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26th June 2006, 07:20 PM  #1 
diyAudio Member
Join Date: Oct 2004

promitheus inrush limiter
hi all!
maybe a lot of people uses the Promitheus circuit to limit the inrush current in the transformer when they power up their ampli, and i'd like to do it too, but i got some problems using the values he gives: the resistors blow up! so let's try to do some calculations... i have an 800VA transformer, 2x25V outputs, 2x 20.000uF, 220V line, so max current is 3,5A and to limit it now i'll use 80 ohm resistor, but how many watts?! and not less important...the RC doubles, so maybe I have to change the delay, but which are the components that determine the delay??ok, the 2 capacitors of 470uF but which resistor? please, if someone can give me some more specific formulas to fit this circuit i'll be very happy because i'm stopped with my diy amp!!! 
26th June 2006, 07:33 PM  #2 
diyAudio Member
Join Date: Sep 2003
Location: Upstate NY

What are you using now?
The resistor will initially dissipate 3.5 A x 240 V = 840W. Obviously you aren't going to put a kilowatt resistor in there. How much power handling you need depends on the delay before they are removed from the circuit and the resistors' thermal characteristics. You should probably be in the area of 50 W  so three 200R/15+W in parallel should be OK  a little higher inrush but not terrible. Higher power ratings wouldn't hurt. 
26th June 2006, 08:43 PM  #3 
diyAudio Member
Join Date: Oct 2004

sure!my calculation was the same, because he uses 4 series resistors 10ohms,5w, so 40 ohm 20 watts for 1200watts initial power...my intention was to use 4 series resistors 22ohm,5 watts in series, not parallel to have the same power dissipation.
the transformer ratio is 9 so the resistor would be a (80/n^2)=1ohm at the secondary of the transformer, giving an rc of 20ms; the delay is about 0,5 sec(measured listening to the relay...)now, so the transient will be much more than fully completed!!!but i can't understand what's wrong with the first circuit, why the resistors blow up... 
26th June 2006, 08:54 PM  #4 
diyAudio Member
Join Date: Sep 2003
Location: Upstate NY

I don't know the exact circuit you are using, but perhaps a wiring error. If the resistors are across the mains they will blow up.
Verify that the resistors are in series with the mains line and that the relay closes across them when the delay trips. 
27th June 2006, 06:44 AM  #5 
diyAudio Member
Join Date: Oct 2004

this is the circuit:
http://mitglied.lycos.de/Promitheus...for_toroids.htm do you know the formula to calculate the delay?and my rc calculation is right or i'm doing someting wrong? 
27th June 2006, 06:45 AM  #6 
diyAudio Member
Join Date: Oct 2004

ups...my first account is now working!raikkonen and morpheus82 is the same person...

27th June 2006, 07:37 AM  #7 
diyAudio Moderator Emeritus
Join Date: Jan 2003

Which one would you like to keep?
/Hugo 
27th June 2006, 07:39 AM  #8 
diyAudio Member
Join Date: Oct 2004

if it will work also at home my old morpheus82, cos with the new one i'm under moderation...

27th June 2006, 08:04 AM  #9  
Electrons are yellow and more is better!
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Quote:
We have talked pretty much about this subject. http://www.diyaudio.com/forums/searc...der=descending
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27th June 2006, 08:07 AM  #10 
diyAudio Member
Join Date: Oct 2004

thanx a lot!but what did you change to obtain the delay you need?the 470uf capacitors?or a resistor?

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