Two power supplies blow fuse

[h_andree]

I'm building a rather complex plate amp.
I have two channels of LM3875 on a plate amp.
I also have two electronic cross-overs.
I made 2 power supplies: One for the plate
amp and one for the cross-overs. I tested them
separate and they work fine. When I connect the
2 power supplies both to the toroid (250 VA)
tansformer I blow the AC fuse (4 amps slow-blow).
I think the fuse should be high enough for this
project.

Is there a special wiring diagram for 2 power supplies?
Both power supplies have 2 rectifier bridges.

Help appreciated.

Thanks Harry

[Nuuk]

Without seeing your wiring diagram it is difficult to guess where the fault may lie.

[h_andree]

Here is the wiring diagram. I left out the details
of the chip amp and the connections between
the cross-over and the chip amp since even if
they are not connected, the fuse blows.

I also tried putting in a CL2 thermistor and
a 2.5 A fuse. Even with no load on the amps
the fuse blows.

Harry

[Nuuk]

Why not just take your Xover supply after the rectifier stage on the amp supply?

I personally would prefer a separate transformer/PSU for the crossover supply but if I was using just one transformer that's how I would do things.

[h_andree]

That's what I orginally had. I got some hum and thought
I would be able to solve it by adding a rectifier.

I guess I can get rid of it and try it again.

Harry

[Upupa Epops]

Which types of rectifiers are here used ? Which value have PS caps ?

[sek]

even if
they are not connected, the fuse blows

Might it be possible that you just grabbed the wrong secondaries for the second pair of rectifiers (and thus cross-connect the secondaries by connecting the two circuits)?

This would explain why they work separately. When you connect the two circuits, what exactly happens? Does it really 'blow slow' (just slightly over 4A), does it blow instantly (way over 4A) or is there even arcing anywhere (e.g. in the switch)?

Sebastian.

[h_andree]

I'm sure I did not connect the secondaries to eachother.
I'm using about 6800 uF per rail for the amps and
5800 on the cross-over.

I cannot tell how it blows. I turn on the switch and
almost immediately the fuse blows. The rectifiers
are 400 V 25 Amps on the amps
http://www.partsexpress.com/pe/showd...number=050-060
and 25 A 50 V on the
cross-over (I ran out of the higher voltage ones) from Radioshack.

Harry

[Upupa Epops]

50 V retifier on crossover is not enough - try there typ with higher voltage.

[h_andree]

Hello all,

I continued testing. One test I did not do yet is connect
the amplifier rectifiers directly to the transformer. This
also blows the fuse. So it's not the fact that I have
2 power supplies connected. When I connect a second
power supply to the +, gnd and - the amps do not blow a fuse
and seem to work fine. I must be doing something simple
wrong but I don't see it.

As for the secondaries. I'm using the Avel transformers
and have connected yellow and Orange to one bridge
and Black to Red on the other. I'm getting 20.6 V AC over both
secondaries (Without rectifier connected). The fuse I'm blowing
now is a 4Amp slow blow. I think it takes about 1- 2 seconds.

Thanks

Harry