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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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