Digital PSU resets when powering DC motor - Page 1
Forum section: electronics / beginners

author: argintviu
author_rank: Contributor
timestamp: January 03, 2017, 09:13:08 pm
content: Hi all, beginner in electronics here.I have built a digital power supply some time ago (created by Tuxgraphics) for use with general microcontroller development. I think you might be familiar with it, there have been some discussions about it on the forum. Recently I have tried to power a small drill with it to make some holes in a PCB and for some reason the MCU either reset or weird characters appeared on the screen as soon as the drill was turned on.Another interesting thing with it while powering the motor is that the supply became unstable, e.g. the voltage and current were all over the place (it's like the MCU couldn't decide how to set them). The drift on both of them was around 20-30%. This doesn't happen with other load types.I am new to electronics (programming background) but I suspect that there are some voltage spikes feeding back into the microcontroller via the ADC ports (as the drill motor is an inductive load). What I don't understand is how can this happen, as there is an antiparallel diode mounted on the output of the power supply so the spikes should go back to the motor. I tried to add an RC snubber on the output with 1uF non-polarized cap + 150R resistor, but it only reduced the occurrence of the problem.Is there any way to improve this design and get rid of the issue?Thanks for reading!

author: Benta
author_rank: Super Contributor
timestamp: January 03, 2017, 10:26:19 pm
content: I won't comment on the tuxgraphics design.But for a PCB drill supply, a transformer and a bridge rectifier is normally all that you need, not even a filter cap.Cheap drills run at 5000...10000 rpm, which will accommodate all drills up to 2 mm.For PCB drilling with 0.6...1.0 mm drills, 20,000 rpm would be better suited.If you need variable speed, look at adding a filter cap and an LM338.

author: argintviu
author_rank: Contributor
timestamp: January 04, 2017, 01:43:59 pm
content: Thanks for the answer.I already used a transformer for the drill, it worked fine, just wanted to use the bench supply out of convenience (e.g. having an universal supply for everything).It's strange how a complicated design like this can be made useless by a simple motor. I wonder how this problem is circumvented in commercial bench power supplies.I'd like to build a new bench supply, one which has the ability to also run motors and be a bit more stable (I'll keep the tuxgraphics one for lighter work). Would this design be more suitable?https://www.eevblog.com/forum/projects/amspire's-power-supply-in-a-pcb/

author: Zero999
author_rank: Super Contributor
timestamp: January 04, 2017, 01:58:53 pm
content: It could be numerous things: perhaps the surge when the motor started or the noise from the motor interfered with the power supply?You could try better RF filtering on the output of the PSU.

author: Short Circuit
author_rank: Frequent Contributor
timestamp: January 04, 2017, 08:40:37 pm
content: Maybe check their latest firmware versions to start with?Quote

author: free_electron
author_rank: Super Contributor
timestamp: January 04, 2017, 09:18:37 pm
content: that's not a power supply ... that is a DISASTER !

author: Benta
author_rank: Super Contributor
timestamp: January 04, 2017, 09:24:02 pm
content: Quote from: free_electron on January 04, 2017, 09:18:37 pmNow you know why I did not want to comment on the tuxgraphics design.

author: argintviu
author_rank: Contributor
timestamp: January 04, 2017, 10:21:30 pm
content: While tinkering with the resistor divider from the voltage sensing circuit I noticed that when the voltage on the output rail gets bigger than what the supply is set to provide, instability starts, e.g. the voltage drops randomly. Gotta read the code, this might be a software quirk or even some kind of protection. This also explains the instability when powering the motor (which raises the output voltage with those spikes).I'll keep searching for a way to prevent the voltage feeding back into the loop. For starters I'll borrow the MOSFET + zener protection from amspire. Hopefully I'm not beating a dead horse.Quote from: Hero999 on January 04, 2017, 01:58:53 pmIndeed, the motor is creating voltage spikes that make their way back in the supply via the ADC that is used for sensing voltage.Quote from: Short Circuit on January 04, 2017, 08:40:37 pmI just installed the latest firmware, but it doesn't change anything. Most likely a hardware problem.Quote from: free_electron on January 04, 2017, 09:18:37 pmHeh, in a way it is, but it's good for beginners (e.g. as a first MCU project). I'd dare say it's the DSO138 of power supplies.