Inrush Current Limiting Circuit

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I found this passive inrush current limiting circuit in a back issue of EDN and thought somebody might be interested in it.

http://www.reed-electronics.com/ednmag/contents/images/92602di.pdf

What's interesting to me about this circuit is that it doesn't require a timer (it automatically switches out once the cap has reached a certain charge voltage) plus the relay contacts don't carry the full supply current (just the capacitor currents). Another benefit of this circuit is that it automatically bleeds off voltage from the caps without seperate bleeder resistors constantly wasting power and generating heat any time the amp is in use.

Phil
 
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Bill Fitzpatrick said:


Is that the name of an all girl rock group?
No, it's 5-6-7-8
 

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haldor said:
I found this passive inrush current limiting circuit in a back issue of EDN and thought somebody might be interested in it.

http://www.reed-electronics.com/ednmag/contents/images/92602di.pdf

What's interesting to me about this circuit is that it doesn't require a timer (it automatically switches out once the cap has reached a certain charge voltage) plus the relay contacts don't carry the full supply current (just the capacitor currents). Another benefit of this circuit is that it automatically bleeds off voltage from the caps without seperate bleeder resistors constantly wasting power and generating heat any time the amp is in use.

Phil

One disadvantage I see is that both the load ripple current and the cap charging ripple current (in normal operations) go through the switch contact. These currents will be less than the initial inrush current, but they may be high enough to be aware of when selecting the relay.

Jan Didden
 
haldor said:
I found this passive inrush current limiting circuit in a back issue of EDN and thought somebody might be interested in it.

http://www.reed-electronics.com/ednmag/contents/images/92602di.pdf

What's interesting to me about this circuit is that it doesn't require a timer (it automatically switches out once the cap has reached a certain charge voltage) plus the relay contacts don't carry the full supply current (just the capacitor currents). Another benefit of this circuit is that it automatically bleeds off voltage from the caps without seperate bleeder resistors constantly wasting power and generating heat any time the amp is in use.
I have tested ideas like that but you don't need them, unless, you have rather weak diodes and HUGE capacitors.

Normally you only want to save fuses and be able to have "normal" primary fuses and for this the circuit above has the disadvantage of long reset time. I think an electronical device with a < 1 sec delay is better and it works with prediction. What happens if you have short power interrupts? = burned fuses!
 
Re: Re: Inrush Current Limiting Circuit

peranders said:

I have tested ideas like that but you don't need them, unless, you have rather weak diodes and HUGE capacitors.

Normally you only want to save fuses and be able to have "normal" primary fuses and for this the circuit above has the disadvantage of long reset time. I think an electronical device with a < 1 sec delay is better and it works with prediction. What happens if you have short power interrupts? = burned fuses!

I've see some "huge" capactor banks used in the Pass section (0.2F). I also like the off-line bleeder resistors, that is not something I've seen before.

A short power interruption should not result in burned fuses. When the voltage drops below the zener diode threshold, the inrush current limiter will be switched back in. If the power comes back on there is nothing to cause a current surge bigger than the normal turnon surge.

Phil
 
Re: Re: Re: Inrush Current Limiting Circuit

haldor said:
A short power interruption should not result in burned fuses. When the voltage drops below the zener diode threshold, the inrush current limiter will be switched back in. If the power comes back on there is nothing to cause a current surge bigger than the normal turnon surge.
What happens if you turn on a toroid without any load?

What happens if you turn on a toroid with charged caps?

Someting to think about.

Also the rectifier bridge can take 200-300 A, short term which would not destroy them.

I can give you the answer about the toroids later when you have thought about it.
 
Another disadvantage off the circuit is if there accures an error and there is a short then your fuse will not blow but your resistor will after 10...30 seconds, with a timer after less then 1 second your fuses blow. But hen again your amp is already dead, one more resistor doens't matter that match at that point :bawling:
 
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Re: Re: Re: Re: Re: Inrush Current Limiting Circuit

peranders said:

.... and the answer is: You will still get these inrush currents regardless of the load.


Indeed. And worse than that, your inrush current depends on the point on the 50Hz that you switched the transformer off last time. They maintain what is called residual remanence. If you then switch them on at the wrong point in the wave form with the field reversed, you get inrush current much higher then expected. So, the fuse may blow sometimes, not blow some other times, unless the fuse is dimensioned for the max possible surge.

Jan Didden
 
That explains exactly why I'm popping fuses 1 out of 10 times. Everything's fine until the rare case where you get the light show on power-up. :)

Is there a way to calculate what amperage you need that should be safe in all cases? I have a 110V primary and dual 22V secondary toroid that is rated at 225VA. That works out to about 10A or 5A on the secondaries, depending on whether you're wired parallel or in series.

What fuse should I use on the primary?

Should I bother using fuses on the secondary to protect my actual amp, or is it safe from the inrush current?

Chris
 
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