Soft/Slow start - high current - low voltage class A PS for a Pass F5 amp

[sekess]

Hi,
I am posting what I am possibly going to use as a soft start on a modified Nelson Pass F5 power supply.
I felt that I was beginning to hog up too much space at the F5 thread talking about sost starts. So, I am expalining what I am thinknig about using for this soft/slow start on this thread. Please post if you have any questions and/or suggestions.

My power supply is dual-mono (2 fully separate power supplies in the amp case). Each transformer is 600VA. There are two soft recovery rectifiers on each supply. There are four 68 000uF caps in each power supply (total of 272 000uF in each power supply) -- the first cap on each rail is followed by a choke which is followed by the second cap. For my line voltage I ran a 240Volt for just the amps. The amp rails are about +/-24volts and each amp pulls in the area of 2 amps continuous (class A). The bias is 1.3 amps actually. But I assume there is a little more drawn from the previous stage. So, I conservatiely am saying 2 amps. Plus, I could always bias a little higher.

So, we have relatively large transformer and a ton of capacitance. From snooping around and suggestions I received over at the F5 thread, I was thinking about using a one 50 ohm resistor feeding both transformers which would be bypassed very quickly (1/2 second or so). Then, I was palnning on using a 10 ohm thermistor on each secondary (4 of them - dual secondaries - 2 on each transformer). Then, I would bypass these guys after 5 or 10 seconds. I'm not sure exactly on the 5 or 10 seconds. My plan was to keep a volt meter on the rails at start-up and see how long it would take for the caps to reach full rail potential. Whatever that time turns out to be, that is the time I was going to set as the secondary bypass.

My hope for this scenario is that the primary resistor will reduce the primary current surge and that the thermistors on the secondaries would slowly charge the capacitor banks.

Instead of a resistor on the primary side, I might go with a 50 ohm thermistor -- not sure on this yet.

What do you guys think? Does this sound ok?

Thanks for looking,
Steve

[Zen Mod]

over engineering

Nelson is using one CL60 for 110V mains , for almost everything ;

two CL60 in series , cleverly arranged instead of wire bridges , for choosing 220V primary arrangement .

use them as you wish - same as Papa , or implement them instead of fixed resistors , combined with relay soft start circuit ;
in that case you can use even 3 CL60s in series .

[sekess]

Hey Zen Mod,
Yes I know it's way over engineered. 2 reasons --

1. I tried to accumumlate parts that I could get good deals on over time.

and

2. I think I may eventually try bridging 2 pairs of F5's (I have 16 ohm drivers). And I thought that I may run twice the normal number of amps on each supply.


My reason (concern) for considering not simply following Nelson's CL-60 recommendation is because I have so much capacitance as compared with the standard First Watt power supply. I felt that I may need to ease up to full power a little slower than normal.

Thanks,
Steve

[Zen Mod]

I wrote that Papa obviously use that scheme on many designs , including largest ones ;

at least that's what I can see on pics ;

anyway - that's tried by yours truly - with 800VA toroid and 300mF ; two CL60 in series , for 220Vac mains


no CLC , no CRC - pure C

[Magura]

I run a PSU about the same size as yours, on two CL-60 thermistors.
Works fine.

If you want to double the supply, just throw in a couple CL-60 on the secondaries.

Magura

[sekess]

Ok,
Thanks guys -- that is a lot simpler than what I was contemplating.

You know -- I really like Duncan's Power Supply designer II software. But it would be real nice if you could place resistors or thermistors on the primary and/or secondary side of the transformer to simuilate soft start conditions. It would be a nice way of graphically seeing how the various components respond to the soft start components. As it is now, I don't think you can add components in thos locations in his software. I would think that it wouldn't be too difficult of an program mod.

Thanks,
Steve

[Zen Mod]

PSUD have "soft start" function under "options"

[Zen Mod]

Originally Posted by Magura

... just throw in a couple CL-60 on the secondaries.

Magura

that way - you'll kill them fast

new circ - instead soft start you'll have hard stop

[sekess]

Hi Zen Mod,
I tried that -- but it only seems to show you the result of the circuit which has been soft started over some time. It does not tell you what components were used in soft starting. And more importantly, it does not let you design the soft start portion.

At least as far as I can determine.

Thanks,
Steve

[Zen Mod]

sorry - I didn't realize that you are thinking about customizing that