Power Supply blowing fuses

[cm961]

Hi,

I've got a nice big power supply that I'm trying to use for an amp. Its got about a 1000VA transformer, with two 10,000uF caps. Anyways, when I leave the bridge rectifier open (don't connect to the caps) it doesn't blow the fuse. But if I connect to even one capacitor, the main line fuse blows.

Is there a large startup surge current in an unregulated design, large enough to blow a fuse? (like 3A)

Or is one of the capacitors shorted? They seem to charge/discharge fine I think.

Thanks guys,
Pete

[halojoy]

Even though there is a momentary very high current
it should not blow the fuses.
Not with 10.000uF.

But the fuses maybe are FAST.
In power supplies, for protecting the PSU,
there should be some SLOWer fuses.

All I can think about for now.

/halo

[AudioFreak]

Yes there is a massive current surge on start up when using a toroidal transformer of that size connected to a bank of capacitors. Best solution is to use a slow blow fuse and a thermistor in series on the primary winding or instead of the thermistor, you can use a relay and a power resistor, it's up to you. This will keep the surge current lower and minimize the number of fuses you blow.

[cm961]

Thanks guys,

I should have mentioned, the amp is rated for a 10A fuse, and I was using 2.5s and 3s. So maybe even if they are slow blow, they can't handle the power on surge. I'm going to try a 7.5A fuse, that surely must be able to handle the surge.

And just to get my theory right here...

Is there a surge because the caps aren't charge and are thereflow low impedance paths, with increasing impedance as they charge?

Pete

[cm961]

Thanks guys,

I've fixed the problem. It was just too small a fuse for the surge current. I have a 7.5A in there now. When I get everything working I'll replace it with the 10A. For curiosity's sake, its a Carvin DCA800 that I'm rebuilding. The amp channel boards are pretty bad, but I've been repairing them and replacing parts. When I get this thing going its going to be great because it only cost me $100 Canadian on ebay. The transformer is probably worth that alone (well maybe new). I know one channel has the +70 VDC shorted to the speaker output... not a good thing!

Thanks for all the help, diyaudio has helped me so much, and taught me so much.

Pete

[halojoy]

I take for given, that you have checked out
so there is no shortcut anywhere.
That thwe wiring is okay.
And that there is no Shortcut in the CAPS?
If they have resistance 0 Ohm, it is not very good.

And about the Caps you are right.
They need to be loaded up to full voltage.
and especially low impedance caps
have very high currents in a short time.

But anyhow, I find it a bit strange that slow fuses
should blow...
But it is a BIG TRAFO, you've got there.
how high voltage?

/halo

[dhaen]

Halojoy,

Torroidal transformers are a "special case" when it comes to surge current.
I can't remember the exact reason, but it is to do with momentary core saturation, and the core material. The surge current is considerably greater than with other types of transformers.

Cheers,

[cm961]

I guess it was the surge current as I'm using a large fuse now and its ok. It must be a pretty high surge though to blow a slow blow fuse of 3A. Then again, the lights do dim...

The transformer has 70 + 70 secondaries, and must be rated for pretty high current since the amp claims 800W RMS when mono bridged. Its a class B amp. Let me check the topogy on it.

G. Randy Slone calls it The Quasi-Complementary Output State. It uses all NPN output transistors, which is pretty cool because the MJ15003s are $2.96 at Allied electronics. I paid about $15 each for 2SK1058, although I think I saw them for $7 or $8.

The next thing I need to build is a nice hi-fi preamp... It never ends.