Adjustable current limit for bench power supply - Page 1
Forum section: electronics / beginners

author: byoungblood
author_rank: Contributor
timestamp: February 26, 2018, 06:41:03 pm
content: I'm designing an adjustable bench power supply. Output will be 2-30v @ 5A max.I would like to incorporate an adjustable current limit (not constant current) into my design. Can I adapt the common sense resistor/shunt transistor arrangement (see photo) and simply replace the sense resistor with an appropriate potentiometer to obtain an adjustable current limit? It doesn't need to be extremely precise, +/-20mA would probably be sufficient. Basically just something I can attach a load resistor across the output and adjust as required.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 26, 2018, 07:00:26 pm
content: There are essentially not pots to be used for this purpose. So for the more normal range, this is not a good option (it might work in the µA range and with switches / relays). The more usual way is to use a fixed shunt and add / subtract some voltage to adjust the current limit. For a large range one might consider switching between a few different shunts (e.g. 1 :10 step for something like a 5 A and 0.5 A range).With adjustment one may need a more stable reference than the UBE of a transistor to get a reasonable good adjustment. Making the shunt larger is not a real option, already with 0.6 V drop like in the very simple circuit, the power loss at 5 A will be rather high.The more burden voltage is in the 100 mV or 200 mV range, especially at higher currents.

author: cowasaki
author_rank: Frequent Contributor
timestamp: February 27, 2018, 10:17:30 pm
content: I’m on my phone at the moment but Dave has done three eevblog videos on that very subject. He goes into great detail with examples. I have them saved as links at home but they shouldn’t be hard to find.

author: byoungblood
author_rank: Contributor
timestamp: February 28, 2018, 04:56:58 am
content: Thanks, I’ll look them up!I’ve seen some rather elaborate designs out there, but I’m looking for something a bit more simple so I can wrap my head around the operation of the circuit and actually know what is going on rather than just following someone else’s schematic.I was watching a video on YT yesterday, uses a couple of op amps, one as a differential amp, the other as a comperator to drive the feedback on a switching regulator, which I assume could also be used to feedback into my linear regulator to cause it to reduce the regulated output when it reaches the current limit? Here is a link to the video, the schematic is at about the 8 minute mark:https://youtu.be/EVL7TzCde8ITo obtain my 5A limit, I’d have to substitute a 200 milliohm resistor in place of the 1 ohm current sense. To obtain adjustment down to about 10mA I’d just use the 1 ohm. As suggested before, maybe incorporate a rotary switch to switch the current sense resistors to obtain the desired limit range.Should this work, any suggestions for improvement?

author: VictorTito
author_rank: Newbie
timestamp: May 29, 2021, 04:48:05 pm
content: I know it has been a long time, but you may like to see this solution:It is a current limiter, not a constant current, circuit. It will shutdown if the current limit is reached.Thanks.Victor.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 31, 2021, 02:41:59 am
content: something like this ?

author: VictorTito
author_rank: Newbie
timestamp: May 31, 2021, 06:55:55 pm
content: Comoronadalix, the circuit you show is of a constant current circuit not a current limit circuit. A constant current circuit maintain the current by dumping into the pass transistor the excess power by increasing its collector-emitter voltage (Vce). This heats up the transistor to the point of burning it, unless you use a proper heat sink device. The current limit circuit, on the other hand, as the one presented in the above YouTube video, does not need a heat sink, it shuts down if the current goes beyond a certain limit. Like a fuse.