Linearly adjustable current limit - Page 1
Forum section: electronics / beginners

author: Mandelbrot
author_rank: Regular Contributor
timestamp: June 17, 2013, 03:26:44 am
content: Hello again!I'm making the final touches to my design for a linear power supply and I am currently unhappy about the circuit that limits the current because the limit won't change linearly when the potentiometer knob is turned. I've been trying to come up with other designs involving op-amps and transistors but I'm having trouble with that too. Can anyone give me an idea of how I could do this? I guessed a differential amplifier could be used across the potentiometer, but I can't get that to work.Thanks,Ryan

author: edavid
author_rank: Super Contributor
timestamp: June 17, 2013, 03:55:53 am
content: A simple, but fairly expensive way is to convert the voltage across the current sensing resistor to a ground referenced voltage. This is done with a current sensing IC, or with a differential amp or instrumentation amp. Then you can easily connect that voltage and a voltage from the current limit pot to your current limit comparator.

author: Paul Price
author_rank: Super Contributor
timestamp: June 17, 2013, 09:31:39 am
content: Potentiometers have log, semi-log and linear tapers. You might just be using a pot with a log taper so the current limit is not linear in regard to rotation degrees. Is that your case?Can you draw your power supply circuit and post it?

author: Mandelbrot
author_rank: Regular Contributor
timestamp: June 17, 2013, 10:23:55 pm
content: It was very poorly designed using an LM317 and I was using a linear potentiometer. When I graph the current limit vs. the position of the potentiometer, it isn't linear at all. The rest of the circuit has changed a lot, but the current limiter using U1 in the schematic is what I was using. Really, I think I need to throw that part out anyway for a different circuit. The current limit should be 1.25/(1.25 + value of pot) for the circuit attached.

author: Paul Price
author_rank: Super Contributor
timestamp: June 18, 2013, 01:54:44 am
content: Try this one. Your circuit is a very poor idea because you almost never want to use a 1k potentiometer to control high currents unless it is a high wattage potentiometer, en even then, there are better solid state ways to achieve the same result cheaper and more reliably.You will need to add a low cost op amp and a few more parts but it will give you 0-1.6Amp Linear Constant Current Adj. and output down to just about 0 volts.R1(220-ohm 2W) and Z1(5V zener) could be replaced with a 78L05 voltage regulator.I am fairly certain the LM358 worked in this circuit but I may have substituted a faster op-amp in my final circuit. I will try tobreadboard this again to check on this, but if my memory serves me, this should work the way it is drawn here.The output of the power supply is across the electrolytic capacitor on the far right of the schematic.

author: Mandelbrot
author_rank: Regular Contributor
timestamp: June 19, 2013, 09:58:48 pm
content: I'm having a hard time following the circuit you drew. I'm not very good at this yet.I eventually came up with a new design for a current limit that is (nearly) linear.The diode is to keep the output voltage of the op-amp above 1.25V. I used ideal parts in my simulation because my free student version won't let me pick actual parts. I was planning on using an LM741 because I have loads of those and no other op-amps, and a TIP120 for the transistor because I have one of those already. I'm aware the current can't go all the way down to zero, but it's close enough for me!Will this design work? If not, could you explain what parts of the schematic you posted work to control the current?

author: c4757p
author_rank: Super Contributor
timestamp: June 19, 2013, 10:22:20 pm
content: Quote from: Mandelbrot on June 19, 2013, 09:58:48 pmThat's because - sorry, Paul - while it's a fine schematic for more experienced people who know what they're looking for, it's a ghastly schematic to give a real beginner and expect him to be able to see the signal and current paths. Everything's crammed in tight. Paul, we don't charge by the pixel here... Try the attached version. Hopefully I didn't miss anything. (Edit: I did - I didn't connect power to the op amps. Oh well, I assume you know they need power.) (And I missed the output capacitor. Stick it right over by the "OUT" connector.)QuoteIdeal op amps in a simulation is abigno-no unless you really know what you're doing. Real op amps areveryfar from ideal ones. You should use the free student version of LTspice - I hear it's really quite complete.I'll leave looking at your circuit to Paul, it's time for me to make dinner...Edit:Give me one minute to correct a big error.OK, done. Sorry about that. Migraines and schematics don't mix well.Goddammit, it's taken me something like four tries to get this right.

author: Paul Price
author_rank: Super Contributor
timestamp: June 19, 2013, 11:50:43 pm
content: C4757p:You have copied my schematic perfectly. It is easy to read and accurate.Nice schematic drawing program you have. The color adds to make it easy reading. I use the a free schematic capture program, and I have re-posted revisions that are clearer.I have been reading schematics so long that I have forgotten how years of experience has made it so easy for me and how difficult it was for me as a beginner to try to read one.Thanks for reminding me!