Inrush Current killing fuses

Wynand · 14th February 2006, 07:49 PM

I'm building a LM4870 audiosector amp.
I've got the amplifier itself up and running with a temporary power supply. My problem is that the power supply I've got consist of a 500VA transformer (dual 20v secondaries) rectified seperately. With two 10000uF caps on each rail...

The inrush current is killing me.

I've got an Idea on limiting the current with a RC time delay activating relays to bypass current limiting resistors.

I'm just wondering if anyone knows of a similar or better design to look at/compare.

I must go now unfortunately but will check back tomorrow to see if anyone could help me.

Thank you to anyone who could.

PS. I have been searching for something but could not find the right circuit.

Craig405 · 14th February 2006, 08:01 PM

Hi,

I had a similar problem with a 1KVa transformer.

Relays will work switching in resistors OR..

you can use 'inrush limiters' which are beefy NTC thermistors, when they heat up they lower in resistance. Therefore your transformer can only draw full current once theyre hot.

They are cheap and really simple. Make sure they can handle your current requirements (you can parallel them up) and put them in a sensible place (they do heat up a fair bit).

I know RS have them under 'inrush limiters' because thats where i got mine.

Craig

bikehorn · 14th February 2006, 08:02 PM

soft start circuit?

http://sound.westhost.com/project39.htm

Nordic · 14th February 2006, 09:11 PM

Wynand, you are using T-fuses right?

quasi · 15th February 2006, 04:58 AM

I developed a soft start circuit for an amp I was working on.

The schematic is here; http://www.diyaudio.com/forums/showt...687#post784687 - post 224.

The PCB layout is here; http://www.diyaudio.com/forums/showt...781#post783781 - posts 217 & 218.

I like relay contacts because they offer the ultimate in on resistance.

Use it if you want to.

Cheers

Wynand · 15th February 2006, 04:59 AM

I'm using 20mm fuses, slo-blow on mains input

I've seen the circuit on ESP but want something simple.

And I don't know which NTCs to choose, even after looking at some datasheets.

Wynand · 15th February 2006, 05:16 AM

Aswell. Ntcs work by warming up. If you put the amp off for a couple of seconds, the caps discharge, but their still hot and thus not going to stop the inrush.

Relays will work everytime.

If nobody has one I'll work on my design and try to post it.

peranders · 15th February 2006, 06:30 AM

Here we have a typical trade off:

1 Inrush current
2 We must have fuses and they should also protect, meaning they should be chosen after the load.

The question is how you solve it.

There are a couple of threads, one of them is here:
Toroid soft start

peranders · 15th February 2006, 06:47 AM

Quote:

Originally posted by Wynand
I have been searching for something but could not find the right circuit.

http://home.swipnet.se/~w-50719/hifi/sst01
http://www.hypex.nl/docs/softstart.pdf
http://sound.westhost.com/project39.htm
http://lcaudio.com/index.php?page=12 in the middel of the page

Samuel Jayaraj · 15th February 2006, 07:33 AM

For those who are facing the problem of inrush current, here is a link to the solution Amp Protection Circuit - Here! which in addition protects speaker thumps, detects DC to cut off speakers and has thermal protection if you wish to add.

Cheers,

14th February 2006, 07:49 PM	#1
Wynand diyAudio Member Join Date: Mar 2005 Location: Cape Town	Inrush Current killing fuses I'm building a LM4870 audiosector amp. I've got the amplifier itself up and running with a temporary power supply. My problem is that the power supply I've got consist of a 500VA transformer (dual 20v secondaries) rectified seperately. With two 10000uF caps on each rail... The inrush current is killing me. I've got an Idea on limiting the current with a RC time delay activating relays to bypass current limiting resistors. I'm just wondering if anyone knows of a similar or better design to look at/compare. I must go now unfortunately but will check back tomorrow to see if anyone could help me. Thank you to anyone who could. PS. I have been searching for something but could not find the right circuit.

15th February 2006, 04:58 AM	#5
quasi diyAudio Member Join Date: Sep 2004 Location: Adelaide, Australia	I developed a soft start circuit for an amp I was working on. The schematic is here; http://www.diyaudio.com/forums/showt...687#post784687 - post 224. The PCB layout is here; http://www.diyaudio.com/forums/showt...781#post783781 - posts 217 & 218. I like relay contacts because they offer the ultimate in on resistance. Use it if you want to. Cheers __________________ http://sites.google.com/site/quasisdiyaudiosite/

15th February 2006, 06:30 AM	#8
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	Here we have a typical trade off: 1 Inrush current 2 We must have fuses and they should also protect, meaning they should be chosen after the load. The question is how you solve it. There are a couple of threads, one of them is here: Toroid soft start __________________ /Per-Anders (my first name) or P-A as my friends call me

15th February 2006, 07:33 AM	#10
Samuel Jayaraj diyAudio Member Join Date: May 2001 Location: Bangalore, India	For those who are facing the problem of inrush current, here is a link to the solution Amp Protection Circuit - Here! which in addition protects speaker thumps, detects DC to cut off speakers and has thermal protection if you wish to add. Cheers, __________________ Sam

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14th February 2006, 08:01 PM	#2
Craig405 diyAudio Member Join Date: Jul 2005 Location: Sussex	Hi, I had a similar problem with a 1KVa transformer. Relays will work switching in resistors OR.. you can use 'inrush limiters' which are beefy NTC thermistors, when they heat up they lower in resistance. Therefore your transformer can only draw full current once theyre hot. They are cheap and really simple. Make sure they can handle your current requirements (you can parallel them up) and put them in a sensible place (they do heat up a fair bit). I know RS have them under 'inrush limiters' because thats where i got mine. Craig

14th February 2006, 08:02 PM	#3
bikehorn diyAudio Member Join Date: Aug 2004 Location: toronto	soft start circuit? http://sound.westhost.com/project39.htm

14th February 2006, 09:11 PM	#4
Nordic diyAudio Member Join Date: Sep 2005	Wynand, you are using T-fuses right?

15th February 2006, 04:59 AM	#6
Wynand diyAudio Member Join Date: Mar 2005 Location: Cape Town	I'm using 20mm fuses, slo-blow on mains input I've seen the circuit on ESP but want something simple. And I don't know which NTCs to choose, even after looking at some datasheets.

15th February 2006, 05:16 AM	#7
Wynand diyAudio Member Join Date: Mar 2005 Location: Cape Town	Aswell. Ntcs work by warming up. If you put the amp off for a couple of seconds, the caps discharge, but their still hot and thus not going to stop the inrush. Relays will work everytime. If nobody has one I'll work on my design and try to post it.

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