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Riddle: Mystery voltage, 70Volts, comming from my switching power supply


taito

Question

INTRO:

 

This baffled me for a week. ha. The answer is at the bottom of the post, so if you want to think about it for a while, for fun, overt your eyes. I reversed the words so you can't accidentally see the answer.

 

STORY:

I was working on various B&W game boards, I don't have a B&W monitor so I just use the TV set composite in RCA (yellow plug) to get a picture, and I noticed I was getting a tingle of the board as I bushed it here and there. This happened a lot as I connected up boards. I had my fingers on top of the Switcher and my screw driver on GND and I got a tickle. :blink:

 

This was concerning so I checked the supply. There was indeed 60-70 volts between GND and earth. The voltage was there even when I un-pluged the supply. So I figured the supply was storing a charge. :o

 

So what could be???? Where was this Phantom charge coming from???

 

 

 

ANSWER: .Dng vT eht mOrf gnimmoc SAw eGatlov Eht

Edited by taito
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Yeah I get ya point @wiredoug, but anything that has a potential above earth (FG on the supply) is IMHO a voltage source. Just depends on how you look at it. :)

 

Think I was just surprised that the consumer exposed socket was pumping out 70V. I worked on an old valve chassis at it's GND is EARTH, so no worries there.

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Might be worth noting chassis trimmer pots, the square metal type used on many chassis are not suitable for adjusting with a metal jeweler's screw driver or if that is your tool of choice, don't earth yourself. You are "supposed" to use purely plastic chassis adjusting tools as not only does the metal conduct power, the metal also affects the adjustment as you find when you remove the metal tool from inside the cap.

 

Chassis adjusting tool kit.....

 

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Might be worth noting chassis trimmer pots, the square metal type used on many chassis are not suitable for adjusting with a metal jeweler's screw driver

 

Yeah good, point! Will look into that. Thanks. :)

 

The voltage adjustment pot is plastic. Just to clarify to everyone who may be wondering how it happened, I actually got my tingle because I had my hand on the case, (FG, EARTH), and I was tightening the GND wire to the switcher. The TV was powered up and also connected to GND at the switcher, so that's where the 70 volts tingle was coming from. :0

 

And as @wiredoug pointed out these devices have a floating grounds and have different potentials depending whether they are powered up or not.

 

- - - Updated - - -

 

In the USA we get that if you reverse hot and neutral when wiring things up. It can often pop fuses or trip GFCI breakers so ops do the typical stupid thing of ripping the ground pin off the power plug rather than fix their shit right.

 

Dam, that's dangerous eh! I once got a table where the operator replace the power cord/lead. They put active to neutral and neutral to active. This meant that the games power switch only switched the neutral. The transformers were all live. I got big jolt. Tripped the RCD, residual current device.

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