Cheap and Easy Soft Start?

[samsagaz]

Hi guys, after search the forum i really cant find an cheap soft start to build. Im building an 500Wx2 Power Amplifier, i really want to use an soft start to charge the capacitors.

Can someone please point me to some simple design of that? some IC that help me to that?

Thank in advance

[BobEllis]

Use the advanced search function and search on titles only. There are lots of threads on soft starts. diyAudio - Search Results

The easiest one of all is a CL-60 in series with the transformer primaries. A couple of bucks and you're done.

[samsagaz]

thanks! will search about the CL60!

[BobEllis]

CL-60 GE Sensing / Thermometrics Thermistors - NTC

[nigelwright7557]

Originally Posted by samsagaz

Hi guys, after search the forum i really cant find an cheap soft start to build. Im building an 500Wx2 Power Amplifier, i really want to use an soft start to charge the capacitors.

Can someone please point me to some simple design of that? some IC that help me to that?

Thank in advance

The simplest and cheapest is a switch and a power resistor.
Once powered up the switch shorts out the resistor.

Otherwise it will be relays and delay timer.

Personally I try to avoid them as I have seen so many threads where the soft start has caused a problem.

I run a 1000VA transformer with no soft start just a 13 ampo mains fuse.

[BobEllis]

Sam,

For the sake of safety, I strongly suggest not just upping the fuse until it stops blowing on turn on. Nigel's fuse allows an extreme overload of the transformer (2800+VA) without blowing the fuse. That's fine for Nigel as long as he is comfortable with it, but I don't believe that it is good practice.

A CL-60 will limit the inrush, heat up and be almost transparent. It will not protect against inrush if the amp is cycled off and back on before it has a chance to cool. To me that is a small price to pay for no moving parts reliability.

Some would suggest an improvement in sound quality using a switch or relay to bypass the CL-60 after start and allow it to cool and be ready to protect a rapid cycle. My amps that use them do not have this feature and I haven't felt a need to investigate whether that would improve the sound quality. I use a CL-30 and a 6A slow blow fuse on my amps with 1KVA transformers

[Wavebourn]

I use 20 Ohm/12W high temp resistor and a relay powered from DC output. As soon as all voltages are proportionally up the relay shorts the resistor up.
It will protect from inrush when cycled on/off: if relay is still on that means capacitors are charged, so no inrush current. If they are discharged, the relay is off. However, on/off voltages for relay are different, but it is not a big deal, according to my experience.

[ostripper]

I use one of these ...

15A NTC thermistor , as current passes through resistance goes down to 2-3R. Circuit below, ( magnetron relay - 20A and 12.8V trafo from broken micro oven) triggers relay across the thermistor after 3 seconds.

All junk parts , has lasted for 3 months now to keep inrush from 1KVA trafo at bay.

OS

[djk]

A CL-60 is not the best choice without a relay bypass.

CL-60
Manufacturer: GE Sensing / Thermometrics
Description: Thermistors - NTC 5 Amps 10 Ohm

The resistance is too high, and the current rating is not high enough. I have had a CL-60 blow to pieces from the inrush current of a Hafler DH500.

For a 500W x2 amplifier I would choose a device between 1R0~2R5 with a current rating of 15A~20A.

Digi-Key - 570-1041-ND (Manufacturer - SL22 1R020)

Digi-Key - 570-1047-ND (Manufacturer - SL22 2R515)

Digi-Key - 570-1044-ND (Manufacturer - SL22 2R018)

[samsagaz]

wow, thanks too much guys! a lot of info to understand and learn from