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Power supply voltages. ???

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Righteous

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Heya all,

A long story made short...

Basicly i have a power supply that gives out 18 Volts DC...

During the long story i discovered that if you go from the negative or positive of this power supply down to earth there is a potential diffrence of 110 V AC.

Now i cant understand why could anyone point me in the right direction ??

The power supply seems to operate okay give out the 18 volts DC is suppose to and its a double insulated device.

So im using the house earth when measure the potential.

Is this 110 V AC created from the PWM modulation devices that help keep the DC supply stable ?? or is there something wrong maybe capacitor causing this problem or something ??

EDIT * Also if you measure potential diffrence in AC across the postive and negative its 1v or something so shows its in phase on both conductors too*

At first i thought the bridge rectifier was defective but test and all is okay.

Cheers for reading and any advice or comments you have :).

Tom.
 
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I have attached a quick drawing of what mean if it makes things easier to see :).
 

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The third prong of the power supply is not connected to earth (like it should be).
 
Ah yea its meant to be like this mike its double insulated which means it does not require the earth its an official standard items can be made too. :)
 
Well, then you are just measuring the AC voltage divider due to stray capacitance between the primary winding and the secondary winding. Put your multimeter in the AC mA mode, and measure the "ac leakage current" where you previously measured AC volts. I'll wager that your meter will have a tough time measuring the leakage current. Bottom line, dont worry about it. If it matters to you, earth one end of the DC side if the supply.
 
I agree 100% with MikeMl.

This is a common problem with these types of power supplies.
Depending on what the power supply is powering, I have found that it can give some odd results with some circuits.

JimB
 
ah yea well reason i found it is cause i had a electric shock from it.

The power supplys negative goes to a chassie.
At the time the chassie was not earthed and i touched the chassie and a earth and got shock.

I was just wondering if its normal operation as like to know how stuff works :).

And okayy cheers Mike so its induced from that little transformer that cuts it down hmm i wonder if theres a rectifyier on the low voltage side output on the transformer surely that would stop the ac voltage in theroy ill have a look at it when get chance.

Cheers :).
 
If you got a shock something is wrong, stop using the supply until problem is found ( unless just a static electric shock ). Check the leakage current. It should be in micro amp range or less. The double insulation may now be no insulation.
 
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