What would this motor circuit do to a 5V SMPS for capacitive loads ? - Page 1
Forum section: electronics / repair

author: MathWizard
author_rank: Super Contributor
timestamp: September 21, 2025, 04:12:07 pm
content: I have this brushed DC motor that ran off a 7.2V battery and I'm converting it to wired power. I scoped the current with a 0R1 resistor, the motor mosfet can do a few different PWM, like 75% and 90%, at 15kHz, and slower ones around the 2Hz range.Probing right off the wire from my bench PSU, set to 5.1V and 3.2A max, it looks pretty bad. IDK how much cleaner it would look from a battery. There are lots of voltage spikes over 16V, I'm guessing your average 5V forward converter probably uses 16V output caps. The Vcc voltage goes well below 5V all the time too, down around 1V. And the current spikes are very high too, up around 7-8A on startup. So it may trigger OVP/UVP and OCP, depending on how some SMPS filter senses it's output.I've seen circuits that put a current limiting resistor in series with the motor circuit, and use another mosfet to short the Rlimter once the mototr gets up to speed and has enough back EMF.I was trying to make a basic BJT diff-amp circuit to rob voltage from the motor's MOSFET gate, but it's not the RMS voltage or current that may be an issues, it's the peaks and dips, when the gate turns on/off. Even the startup current itself may not be that bad, it's only over 5A for a few milliseconds.The motor has a reverse biased diode across it, and some 0806 cap of some size. What else could I do to improve the spiking ? And is it likely to damage or mess with an average 5V SMPS ?In the pic's below, Red is the 0R1 voltage, it's below the motor MOSFET, and above the (-) PSU wire, and Green is Vcc from PSU(+), attached to motor+. I've been measuring and plotting stuff, I worked out the RPM as it accelerates, now to try and get LTSpice to match some motor models.

author: Swake
author_rank: Frequent Contributor
timestamp: September 23, 2025, 08:41:19 am
content: Mate, you're complicating life.If it asks for 7.2 VDC (= battery) then feed it with a 7.2 V power supply not 5 V. If the power supply is crap cheap stuff then eventually put a big cap on it, something like 1000µF per amp the motor pulls and case closed.

author: MathWizard
author_rank: Super Contributor
timestamp: September 23, 2025, 06:33:08 pm
content: IDK why the battery died, but I was hoping I could use some old SMPS in my junk box, and the motor is overpowered for my needs, 5V is fine.Now for making things more complicated, I could use a 12V SMPS and try making a 5V higher current supply, either like one of my bench PSU's, or like an E-load I'm working on. I have heatsinks. But yeah if I have to order any amount of parts like caps (I have most everything else)....price-wise it's just aswell to order some PSU for motors.But as a learning challenge, I'm trying it in LTSpice.Maybe it wouldn't be that hard to make a simple, largely discrete, buck converter from 12V to 5V, that can work with this motor. I've dabbled with them a little bit. I have a bunch of donor caps from a Plasma TV, and other TV's/etc.I have a pretty good electrical and mechanical model of it LTSpice, and just adding 100uF on the Vcc rail, and some same cable resistance and inductance, after it, I can get a fairly similar ugly waveform on Vcc. I haven't tried adding models for brushes and multiple winding's or anything, that's ok.