Simple DC Load oscillation query - Page 1
Forum section: electronics / beginners

author: rfengg
author_rank: Regular Contributor
timestamp: October 21, 2024, 09:07:46 pm
content: All,I assembled the attached schematic for a simple DC load on a breadboard. Everything works perfect upto 5A when I connect a DC power supply between the nodes indicated as Load 1 and Load 2 as I am able to vary the load current between 100mA to 5A with the 10K pot P1.However when I attach a 1uH inductor in series with the DC power supply at Load 1 -Load 2, the whole circuit starts oscillating and it does not work as a constant current source anymore. I tried putting a 47nF cap between pin 1 and pin 2 of the LM358 as suggested on another forum, but it does not help.Any suggestions on how to stop the circuit oscillating with a inductive load in series with the DC power supply?

author: Benta
author_rank: Super Contributor
timestamp: October 21, 2024, 09:13:51 pm
content: Shouldn't "LOAD-2" got to a positive voltage?As you've drawn it, it won't work.And what's the purpose of C4?

author: rfengg
author_rank: Regular Contributor
timestamp: October 21, 2024, 09:20:47 pm
content: Thanks Benta....the LOAD-2 is indeed connected to the negative of the power supply to be "loaded".C4 is a decoupling cap I guess......the original circuit was obtained from Codytronics and not designed by me.....but other than the oscillation with a 1uH in series with the power supply to be loaded, the circuit works perfectly fine.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 21, 2024, 09:31:30 pm
content: 1 - add a 100R resistor between pin 1 and the MOSFET gate.2 - add a 1k resistor and a 1nf capacitor, as shown in the image.

author: rfengg
author_rank: Regular Contributor
timestamp: October 21, 2024, 09:33:18 pm
content: Thanks Avelino Sampaio......tried this yesterday but it didn't help

author: iMo
author_rank: Super Contributor
timestamp: October 21, 2024, 10:24:48 pm
content: It cannot work "perfectly fine" with powering the opamp with 5V, imho.The IRL540 needs a much higher voltage than the max 5V from the opamp's output when with the 1ohm in the source (1ohm * 5A = 5V).Also the opamp will not work fine with 5V input when powered by 5V.It should be something like 10-15V..PS: at 5A the voltage at the 1ohm resistor will be 5V.With 5V max output from the opamp and 5V at the 1ohm the Vgs=0 and the IRL540 will be closed. See below the graph.

author: Andy Chee
author_rank: Super Contributor
timestamp: October 21, 2024, 10:39:04 pm
content: Can't help with the oscillation.But perhaps the reverse engineering of an eBay load might help inform your own build?https://www.voltlog.com/pub/dummy-load-sch.pdf

author: rfengg
author_rank: Regular Contributor
timestamp: October 22, 2024, 01:53:49 pm
content: Quote from: iMo on October 21, 2024, 10:24:48 pmMy apologies iMo....you are absolutely correct......since I had copied the schematic from the web, I forgot to change the PS voltage indicated on the schematic from 5V to 9V as I was indeed driving it with 9V.....Were you using LTSpice to simulate this? Are you seeing any oscillation at the gate of the IRL540?

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 23, 2024, 06:02:31 am
content: Hi!I assembled a similar load circuit for an AC source. First modeled in Microcap (please see attachments).The scheme worked immediately.Maybe you need to check the PCB trace. Perhaps the problem of oscillation lies there.

author: MarkF
author_rank: Super Contributor
timestamp: October 23, 2024, 08:10:02 am
content: Take a look at@Jay_Diddy_B Electronic Loaddesign for a damping circuit he has on the output:You need the extra resistors and feedback capacitor around the opamp as @Avelino Sampaio suggested. You can try upping the capacitor to 10nF to see if that stops the oscillation as a test. That high of capacitor will really slow down the responsiveness.You can seewhat I did, but it has its own issues.

author: RFDx
author_rank: Regular Contributor
timestamp: October 23, 2024, 08:24:33 am
content: Quote from: rfengg on October 22, 2024, 01:53:49 pmYes, it does oscillate in simulation because of the 1uH in the load line and with C4 (1uF) being an ideal/low ESR cap. Dampening the op amp doesn't help. Give C4 and/or the 1uH inductor some ESR (e.g. >1 Ohm).

author: iMo
author_rank: Super Contributor
timestamp: October 24, 2024, 02:00:31 pm
content: Quote from: RFDx on October 23, 2024, 08:24:33 amI do not see an oscillation on the gate in a static regime at 5A.The ESR of C1=0ohm and L1=1milliOhm (default in LTspice).There is a small oscillation with a step response because of the LC in the drain, what could be expected.The simulation results depend on the models used, thus it may or may not oscillate. Spice is just a tool to understand how the circuits basically work, you have always to verify them in the real hw.Better to introduce some contra-measures as described above..