22000uF filter caps. Fuse blow question

I replaced some 15000uf filter caps in my mixer's power supply with 22000uF. The rest of the power supply was recapped and working fine prior to this. So it was the only operation I did before I loaded it down with one channel and two buss channels and powered up. I get a blown main fuse .

I know the extra inrush current can cause fuses to blow, but question is how long does this take? Like 2 or 3 seconds? Or say 30 seconds? Because I powered it on, and it took about 30 seconds to get over to the board, arm input channel and buss and only when I armed the channels and pulled up the faders did the fuse blow. The seeming simultaneousnous of arming the channels (i.e. completing the circuit that powers the channel arm bulb) makes me wonder if I didn't pull a wire or something in the meter lights circuit. I wouldn't think that would cause the power supply mains fuse to blow though. Maybe wrong.

But anyhow, main question is, how long would it generally take to blow a fuse if my filter caps are too large? Upping from 15000 to 22000 I thought would be okay. It's a little less than 50% larger and convenient in that they have the same diameter.

Are you using anti surge fuses ?   

Is the transformer toroidal ?  They draw a lot more current on power up anyway.

Look into adding an inrush current limiter. 

Back in the 70's when I built my big dog DITY amplifier I repurposed an old tube PS power switch that had a two stage on capability. I connected the power transformer through a resistor to charge the caps before hard connecting it. I also powered the speaker relay from the second on position so speakers didn't click/pop.

JR

would using low esr caps in any way cause issues in a power supply under any circumstances?

scott2000 said:

would using low esr caps in any way cause issues in a power supply under any circumstances?

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Low esr caps will make inrush current worse.

JR

So, the inrush current limiters are in series between the big caps' (+ lead) and the board?

Schematic attached of power supply.

By the way I was using the original (main) fuse @ 4A/250V. So, not sure if it was slow blow or not. But, at around 30 seconds, one might assume it was ? ? ?

JW said:

So, the inrush current limiters are in series between the big caps' (+ lead) and the board?

Schematic attached of power supply.

Click to expand...

All the inrush limiters I've seen were in series with the line cord.  Even transformers can cause current spikes depending on the phase of the initial mains power application. (I've also seen thyristor power switchers so transformers turn on at zero crossings).

You should be able to find app notes around the WWW.

JR

I have seen psu's do this  before but it usually was fault.  Last time I saw this it was a failing bridge rectifier.

pucho812 said:

I have seen psu's do this  before but it usually was fault.  Last time I saw this it was a failing bridge rectifier.

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A shorted rectifier diode will draw huge current trying to drive full AC voltage into a large reservoir capacitor.

OP noted that he increased the size of his reservoir caps which will draw more current at turn on.

JR

JW said:

I know the extra inrush current can cause fuses to blow, but question is how long does this take? Like 2 or 3 seconds?

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Just a few cycles of mains, so maybe 100ms tops. You must look elsewhere for abnormal steady-state current draw.

Okay,

Found fault number 1 and it is so embarrassingly dumb I can't mention what it is. Okay, I will. I flipped one of the big filter caps around. Doh.

Corrected that and p.s. is functioning fine except for phantom circuit  is blowing fuses. (unloaded so not a short in the board)

Thought I'd post close up of 48V schematic and ask what component you guys would be looking at first.

Tr6; check orientation

JW said:

Okay,

Found fault number 1 and it is so embarrassingly dumb I can't mention what it is. Okay, I will. I flipped one of the big filter caps around. Doh.

Corrected that and p.s. is functioning fine except for phantom circuit  is blowing fuses. (unloaded so not a short in the board)

Thought I'd post close up of 48V schematic and ask what component you guys would be looking at first.

Click to expand...

Thank you for sharing, while embarrassing it can help other people learn from your mistake.

JR

Checked orientation of T6,7, and 8. Actually replaced T6 and T8 because I had them on hand from other Yamaha gear.

Hard to test voltages when the fuse keeps blowing. The transformer AC windings test fine.

Hmmm.

Surely check the bridge and snubber caps first ...?

MagnetoSound said:

Surely check the bridge and snubber caps first ...?

Click to expand...

Indeed!

JW said:

Hard to test voltages when the fuse keeps blowing.

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Replace fuse with resistor; I would start with about 100 ohms 1-10W or whatever approaching. You may have time to check some voltages before the resistor gives smoke.

Diodes will often fail as a short circuit from over heat...

JR

Hi JW & All
I tell everyone you should be a BOY SCOUT and be prepared.
I recommend that if you plan to swap out capacitors (or?) and the system is working measure the input power for reference. Then make a sketch of the orientation of the parts before removing them.
For the power measurements buy a Kill O Watt meter, typ $20 @ Harbor Freight or similar.  The meter is then used before and after the modifications. Everything I test, debug or repair is powered on through a Wattmeter.
Good hunting Duke