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Paul Ebert 31st May 2008 02:54 AM

Pearl PS blowing fuses
I have a Pearl that I'm assembling and I'm running into what is probably a common noob problem. I have the primary wires connected (directly) to the IEC power module on my PS enclosure with the secondary wires not connected to anything. About every second or third time I switch the power switch on it blows the 1A fast acting fuse.

When it does not blow the fuse, the secondary voltages seem OK (about 34 volts).

I realized that my transformer puts out 100 VA. The one in the Pearl article puts out 30 VA. Is this my problem? If so, what should / can I do about it?


wirewiggler 31st May 2008 03:43 AM

If you are using a toroid transformer, I have had that problem with inrush current when caps are charged up. You could put a bleeder resister across caps to discharge them, good safety precaution with caps anyway.


Paul Ebert 31st May 2008 03:47 AM

It is a toroid, but there are no caps connected to it, at this point. Is there a way to control inrush on the primary side?

Nelson Pass 31st May 2008 04:30 AM

Likely the polarity of one of your primary windings is reversed. That
will do it every time.

wirewiggler 31st May 2008 04:35 AM

I would not sweat it, you may want to use a slow blow fuse anyway. My transformer is about the same size as yours (overkill but the only one from AnTek had at that voltage). When I turn off my power supply the power Led will stay lit for a minute, I don't flick the power off and on right away cause I know it will blow the fuse ( to lazy to put a bleeder resister in).

wirewiggler 31st May 2008 04:37 AM

Thank Mr P. I have not had run into yet , good to know.


Paul Ebert 31st May 2008 01:03 PM

Thank you, Nelson. I would never have thought about that possible cause. I'll give that a try.

Bill, it sounds like I will want to put in some bleeders. Where should I put them and what size (ohms and wattage) would be good?


AndrewT 31st May 2008 02:32 PM

adding bleeder resistors will not help the fuse blowing problem.

The usual fuse rating for a motor or transformer is approximately three times the rated maximum supply current.
For a 100VA transformer this is 3*100VA/110Vac~=T3A.
You may get by with less than this.

If you want to close rate the fuse then consider adding a soft start using either a Power resistor or Power thermistor. The resistor must have a delayed bypass and the thermistor version all benefits from the added bypass.

Paul Ebert 31st May 2008 06:08 PM

OK, so I may have a primary winding which is out of polarity and I should have bought a smaller transformer. I may want some thermistors (need to read up on the F1 service manual...) and some bleeders.

To start, and until I can get parts in, could I simply use only one of the primary windings? Wouldn't that cut my current in half and eliminate the polarity mismatch?

And here I thought the PS would be the easy part to debug! :bigeyes:

AndrewT 31st May 2008 06:45 PM

if a dual primary is wired in parallel for 110/120Vac and one of the windings is out of phase with the other then the transformer acts as resistance of just a few ohms across the mains voltage. Bang goes the fuse every time (just as Nelson said).

If it only blows occasionally then the windings must be correctly wired, but the start up current is too high for the size of fuse.
Either fit a bigger fuse and take the risk that goes with that or reduce the start up current using a soft start circuit.

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