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Isolation


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#1
samplerdave

samplerdave

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Hi all,

 

Here is the problem. I'm running a series of charge/discharge tests on an old sulphated car battery and wish to record the results using my Pico AD40.

 

I have no problems with  the discharge recording.  Just place a load across the battery and put the AD40 (via 3:1 attenuator) across battery.

 

But the charging part may be a bit different as I am worried that the negative side of the charger may not be at the same (zero) potential as the earth of the computer.

I do not wish to damage the AD40.

 

So,  can anyone recommend a simple way of isolating the AD40 to overcome this risk.  Maybe some kind of opto coupler; whatever.

 

I've not had to deal with this problem before,  so any advice is welcome.

 

David.


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#2
Plastic Nev

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Not familiar with the AD40, however for safety of both yourself and the equipment, it would be simpler and safer to make sure that both the negative of the charger and your other equipment is connected to a good earth point.

Running equipment isolated from earth has the possibilities of equipment cases being at mains voltage and therefore an electric shock risk, don't do it that way.

 

Nev.


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#3
samplerdave

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Nev,

 

Thanks.

 

After posting I got around to thinking that as long as the charger case, transformer frame and the output DC negative were all tied to the mains earth then there should be no problem as the computer would naturally share the same earth as the rest of the bits.

 

Does this tie in with your thinking ?

 

David.


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#4
Plastic Nev

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It does to a certain extent, though the negative out from the charger just may be floating and neither negative or positive connected to earth, just the metal case of the charger. The manufacturers of the charger will have done this as a possible prevention of a short circuit if the charger is sat on the vehicle and is one of the older positive earthed cars.

For your purposes, if you have a meter that can be used as a circuit checker, or has a low value ohms scale, check if there is a good low resistance connection between negative and earth. Check it in both directions in case the rectifier shows open circuit one way and a closed circuit the other.

 

If it does show as open circuit in either direction, then an extra length of wire from negative to one of the charger case screws will bring the negative to earth potential.

 

Nev.


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#5
samplerdave

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Nev,

 

Yes,  that's the route I'll take.

 

This business of 'floating voltages' is a nuisance when using any earthed equipment to measure/monitor anything that does not have one end at earth potential.  Easy enough using a VOM.

 

Thanks for the advice.

 

David


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