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Here is the circuit in question: schematic (https://i.sstatic.net/qd0cD.png)

The circuit is an adjustable power supply that I have put together (can be bought here) and I am working on putting it all together inside of a case now.

One thing that I wanted to do is display the current limit set by the current control potentiometer. The output voltage is easily seen when changed because whatever is set will be the output. It is different for the current, however, because the circuit will only draw what it needs despite what the ceiling is set to. Even though I know I could get by without it, I would like to visually see the current limit change as I turn the corresponding potentiometer.

I have already successfully assembled and tested the power supply. I have one ammeter set to the output and is powering a small fan I pulled out of a dead xbox 360 all through a breadboard. I will be able to test any suggestions so even if you only have an idea of where I should patch my ammeter in, please let me know and I can give it a try.

Note: The current limit potentiometer is linked to the op-amp facing left

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2 Answers 2

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The current limit setting is proportional to the voltage at the non-inverting input of IC1, which is set by the potentiometer P1. Measure that voltage. Calibrate the meter in a way that @Ecnrewal is describing.

enter image description here

p.s. The original schematic is somewhat badly drawn (although it could have been worse). No wonder you are having difficulty with figuring out where to measure. Guidelines for drawing good schematics can be found here, if you are curious.

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  • \$\begingroup\$ I actually found this schematic while researching the kit this came from. Thank you for the guidelines though - that's good to know for the future. I'll look into making a custom display based off of the pot voltage - great suggestions! \$\endgroup\$
    – Daniel
    Commented Jan 10, 2016 at 3:52
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Put a blank paper scale under the potentiometer knob. Put the ammeter on the output, set the amp limit and voltage to minimum, and use something like a 120VAC 100W light bulb as a "soft short" or heavy load - about 9 ohms when cold.

Observe the amps, and turn up the volts to see if the amp limit is working (if it is the volts should not rise and the amps should not either) - record the amps, turn up the amp limit, observe while adjusting volts, record the amps again (record by writing on the paper label under the pot knob.)

I don't see another practical approach to having the LIMIT "displayed" (displaying the actual amps is easy.)

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  • \$\begingroup\$ I suppose it was naive of me to assume that somewhere in this circuit is the actual current limit. Thank you for your response. \$\endgroup\$
    – Daniel
    Commented Jan 10, 2016 at 3:43
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    \$\begingroup\$ You could possibly calibrate a voltmeter reading the control voltage to reflect the limit amperage (calibration method same, just a difference in where you are writing it down) but it's more difficult in practice (have to find an analog meter you can open up and put a blank face on to place your scale, or head off into microcontroller-and-display-land for the moderne approach.) Right - that is Nick's answer. \$\endgroup\$
    – Ecnerwal
    Commented Jan 10, 2016 at 3:48
  • \$\begingroup\$ I've got an unopened MSP430 that might just be perfect for this. Thank you! \$\endgroup\$
    – Daniel
    Commented Jan 10, 2016 at 3:52

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