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Rodeodave 16th September 2005 08:34 PM

Fuse popping
I built an unregulated 40V DC power-supply using bridge (full-wave) rectifiers and 30000F electrolytic caps. The primary and the secondary winding are protected with fuses.
Everything worked fine, until I connected the supply's ground with the earth wire (directly, no resistor, diode, cap,...nothing, just wire).

Now what' wrong? When the circuit was not connected to earth, the first value that would not always pop was somewhere above 5A, so I put in a 6.3A slo-blo rated fuse.

Now that the circuit' ground is connected to the earth-wire, the 6.3A fuse blows like nothing.

What happened? Is this normal?

peranders 16th September 2005 09:16 PM

How big is the transformer and where have you located the fuse?

Rodeodave 16th September 2005 09:38 PM

The transformer has three secondaries, but I'm only using one (the one providing the most A). I think it's located somewhere around 800VA, minimum, that thing sure is heavy. It's from an old Sony surround-amp.

One pair of fuses is between mains and transformer and the other pair is between transformer and rectifiers. And since I've connected ground to earth the second pair (6.3A slo-blo) keeps blowing...before it didn't.

The input is 230V AC (Europe...) and secondary is around 50V AC before rectification and smoothing. The inrush-current must be quite high since there are several big cans...but why does connectig gnd to earth make it higher? Less resitance?

peranders 17th September 2005 06:14 AM


Originally posted by Rodeodave
And since I've connected ground to earth the second pair (6.3A slo-blo) keeps blowing...before it didn't.
Do I understand you right:

3 secondaries

One of them connected to a rectifier bridge + caps?

The second is grounded, both wires?

The third is unconnected?

If yes, can you explain why you do this?

DaBit 17th September 2005 06:28 PM

You must have done something wrong, or the transformers' insulation is dead.

Try measuring the resistance between primary and secondary winding. It should be very high.

AndrewT 18th September 2005 08:53 AM

I'm confused just like you.
You should have ONE fuse on the mains input (live only).
You should have ONE fuse on each used secondary winding.
You should have 4 insulated and unconnected wires coming from the unused pair of secondary windings.
You will get about 73Vdc from the 50Vac winding when the PSU is unloaded. This will drop slightly when you load it up. You will not get 40Vdc from here unless you regulate it.

sndscientist 23rd September 2005 04:48 PM

just out of curiosity why would you ground the secondaryto equipment earth ground? that kinda eliminates the isolation properties of the transformer in keeping mains out of the secondary. old sony surround amp? what model? i have mountains of old sony stuff around here. it kinda sounds like this one transformer that had a set of isolated mains on the output side, the amp needed to see the primary voltage and the frequency to adjust itself to various operating conditions.

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