Electronics > Beginners
Alternative size current sense resistor
(1/2) > >>
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.
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod