Why won't this constant current circuit work?  - Page 1
Forum section: electronics / beginners

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: May 08, 2023, 09:13:37 pm
content: This is essentially a follow-up post tohttps://www.eevblog.com/forum/beginners/driving-this-dual-coloured-cob-led/Circuit in question:http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWEBmAHAJmgdgGzoRmACzICcpkORKWISCkdApgLRhgBQyWjRcILZOhB9GYHOChQuPEfypyxRahEaQOfUuFwjUIdKj1E9yKNAktMOSgjRYEpMPkhEspmJHTIw8X38-SYFjUxiCojKQSYuDwZghEpAz+vhAASkwAzgCWGQAuAIYAdgDGTBwA7iCRKMJBEkLSAGZ01gKh1nosoQ1g0Ege6BX6hrooCMKh6pWi4BIzDeoA5qPho9VqHMvo6NTV2ypOUkuK+pBG-PtHHABOJ8QhF2dH0MjIHKn6OwLijJdsOmo5DEjnEhnUQApwfdpJUoco7vD1LchGJDiiIdQxL4hpc9iN1jivqtcVEOLkxhNzrw9NFBDhoD9kDgwMgEAggq9VNhSNsWS5XARSKhaD5GAATJiNfIAVwANrlCdQ2K1LuIYZ8lT9RsqNtN+DqTixVlNtVrQsrAnhDasZoJhMIJVK5bkWLKmGLJNFrOoPrazfrwYC+EDATAEBwgAI'm trying to essentially choose the correct value potentiometer for the constant-current circuit above (and being able to vary the current from 0mah to 600mah, for the LED), but no matter what potentiometer I use I am not able to vary the current flowing through the LED. I would like to keep using the 0.33ohm power resistor as I have many on-hand currently, as well as have the same voltage going into both the op-amps power/supply input and non-inverting input ("+" input) for a (seemingly?) simpler circuit.Please help? I'm really stumped.

author: srb1954
author_rank: Super Contributor
timestamp: May 08, 2023, 09:33:10 pm
content: What op amp are you using?

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 08, 2023, 09:49:06 pm
content: The LM741 won't work. I changed it to a LM324 in your simulator and added a 47K resistor for scaling. (LM358 would work too)

author: Benta
author_rank: Super Contributor
timestamp: May 08, 2023, 09:58:54 pm
content: Yup. You need an opamp that'll accept a common-range input voltage to ground.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: May 08, 2023, 11:41:23 pm
content: Woops I had no idea I could change the selected op-amp! I am indeed using the LM358 so the LM324 works perfectly in the simulation. It's all working now, thanks!

author: EPAIII
author_rank: Super Contributor
timestamp: May 09, 2023, 05:41:32 am
content: I get the OP amp change, but am wondering about the two numbers on the ends of the pot (509.9 & 490.1). There are no units. What do they indicate?Quote from: Kim Christensen on May 08, 2023, 09:49:06 pm

author: timenutgoblin
author_rank: Regular Contributor
timestamp: May 09, 2023, 07:28:48 am
content: Quote from: EPAIII on May 09, 2023, 05:41:32 amThe two values look like the resistance either side of the wiper of the potentiometer. It's likely a 1k (509.9 + 490.1 = 1000.0) potentiometer. The high side is 509.9 ohms and the low side is 490.1 ohms. I agree, it should have units as the other values on the circuit diagram show.

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 09, 2023, 02:52:44 pm
content: timenutgoblin is correct: If you click the OP's link to the original simulated circuit, there is a "resistance" slider on the right side of the screen. Moving that changes the values.