I have got a 12V dc power suppy for external hard disc. when i was connecting the harddisc USB connector to the computer i got a shock. On checking I found that power supply out put gives 12V dc and 25V ac. However on interchanging the the terminals it shows only dc and no ac.
It is puzzling to me can any one throw some light :?
sunil

 

It sounds like you were just getting stray pickup?, presumably you were using a high impedance digital voltmeter?.

It's still far more 'reliable' to use old analogue meters, like the old Avo 8's, they don't have such problems - but you do need to be aware of their lower impedance!.

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I did check even with an analog multimeter but the results are same.
A friend of mine told me that it is a SMPS that is why it is giving ac output on no load but it won't be ther on load. :?: I don't know about SMPS :?

 

I see no reason why a SMPS would output AC with no load?, do you have access to a scope you could put on the output?.

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OK lets examine the above statement:

"I got a shock", do you mean an electric shock or a surprise?

"12v DC and 25v AC", Where were you measuring the AC?

"interchanging the terminals", did you reverse the mains plug in the socket, or reverse the meter connections?


I just measured a "wall wart" supply for my external disc drive and I got 12v DC output as expected, but if I switched my DMM to AC volts I got a strange fluctuating reading.

When I measured from one of the output pins of the wall wart to mains earth, I got about 115v AC.
This is reasonable, I get the same from the PSU for one of my laptops.

The reason for the 115volts (half of the mains supply voltage), is that there is no earth connection to the wall wart and although the low voltage output is isolated from the mains, there is stray capacitance coupling across the transformer in the wall wart and this lets all the secondary circuits "float" at half supply voltage.

(A bit like your unearthed 'scope Nigel! :lol: )

So Auro, does that help to explain? if not please be a bit more specific with your statement of the problem and we will think again.

JimB

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