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Why this Dummy load is not giving appropriate amps?

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chevymn1964

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Hey Everyone, I found a Schematic on the web for a 0-20AMP Adjustable "DUMMY" Load... I made the circuit exactly to the schematic and when I test I can only get about 0-3.75 amps max? when I try to boost the reference voltage I can get more current draw but then its not constant current... Im not too experienced with op-amps so I was wondering if anyone can check it out and give me some hints? I think that pis 8 should goto pin 2 and reference the reference voltage comparitor to the current feedback?? the project can be found at: http://www-d0.fnal.gov/hardware/cal/lvps_info/curr_load/
 

Re: Dummy Load HELP?

This circuit looks OK but the main issue here is the voltage between positive and negative pins ..
What you may try to do is to supply the opamp (pin 7) of external, say, 12V power supply ..
As the MJ11030 has internal bias resistors (see picture below) you can remove R4 and reduce the value of R3 to below 100Ω, and for test purposes remove this thermal switch ..

Regards,
IanP
 

Re: Dummy Load HELP?

Thanks for the fast reply Ianp.. I tryed to supply the op-amp with an external p/s to no sucess??? I get the same results..... hummmm???
 

Re: Dummy Load HELP?

Maybe hFE of the Darlington is to low and opamp just can't drive it properly ..
Try to add another BJT as shown in the attached picture .. Q1-Q2 represet the Darlington, Q3 is the "enhancement" ..

Regards,
IanP
 

Re: Dummy Load HELP?

Hello, thanks again IanP... I'll give that one a try. I was wondering if that would be a problem.... The only thing is though that when I boost the reference voltage to the pot I could get much more current draw ie 20-35AMPS but then its not constant current???? that was what got me thinking maybe pin8 should goto pin2 to reference the voltage reference from the current feedback?? follow me??? lol.... I will try enhancing the gain of the bjt and let you know! Thanks!

Added after 8 minutes:

Well, I just tryed to boost the bjt with no luck.... same old 3.5amps.... Any other ideas???
 

Re: Dummy Load HELP?

I was also wondering if the value of the current sense resistor is critical? I was unable to find a .01ohm at my local electronics parts house so I used a .05ohm... I have some .01ohm samples on their way.... But in the other hand I tried to hook up 2 .05ohm resistors in parelle with no change? Hummmmmm
 

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