04, 2019, 05:21:25 pm »

When I serviced Fire alarms I carried a 25 watt light bulb in a socket with pigtails so I could load the meter and get rid of the phantom reading.

Indeed, an incandescent lamp with clips is a powerful instrument in the right hands :) I still optimistically carry the 12 V version in my cars, although I admit that figuring out all but the simplest problem in these modern CANBus monsters is pretty unlikely. -F
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Scott Helmke

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Re: 46 VAC on the case
« Reply #7 on: January 08, 2019, 10:18:29 am »

Assuming it's just a parasitic voltage with very little current available, it can still cause *audio* problems if there's a long mic cable run to the mixer. I had that happen in an installation once - the parasitic voltage from the switching PSU was being carried away through the shield of the mic line to the mixer, which was maybe 100 feet away from the wireless rack.  I ended up grounding the receiver to the rack (had to scrape away some paint on the rails) to fix the problem.
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TJ (Tom) Cornish

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Re: 46 VAC on the case
« Reply #8 on: January 08, 2019, 10:28:50 am »

All good guidance above. I'll add that lowering the input resistance of the meter from (typically) 10 M Ohms to something like 10 k Ohms will make the readings far less frightening -- unless there really is a leakage problem. It's surprising to me that more meters don't have a "low-Z" mode for making these kinds of AC line measurements. -F
You need a Fluke 117.
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Frank Koenig

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Re: 46 VAC on the case
« Reply #9 on: January 08, 2019, 11:25:56 am »

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Re: 46 VAC on the case
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