Constant current load - Page 1
Forum section: electronics / beginners

author: glennpierce
author_rank: Newbie
timestamp: November 13, 2014, 08:39:58 pm
content: HiI'm replicating Dave's constant load circuit.I only have used one op amp only and I am feeding the non inverting input witha voltage divider consisting of a 10k 10 turn pot and a 30k resister. The input to drive the op amp is 9v.This provides a nice smooth range of zero to 1.55 amps and all is well.I plan to add a Zener across the input to stop the circuit being driven by more than 9v or use a regulator.Not sure what is best. I will probably also add a 2 amp fuse to the load side as well.I have a couple of questions thoughI tried to parallel up two MTP3055v Mosfets to handle the power better. However, when I did I was no longer able to dial low currents with the pot.The pot's minimum starts at 0 and jumps immediately causing the current load to settle at about 500 mA ? Why is this. Both Mosfets are OK individually.I managed to damage a previous Fet. I believe I did this running 2 Amps through it for a few seconds with no heatsink attached. Does this seem likely as it can handle up to 12 amps ? I don't have much experience with these devices so am not sure on how easily they are damaged.Thanks for any advice and thanks for the circuit Dave .Glenn

author: Thor-Arne
author_rank: Unknown
timestamp: November 13, 2014, 09:27:19 pm
content: How do you try to parallel the mosfets?I made avariation on that cc-load paralleling two mosfets, i used IRF540 though. Works just fine.How are you trying to parallel the mosfets?I don't understand how you are going to use a zener in this, schematic?If you don't use a heatsink the mosfet may get to hot very fast, and possibly fail within seconds.

author: glennpierce
author_rank: Newbie
timestamp: November 13, 2014, 09:43:40 pm
content: I simply connected Gate to Gate, Drain to Drain and Source to Source.The Zener would be directly across the input of the circuit.iehttp://www.learningaboutelectronics.com/Articles/Overvoltage-protection-circuit.phpI got lazy and didn't attach my heatsink. I thought incorrectly it would be ok for a short time (seconds).

author: suicidaleggroll
author_rank: Super Contributor
timestamp: November 13, 2014, 09:58:23 pm
content: Quote from: glennpierce on November 13, 2014, 09:43:40 pmThat won't work unless the fets are perfectly matched, and even then it's not recommended. The right way to do it is how Thor-Arne did, each fet needs its own current shunt and op-amp.Quote from: glennpierce on November 13, 2014, 09:43:40 pmIt would, but not at 2 amps and [I'm guessing] some not-insignificant voltage.

author: glennpierce
author_rank: Newbie
timestamp: November 14, 2014, 12:00:33 am
content: Thanks guys that makes sense.I think I will stick with one FET as it should easily handle 1.5 Amps.My curiosity has been satisfied.

author: Thor-Arne
author_rank: Unknown
timestamp: November 14, 2014, 12:34:00 am
content: Quote from: glennpierce on November 13, 2014, 09:43:40 pmAs suicidaleggroll said, that won't do.You need to us a opamp and a load resistor to balance the load, it's easy to implement but uses a litte more space.See the schematic in my post, the two "channels" is marked by the two blue boxes.Quote from: glennpierce on November 13, 2014, 09:43:40 pm

author: pa3bca
author_rank: Regular Contributor
timestamp: November 14, 2014, 02:25:36 pm
content: Quote from: glennpierce on November 13, 2014, 09:43:40 pmPerfect for self-oscillation of mos/hexfets! That is probably what is happening. Have you looked at it with a scope?At least add individual resistors in the source leads of each FET. That will stop oscillation.And as already noticed, this is not a good way to parallel mosfets (in order to share the load) Remember that the fets are operating in the linear region, where thermal runaway is a serious issue (the opposite is true when fully saturated, where the RDSon has a positive tempco).So at least add series resistors in the sources, or better use the above proposed schematic with a control loop per fet.