Pulsed Width Modulation Controllers

Hi Folks,

Thanks to Bromley Libraries I've got my hands on a copy of the "Complete Book of Model Railway Electrics"/Roger Amos. After giving it a quick scan I'm quite astounded by some of the very simple, easy and workable ideas contained therein ... sidings protection being a case in point, took one look and thought "Doh!" why didn't I think of that etc etc ...

Anyway I was taking a closer peek at the circuit diagrams for pulsed width modulation controllers and thought "I can do that" however after a bit more reading it transpires that PWM controllers can fry motors, in effect giving great slow running, super smooth starts but only if you don't mind the smoke. :-)

baring in mind that the book is the best part of 20 years old I was wondering how well modern motors perform with this kind controller. If using a PWM controller a good idea generally or if there was a better solution.

Comments anyone before I commit myself to a trip to Maplins?

Reply to
Chris Wilson
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I have built the PWM controller from the R A Penfold book and found it works quite well. I find the some of the Amos designs sometimes need a bit of work to get the component layout right.

Try MERG or

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Reply to


The motors that a PWM controller will probably fry will be coreless motors. These motors definitely do not like it up 'em :-)

Basically, the armature of a coreless motor has very little mass, therefore there is little momentum and the armature can come to a standstill in between pulses of a PWM waveform working at low frequency when the mark/space ratio is low. This means that the brush gear has to cope with starting current on every pulse and the motor life is drastically reduced.

You should also control the continuous current that a coreless motor can take - about a third of its maximum current is the usual for long life. Most PWM controllers I have seen don't have any way of limiting the current output to the level that your average coreless motor is looking for - say 100 - 200mA.

Also, if your PWM controller also incorporates feedback and reads the back EMF when the pulses are off, then it sees a coreless motor with an armature stopped, or nearly so, and delivers a hefty boot up the backside to get it moving - even worse for the brush gear.

So don't use PWM if you've got any Portescap, Maxon, Canon or Faulhaber motors in your locos.

DCC does use PWM output from a decoder to control a motor's speed, but some decoders are designed to use a high frequency so that any adverse effect on a coreless motor is negligible. For straight DC working, the same would hold, but most PWM controller designs use mains frequency for their waveform frequency and that's the low frequency that does the hatchet job.


Reply to
Jim Guthrie

DCC loco decoders all work with PWM.

A lot depends on the frequency of the pulse - big motors like Athearn work best at 40 pps, Marklin at 60 pps, old Triang X04s at 100pps, N scale 2-300pps. It seems to be dependent largely on the armature diameter. Pulses based on mains frequency are 100 pps. The higher the pps the closer operation is to smooth DC. Basically, you'll find that low speed control is much better on PWM but the loco will emit a note equal to the pulse frequency (middle C is 442pps (I think)) The volume will be dependant on the wear in the motor bearings.

Regards, Greg.P.

Reply to
Gregory Procter

NOt aware of that one, do you have a title? Cheers.

Yes, I can see that but with a bit of fettling. In terms of performance (rather than ease to build) which do you rate, Amos or Penfold?

Cheers, been there before, very interesting.

Reply to
Chris Wilson

I've no coreless motors ... yet

Ringfields etc then should be OK then? ... assuming that I don't leave them running for long periods? I read somewhere that 20 mins at a stretch is just about the limit, which if true gives me a wide safety margin as the layout is end to end with short runs.

Helpful post, helped put the issues into context thanks.


All block control, no DCC :-)

Reply to
Chris Wilson

Sorry, should have said, I don't use DCC, still very helpful post especially in view of the figures above, could well be useful if I later try my hand at HF coach lighting.


Reply to
Chris Wilson

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