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Alternative size current sense resistor

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queennikki1972:
I have over 40 current sense resistors but of a different size than the diagram. I'm trying to learn to have a current limit on my linear supplies.. I was wondering if this is right if I use
Current Sense Resistor 3W .05ohm or if the mentioned 10ohm is recommended..

minimum current = 1.25/R1  (.05ohm)   means 25ma minimum   ??????

R2= 220ohm resistor
R3= 5kohm pot
Maximum Current is 1+(R3/R2) x the minimum means 593mA maximum   ?????

These are not real numbers just checking the math

MosherIV:
Hi

I would suggest that you do NOT try to use LM317 to implement current limit.

In a linear supply, current limit is done by ORing together the voltage setting with the current limit setting. Best done by ORing together the output of 2 op-amps.
When the current limit is reached, the current limit op-amp drags the output voltage down to limit the current.
This would be very difficult to do with LM317.

Have a look at the 2nd psu circuit from this link
http://www.bowdenshobbycircuits.info/page4.htm
Not the clearest current limit circuit to understand but clearly shows how to OR the output of 2 op-amps together


queennikki1972:
Unfortunately its a little hazy for me to read at my level of experience.... I'll study it and see if i can get the idea.

queennikki1972:
I found this, but as you said its not suggested..

https://electronics.stackexchange.com/questions/100649/lm317-based-power-supply-with-current-limiting

MosherIV:
No problem.

I searched for ages looking for DIY bench PSUs. I studied them until I understood.
Eventually, I found this one :

http://www.microsyl.com/index.php/2010/03/31/bench-power-supply-0-25v-0-5amp/comment-page-2/

I found it by the easiest to understand.
The current reading is done with a ZTX1009, feeds an op-amp which compares it with a current limit setting set by R11.
When the current limit is reached, the op-amp U1B pulls the voltage op-amp down via the diodes D4 & D5.

Personally, I alway prefer high side current sense and power drive.
Additionally, use Bi-polar transistors, then yo can add a switch to the base of the power transistors for a simple low current, output on/off switch. You cannot do that with FET based PSU designs.

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