Oscillations in diy constant current load - Page 1
Forum section: electronics / beginners

author: newbie666
author_rank: Regular Contributor
timestamp: April 18, 2020, 07:25:02 am
content: Hello,I've just built the following circuit (attaching relevant part). Pass element - BTS117 is attached to the pcb with around 5-6cm of wire (it's mounted to the enclosure to dissipate heat).When testing my load (load is attached to E3610 power supply - the only one I have) I see small oscillation on the load input (~130mV, ~15kHz). I suspected that increasing C105 will help with oscillations so I've soldered another 100nF capacitor on top of the original one for the total of 200nF but that didn't change the frequency or amplitude of oscillations so maybe that is not the way to go.Any other ideas how to make the load stop oscillating?

author: newbie666
author_rank: Regular Contributor
timestamp: April 18, 2020, 07:29:58 am
content: Also attaching a photo of the device in question.

author: mikerj
author_rank: Super Contributor
timestamp: April 18, 2020, 08:55:15 am
content: Where did the design come from? A value of zero ohms can not be correct for R105, it means that the compensation cap C105 is providing virtually no feedback as it's shunted by the sense resistors. It might even be making stability worse in this state by providing additional capacitive loading to the op-amp.Raise the value of R105, something of the order of 1k or so.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 18, 2020, 09:02:52 am
content: C105 100nF should work but if it isn't then I agree with previous - use 100R or 1k R105 (1k is better).If everything is done good then C105 have to be just > 2-3 nF sufficient.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 18, 2020, 09:04:09 am
content: And, of cause, you should check your power supply first with some dummy load. (Check that it doesn't oscillate itself).

author: newbie666
author_rank: Regular Contributor
timestamp: April 18, 2020, 09:27:51 am
content: QuoteThat was it, I replaced 0R with 2k and oscillation is gone.It seems I have misunderstood how C105 should function. I thought that loading opamp with capacitor will slow it down, seems it's more complicated then.

author: Gyro
author_rank: Super Contributor
timestamp: April 18, 2020, 09:28:33 am
content: Those wires look quite long and parallel - try soldering (another) R104 right on the gate pin, at the end of the wire.BTW, the thermal conductivity of diecast boxes isn't nearly as high as you might imagine. It's quite easy to create 'hot-spots'. Probably not an issue here, but worth remembering.

author: newbie666
author_rank: Regular Contributor
timestamp: April 18, 2020, 10:27:47 am
content: Max power dissipation won't break any records but the device is pretty stable at 9.5W with pcb mounted (it acts as a lid). I need a load to test and troubleshot power supplies for microcontrollers so it's way over specified for me anyway.I also have two more boards and cases so I could connect up to 3 devices for 28W of power sinking but I don't think I'll ever need thatAttaching a photo of finished, working device - my first design with hammond diecast enclosure + pcb as panel and I'm quite happy with the outcomeThanks everyone for help.