Inrush protection failures.

[Peter Daniel]

During last 6-8 years that I've been using A75 and Zen amps, I had quite a few rectifier bridges failures. It always happened at start up, when one bridge just blew, tripping fuses. I have four A75 and 2 Zens and it happened on all those amps. The fact is, I was using CL-40 as inrush protection (as it was recommended in original article) but my capacitor banks were not really big (80,000u on average). I was also using surplus bridges, however, they were rated 35A.

So now when I'm using that elaborate discreet bridge on my upcoming Aleph X I would like to avoid any future failures and make sure my inrush protection works fine. The total capacitance per channel would be 130,000u. Is CL-60 enough protection? I was experimenting yesterday with resistors, but it looks like with class A amps fixed resistors don't work well, because of a voltage drop. For instance on my A75, 70ohm resistor generated 70V drop and the amp was only receiving 40V AC. So it looks like thermistors are the way to go. I was considering putting two CL-60 in series just to be safer.. Is it worthwile?

This are the bridges I was using so far.

[nar]

I remember a Nelson drawing of an Aleph 4 on the Passlabs and he used 2 CL 60 in parallel. So ? Magic ? Or mistake ?

[Eric]

Peter: Sorry I don't have any advice for your problem, but I do have a question. Where exactly were your fuses located that blew when the rectifier would fail?? I'm in the process of building a power supply and want to make sure it is protected from blown bridges...

Thanks,
Eric

[PMA]

Peter,
I am using 3 Ohm/20W resistor in series with primary winding of 500VA toroidal transformer.
Pavel

[marijan]

When I tryed to solve the problem with my death NTC I read in one of the thermistor manufactor`s manual (aplication notes), that there is no benefit of useing two paralell conected thermistors on inrush curent protection circuit, since one device will tend to take all the curent flow through the AC or DC rail. Anyway there was a posibility of useing one on AC primary rail and one betwen the bridge and filter caps. Maybe Nelsson can explain what is the purpouse of using two paralel NTC`s in A4. There is only one in A2 for example.

marijan

[Jason Hubbard]

XA-200 uses 4 thermistors in parallel as R element in CRCRC filters.

NP advised that current sharing was not an issue. This must be fairly well proven else Passlabs wouldn't intend to sell these amplifiers to the general public.

[Nelson Pass]

On the Aleph 1.2 and products that draw more than
about 2 amps, we put Thermistors on each of the
two primary windings, so they aren't literally paralleled.

I use a lot of the 35 amp bridges, and I never see failures,
even with 200,000 uF. Perhaps there is simply something
defective with your surplus bridges.

[Peter Daniel]

Originally posted by Nelson Pass
Perhaps there is simply something
defective with your surplus bridges.

I guess that must be the only explanation then. Because I had different bridges on my third A75 pair, and indeed no failures there (less caps too, thou).

Eric, I had it mounted on amps heatsinks.

[HDTVman]

Hi Peter

You can use fixed value resistors for soft turn-on with a class A amp. Just try a lower value. I pick a value that gives me about 80 to 95 volts AC after 10 to 20 seconds on time. If you have a current meter and a variac Ohms law will give you the answer, other wise just try a few different values.

Later
Bruce

[Eric]

Originally posted by Peter Daniel

Eric, I had it mounted on amps heatsinks.

Peter, I think you took me too literally: I was interested in knowing where in the circuit did you place the fuses, not where physically.

Some people put the fuse on the primary, others on the secondary, others between the bridge and the caps. Each has an advantage or disadvantage. Just wondering where in the circuit you placed yours...