Unregulated PSU Questions

[Redshift187]

I have two questions pertaining to the unregulated power supply I'm using with my chipamps.

First a description. It's a 2x 18V secondaries transformer, giving between 25V and 27V DC after the capacitors. I've got a dual bridge snubberized setup with 1x 15,000uF cap, 1x 0.1uF ceramic cap and 1x 0.1uF polypropylene in series with a 1R resistor (per rail).

Question 1: I've got about 0.2V of ripple on the rails. What is a "normal" amount of ripple, and what is a "good" amount of ripple for an unregulated supply? In other words, should I be able to do better easily?

Question 2: Lots of people talk about fusing between the power supply and the amp. But if one rail blows a fuse and the other doesn't, full rail voltage could end up on the output of the amp. What about a dual circuit breaker that will cut off both rails when one exceeds the current rating? I haven't seen this mentioned anywhere here, but it seems like a good idea. I would assume the reason they aren't used is due to the cost.

[EBM_dude]

I know this is dumb, but what will happen with one fuse in the GND rail?

[Bakmeel]

Hi Redshift,

Answer #1: (my t)
200mV ripple on a 27V rails is less than 1% steady-state ripple. I'd say that is a good spec. If it gets above 5%, you'd get in the "reasonable" range for less critical apps.
below 0.5% is in the range for regulated supplies.

Answer #2:
Circuit breakers are usually designed for AC aplications. They still work in DC, but be sure to derate them accordingly. If applicable, the manufacturer will specify the DC voltage/current range. Reason they are not commonly used in audio is due to their size (DIN-rail equipment) and, indeed, cost. Also note that they are not as fast as a fast-blow fuse. Maybe that is another killer for brakers in amps.

@EBM-dude,
First of all it wouldn't catch a short between positive and negative rails, and secondly because it would actually just open the ground path but still leaving the amp running. You'd get a big DC on the output. Fusing the ground is never a good idea, unless you are very sure what you are doing. Otherwise, fuse the supply(s).

[Steerpike]

One approach is to use two SCRs to crowbar the power supply (i.e., put a short across both the power supply rails and deliberately blow both fuses) in the event of any DC appearing on the output.

[EBM_dude]

Thanks Bakmeel!

[Redshift187]

Yes, thanks everyone! The dual circuit breaker I had looked at was rated for the appropriate DC voltage. It was a thermal design, so I would probably choose an even lower current rating, depending on the current-time graphs in the datasheet. An example of what I was looking at is here: http://search.digikey.com/scripts/Dk...&name=PB257-ND .

As I have a DC detector/protector on my amp output that disconnects the speakers via relays if more than about 0.6V appears on the output, would it be safe to just use fuses on the secondaries? As in, no other stuff will explode if the amp sees only one rail?