simple soft start

[slr 5000]

hi all im building a daul mono 1000va per side amp and think i may have to use a soft start .id prefer not to but ive heard it will be stressfull for my amp on powerup if i dont .does anyone here know a simple soft start cirtcuit i can make any help would be much appreciated

[rjb]

Simplest soft start is a resistor in series with the power lead to your transformer power supply, limiting the initial chargeup current. This resistor is than bypassed by a relay after a short time delay.

[Stuart Easson]

...the absolute simplest is to wire something called an NTC thermistor in series with the primary...

You definitely want one, fuses or breakers popping all the time takes the 'shine' off the new uber amp pretty quick... ;-)

An NTC thermistor has a higher resistance cold (about 10ohms), than hot. Once the thing hits about 100c it has a resistance of less than an ohm and interferes little if at all...

Nelson pass uses them a lot and I think he knows what he's up to. The ones most people seem to use are called CL60's, and are one convenient size from a broad selection...

I don't remember the rating, but there are others in the range that have lower cold resistance and higher max current, a couple of these in series would have an appropriate hot and cold rating for a 1kva transformer.

I'm not sure where you'd get them down under, but here digikey has them for a couple of bucks, and eBay sometimes has them for a tenth of that...

HTH

Stuart

[slr 5000]

hi all ive looked on rs components and they have heaps of thermisters but i really need to know the exact values if possible .i was also thinking about using a variable resister that i could set to say 10 ohms on powerup and after about 30seconds turn it to 0 ohms .do you guys have any thoughts on this idea it would seem to be a very simple way of doing it . cheers

[bigpanda]

Take a look at Quasi's soft-starter :

http://www.adam.com.au/cgpap/QuasiWeb/index.htm

You think this is simple enough ? I do.

[Nelson Pass]

Class A and very high bias amplifiers work pretty well with the
big Keystone inrush thermistors found in the Digikey catalog, like the
CL60 and CL40 - just pick them by current rating. If your transformer
has two primaries, you can use one thermistor per primary. They can
also be used on secondary windings.

Doing this also has the advantage of stretching out the charge
pulse duration, lowering the noise a bit and the stress on diodes
and capacitors.

If your amp is 1 KVA per channel and Class AB, this is not quite as
good a solution as the small resistance of the hot thermistor still
contributes some limiting of the AC line under high current draw
conditions. For such cases, bypassing them with a relay is a good
alternative.

[Stuart Easson]

Here's the page from the digikey catalog. Check out the column labelled "inrush current limiters". I, and many others, have used KC006L-ND, which is the same part as some vendors call a CL60.

http://dkc3.digikey.com/PDF/T082/P1891.pdf

IIRC you guys have 240v mains, so using one cl60 at 5A max. would 'limit' you to only 1200va continuous from each transformer...

There are a couple of cons to go with the obvious pros...

1) they are designed to run hot, really hot. Don't want them near anything else that might 'care'

2) If you are in the habit of turning your amp on, then off, then on again quickly you will get a surge, basically because the cl60 will not have had a chance to cool down...

HTH

Stuart

[peranders]

Originally posted by slr 5000
hi all im building a daul mono 1000va per side amp and think i may have to use a soft start .id prefer not to but ive heard it will be stressfull for my amp on powerup if i dont .does anyone here know a simple soft start cirtcuit i can make any help would be much appreciated

The biggest problem is probably the mains fuses which may blow now and then. The stress is little for the amp really as long as you have done the design right.

[slr 5000]

thanks for the answers guys its aussie amplifiers latest nxs400 modules and his ps3 power supplys i know my mains fuse can take it as i turn on a 3000 watt electric heater all the time with no probs on the same circuit so i presume the daul 1000va toroids would be ok as well as long as i dont turn the heater on when im using the amp .they are on a 16 amp circuit . i was just worried it may damage the amp .if it wont damage the amp id prefer to not use a softstart

[taj]

>>i know my mains fuse can take it

I think he was referring to the mains fuse inside the amplifier on the primary of the transformer. It's common to pop that fuse regularly due to the copious current that gets sucked through it at start-up.

From what I've read, it's more of an issue with toroidal transformers than EI types.

..Todd