DC Motor Control - Page 1
Forum section: electronics / projects

author: jaxbird
author_rank: Frequent Contributor
timestamp: January 20, 2014, 07:30:56 am
content: I've got a new spindle motor that runs on 24Vdc, and I want to add speed control. Motor is around 4A at 24V, so just below 100W.Designed this little circuit that I believe will do the job:PWM should be around 20kHz and adjustable from near 0% to near 100% duty cycle. The buck output is to run the motor as quiet as possible.

author: kripton2035
author_rank: Super Contributor
timestamp: January 20, 2014, 07:35:31 am
content: what is the purpose of T1 and T2 ?

author: jaxbird
author_rank: Frequent Contributor
timestamp: January 20, 2014, 07:44:13 am
content: Quote from: kripton2035 on January 20, 2014, 07:35:31 amThey just buffer the 555 output when driving the gate, for reasonably fast switching.

author: muvideo
author_rank: Frequent Contributor
timestamp: January 20, 2014, 12:41:15 pm
content: Hello, if I understand correctly your circuit, you are driving Q1's gate between 12V and 24V.With a N-mosfet it will not work, you need it's gate to be driven at an higher voltage than it'ssource. Either use a P-mosfet or put the N-mosfet between out- and ground and supply drivercircuitry between 0 and 12V using a 317.Also, the DC motor will have it's own inductance, so no need to build a full buck converter,use motor's own inductance, remove L1 and C4 (this one can be also simply lowered toa smaller value of few nF) and put D3 between motor's leads to provide a path for thecurrent.

author: jaxbird
author_rank: Frequent Contributor
timestamp: January 20, 2014, 01:26:47 pm
content: Quote from: muvideo on January 20, 2014, 12:41:15 pmHello, yeah, you are right that it drives the gate between 12 and 24V (because of maximum 555 supply voltage and Vgs on the PMOS). The label for Q1 is wrongthanks for noticingit should have said MTP50P03HDL (not particularly chosen for this purpose, but I got a few of them) the symbol and footprint is correct thoughThe reason I want the inductor and cap on the output is because I have bad experience with whining and screaming motors even with highish PWM frequency, better safe than sorry on a one offCheers

author: jaxbird
author_rank: Frequent Contributor
timestamp: January 22, 2014, 02:20:44 am
content: Made a PCB for the little motor controller:I did make a couple of minor changes from the original schematic. First lowering the 555 voltage to around 7V instead of 12V. This makes it a bit more flexible as it will now run from 10V to close to 30V input.Another change is since I don't have any suitable inductors capable of 4A+, I instead use 2 (3A rated ones) in parallel.And assembled:Did a quick test (just with a small load) and it seems to work as intended. I'll do a bit more testing before hooking it up to the actual psu and motor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 22, 2014, 02:49:53 am
content: Follower's not really necessary from a 555, the output stage is notoriously beefy already (notorious because of the heavy bypassing required..). A logic-level PFET is *not* a good idea paired with something that already doesn't pull up all the way, though -- the 555 will only do V_OH ~ Vcc - 1V, and that follower burns up another Vbe. Yes, the 5k pullup helps, but it's reeeeeeeeally slow, and on a logic level FET, you need oomph all the way, otherwise it'll just drool all over (which you might not see from the voltage waveform, because it's inductive switching).Might be worth a ceramic cap (say 1uF 50V X7R) at the end of that trace hooking around the transistor, the source pin I believe, to ground (preferably the ground side of D3). Having ground plane fill is excellent, one nice thing about milled boards I think, laziness pays off. It is a bit thin in places, might be worth patching across the narrow spots just in case, but nothing serious I think, all the high current is flowing in its own region.Think that's all I have to critique about it. Looks nice, reasonably tight layout (only thing to improve that would be a two layer design, but who cares, right?), clean and compact.Tim

author: jaxbird
author_rank: Frequent Contributor
timestamp: January 22, 2014, 04:21:47 am
content: Quote from: T3sl4co1l on January 22, 2014, 02:49:53 amHi Tim, appreciate the feedbackI added the 2 transistors because I'm using the 555 discharge pin output (open collector). Not sure how much it can actually sink, but probably at least 100mA. Still it would require a beefy pull up without the transistors. Agree on the FET choice (MTP50P03HDL) being far from ideal, but I have a few of them and hope it will switch reasonably well. I'll try load it up and get some captures.Yeah might need to add a few ceramic caps here and there. Got a good selection of both through hole and 805s.It is a one off thing, so I'm not too concerned about botching in a few components to improve performance. Two layers would have been nice, but harder to make, so lazy winsCheers

author: jaxbird
author_rank: Frequent Contributor
timestamp: January 22, 2014, 04:40:22 am
content: Quick test with a 20W/12V light bulb load. Works well, no smoke so far:Only problem is that the pot goes the wrong wayBut that is easily fixed, just need to flip the two 1n4148 diodes. It was a 50/50, win some, loose some.

author: jaxbird
author_rank: Frequent Contributor
timestamp: January 22, 2014, 07:14:41 am
content: More load testing:After running like in the picture for some time (10-15 minutes) the inductors get pretty hot (up to ambient +40C), well one much hotter than the other. Looking at them, I think I managed to get 2 different types with different series resistance soldered in. I better check and correct with 2 that are fairly closely matched.Anyway, the PMOS doesn't get hot, maybe 5-10C over ambient, so I believe the switching is acceptable.Another thing I noticed is that the PWM frequency is somewhat lower than what I expected, going from 10kHz to about 17kHz (1% to ~100% duty cycle).

author: jaxbird
author_rank: Frequent Contributor
timestamp: January 22, 2014, 01:45:57 pm
content: Made a couple of minor adjustments, fixed the diodes so the pot goes the right way. Also changed the inductors to a better mached pair (they were 2 different types, same uH, but 25% difference in resistance).And finally I decided to switch the pot from 20k to a 10k because the PWM frequency was a bit on the low side. Now PWM is between 16kHz and 30kHz and the inductors no longer get really hot, plus the pot goes in the right directionHere a capture of the PMOS drain with a 5R load (18V supply as my electronic load is only designed to work with up to 20V)Everything looks pretty good, ready to hook up the actual motor.

author: Zero999
author_rank: Super Contributor
timestamp: January 25, 2014, 06:27:59 pm
content: Is there any reason why you're controlling the high side? It would've been more efficient to switch the negative with an N-channel MOSFET.It's also possible to switch the high side with an N-channel MOSFET with boot strapping. Attached is a circuit which uses bootstrapping to power the 555 and drive an N-channel MOSFET. The disadvantage is the duty cycle can't go up to 100% as the capacitor needs to remain charged. You'd need to add a voltage regulator with a low quiescent current to power it from 24V.