Soft start circuit

[Diode]

Hi all,

I'm sure that this topic has been well covered but to save time in search, I have seen soft start circuits that use one or two MOVs that later get shorted by a relay for full power. I'm looking for a simple circuit that is reliable and places to get parts.
My circuit will be powering an old Peavey transformer from a PV .75K amp that produces 2X +/- 53VAC, current unknown but it powers 2X350WRMS power amps. I will be rectifying both sets of windings and then parallelling them for a 500W amp I'm building. I will have 40,000uF of filtering per rail so I need a good circuit to soft start with.
Can someone please help?

Thanks in advance,
Chris

[EchoWars]

Why an MOV? If you are going to short across it, then a decent power resistor will be fine, and IMHO, probably more reliable.

I have built this circuit and added it to several amps. The resistor is an Ohmite 89-Type, aluminum axial, from Digikey.. Very small, very tough.

I show a DPDT switch, with a resistor to discharge the caps. Not necessary in all apps. I also show the relay coil being powered from both rails. Not very efficient with high rail voltages, but keeps current draw even....if these are high-power amps, a 110VDC relay might be a decent idea...less wasted power.

[EchoWars]

BTW, I lucked out finding a couple of good relays at a local surplus shop. I need more, and am SOL so far. If you find a good source of (preferrably) 48VDC relays, I'd like to hear about it.

[peranders]

The solution will work but have two disadvantages:

1: Slow reset. Sometimes you get a rather short dip in mains volatge, this can create a heavy current spike (if you use a toroid) even if the smoothing caps are charged.

2: Not allowed in Europe due to safety reasons if you mix mains circuit and secondary side in the same relay. If you choose this solution you must use a relay with increased isolation, for instance ELESTA SIRxxx types

[EchoWars]

1: Slow reset. Sometimes you get a rather short dip in mains volatge, this can create a heavy current spike (if you use a toroid) even if the smoothing caps are charged.

Huh? And the spike is smaller with no soft-start?? This sentence makes no sense...perhaps you would care to elaborate...

2: Not allowed in Europe due to safety reasons if you mix mains circuit and secondary side in the same relay. If you choose this solution you must use a relay with increased isolation, for instance ELESTA SIRxxx types

He's in Oklahoma. We got half the line voltage you all do.

This is a very commonly used circuit...stolen from an 'audiophile' amp. No magic here. It would be nice to find a decent relay, and a high isolation wouldn't hurt, but most of the HI relays are excessively large. If you got the room, then sure... But for me, at this point anyway, any suitable high-current relay of 48V or better seems to be damn near impossible to find.

[matth]

What about this cct. I have made a couple of these and they work great.
http://mitglied.lycos.de/Promitheus/...or_toroids.htm

[Helix]

I have built several of these Elektor in-rush limiters, from 300 to 750 watt trannies with large cap. banks, no problems so far (3 years)

[HighTec]

Hi.

Now it's my turn.This is soft start unit I used in projects with Holton's AV400 modules.The schematic is stolen from Elliott's pages,but I've added some things from a professional unit.
As you see there are two relays.Relay 1 shorts the parallel resistors and he acts in approx. 1,5 sec delay.The relay two acts immediately.In fact he is the main switch and you don't need to use those 10-15A switches.The relays are 12V type.
F2 is a thermo fuse.He blows if Rel 1 fails.The fuse is mounted between the R8 & R9.There must be a contact with them.The best way is to stick them together with a plastic strap.
Q2 is BUZ71.A much less powerfull transistor can do the job very well.I've used this mosfet because I have a whole bunch of them.
Bellow is the schematic and the PCB(in case you decide to build it)
Regards.

[HighTec]

And the PCB.

Bottom layer:

[HighTec]

Top silkscreen: