Alternative size current sense resistor - Page 1
Forum section: electronics / beginners

author: queennikki1972
author_rank: Regular Contributor
timestamp: September 23, 2019, 09:20:46 pm
content: 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 useCurrent Sense Resistor 3W .05ohmor if the mentioned 10ohm is recommended..minimum current = 1.25/R1 (.05ohm)means 25ma minimumR2= 220ohm resistorR3= 5kohm potMaximum Current is 1+(R3/R2) x the minimummeans 593mA maximum??These are not real numbers just checking the math

author: MosherIV
author_rank: Super Contributor
timestamp: September 23, 2019, 09:39:13 pm
content: HiI 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 linkhttp://www.bowdenshobbycircuits.info/page4.htmNot the clearest current limit circuit to understand but clearly shows how to OR the output of 2 op-amps together

author: queennikki1972
author_rank: Regular Contributor
timestamp: September 23, 2019, 09:51:20 pm
content: 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.

author: queennikki1972
author_rank: Regular Contributor
timestamp: September 23, 2019, 09:55:34 pm
content: I found this, but as you said its not suggested..https://electronics.stackexchange.com/questions/100649/lm317-based-power-supply-with-current-limiting

author: MosherIV
author_rank: Super Contributor
timestamp: September 23, 2019, 10:12:04 pm
content: 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.

author: queennikki1972
author_rank: Regular Contributor
timestamp: October 10, 2019, 01:45:10 am
content: How do you feel about this circuit?5A constant current constant voltage regulatorhttp://www.circuitstoday.com/few-lm317-voltage-regulator-circuits

author: MosherIV
author_rank: Super Contributor
timestamp: October 10, 2019, 07:07:32 am
content: How do I feel about it?Difficult.I am impressed controlling both V and current limit can be done with LM317.Do you understand how it works?It needs 3 voltage rails, Vin, V+ and -6V.The volt adjust and current limit is not that easy to understand.It may not be easy to add panel meter to show the set voltage and/or set current.If you just want to build a bench psu - this may work.Does it help your understanding - only you can answer that.

author: MarkF
author_rank: Super Contributor
timestamp: October 10, 2019, 01:41:12 pm
content: If you use a rail-to-rail or single supply op-amp, I believe you could make it work with lower output voltage range. Maybe worth trying anyway.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 10, 2019, 04:17:55 pm
content: The current limit is usually set by the resistor and the set voltage to drop at the resistor at most. Going from 10 Ohm to 0.1 Ohms is a large change, getting current 1000 times higher. So the math in the initial posting is wrong.Adding a variable current limit to a LM317 or similar voltage regulator is difficult to get good. It is usually easier to start with a combined constant current / voltage regulator with an external power transistor. So more like using an LM723 or 2 OPs and a reference. A lab supply is a rather common topic - with plenty of poor quality circuits around.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 10, 2019, 04:55:24 pm
content: Using a single supply certainly makes things easier; the original design would have needed 4 power rails, with the +/- rails for the op-amp needing to be fairly well regulated if you want to avoid oscillation, which given the AC coupling of the feedback on the op-amp is likely to make your load regulation, shall we say, 'poor'It may be also a better idea to use something like an L200 instead of an LM317, as it's specifically designed to have variable current regulation, and uses the same TO220 package, albeit a 5-pin variant. It also has a TO3 version, much as the LM317/337 etc do with the K series.TheST datasheetincludes example circuits with extended current capability using a pass transistor, and doesn't include AC coupling in the feedback circuit...