trouble getting high DC after rectification

[EETStudent]

Im in the preliminary stages of wiring a power supply and I keep blowing the fuse. I got my two high voltage legs(390-0-390) going to 4x1N5408 diodes, sets of two in series, noding with the positive of the first filter stage cap. I used what Fender did by stacking two caps(2x80uF) in series paralled by resistors(2x220K/1W). This is all i have done thus far. At first i wasnt getting the 450+VDC that i was expecting, but then i think i grounded the center tap and i got about
-350VDC. Am i forgetting to do something? Then I started blowing the 2A fuse, but im not too concerned with that, probably just overload from the larger diodes. I just want to take it one step at a time, and right now i just want to get high DC that should feed into the standby switch

[Enzo]

The diodes won't overload anything unless they are faulty or wired wrong. All a diode is is a one-way wire. You wouldn't expect using heavier speaker wires would overload an amplifier would you?

From 390VAC I'd expect more like 550 volts.

I would recheck the wiring and especially the caps in series. NOTHING ELSE should connect to the point the two caps meet, with the exception of the bleeder resistors.

[EETStudent]

Now Im getting something else. I just powered up and it started humming and audibly producing a sound like a motor, and also made the lights in my room dim. I have jumper wires connecting the power cord to the transformer primaries, and one of those jumpers blew. I brought the circuit down to just one 22uF cap after the diodes thinking the transformer couldnt handle the capacitance and i got the same thing. Apparently something I have in there isnt capable of being in there. Not sure what though

[stokes]

Disconnect the cap,if it still blows the fuse,disconnect the rectifiers if it still blows it is the tranny,if not it was the diodes.Does the PT have heater taps (5v or 6.3)?Are they touching something?

[EETStudent]

I think I sourced the problem. Two diodes that were in series together were goners i believe. Compared to others that measured .5V/550ohms, these two measured .024V/20ohms. pretty thats a dead short. Dont know how this happened though. Its only those two. What I would suspect that happened is that I had too big of capacitors after the diode, resulting in high pulse current thus damaging the diodes. you suppose thats what happened? Im waiting on the transformer I plan on using to arrive, and its much beefier. I was just experimenting on this smaller one and i guess i caused some damage. well, ya live ya learn

And yes there are 6.3 and 5V filaments, but they were just hangin free touching nothing.

[stokes]

You only had 40uf total,that wouldnt cause the diodes to fail,most likely just a couple of bad diodes.Solid state rectifiers can take a lot more capacitance than tube rectifiers and any tube rect. will handle 40uf no problem.

[Enzo]

Transformers have no trouble with the capacitance, it is the rectifiers we worry about. And even with rectifiers, it is the tube ones that have the more severe limits on cap size. Your diodes were never threatened by this circuit. At least not if the circuit was correctly connected.

As Stokes said, the caps you had were no threat to anything.

Your diodes shorted,and if I had to make a guess I'd say they were not bad out of the box, but more likely you inadvertently shorted the output to ground at some point and that killed them.

I would also recommend that you replace all four of them now. The two that shorted for all purposes become essentially a wire. That means the other diodes were now connected essentially across the winding. That will seriously stress those diodes. FOr the cheap price of a couple diodes, I'd replace them, even though they test OK. You cannot test for a weakened diode - not with a hand meter anyway. And I doubt you have a curve tracer.


Additionally, your 1N5408 has a surge rating of 200 amps. That means it can conduct 200 amps for very brief intervals - such as when the caps first charge up - without harm. When your caps are empty of charge, when you first apply power they appear as a short circuit to the rectifier for a very brief instant. In other words they will take al the current they can find for a millisecond or two. Ideally you should add a series resistance between the rectifier and the filter that will limit the surge current to 200 amps. In this case that would be something like 2 or 3 ohms.

But in a practical application, common sense takes over. One factor, the winding has resistance, and likely more than 2 or 3 ohms, so that limits current here. ANd just in general, I really don't think that transformer winding can even provide a 200 amp pulse. So it is a non-issue.

But I do think the length of time you had the circuit energized with the shorted diodes was way longer than any surge rating duration, adn while the part may not have seen 200 amps or anything close, I bet they did see in excess of their 3 amp rating. So that is why I'd replace them.

[stokes]

I had a bunch of those 3amp diodes I got from a flea market some years ago.I used them in circuits that would normally call for the standard 4007's and half of them either shorted out immediately,or lived a very short life for no good reason,just a bad batch I figure.I ended up throwing away about 50 unused pcs.Did you go thru my trash?

[Enzo]

You are right. I normally would not fear for the diodes, but a bad batch of them is entirely possible. Now that you mention it, I have such a bag myself. 3A diodes that don't hold up to the voltage they are labelled for. SUpposed to be 1000 volt items, I retired the stock to low voltage applications where they work fine.