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Author Topic: Advice on PWM Controllers  (Read 305 times)

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Offline PostModN66

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Advice on PWM Controllers
« on: November 17, 2017, 02:57:17 pm »
I have been lucky enough to acquire a couple of MedVend PWM controllers (thanks @Phoenix) and I am hoping to learn from other people's knowledge if possible before launching on a programme of experimentation!

Anyone used one of these, and if so any general tips?
It needs a DV supply; any problems foreseen if I rectify 16VAC with a smoothing capacitor?
There are jumpers on the circuit board allowing you to select the frequency between 60Hz and 290Hz - what would be best?

On a related note, what would happen if I tried to run a Gaugemaster hand-held controller from a 12VDC supply?

Thanks in advance for any advice

Cheers Jon  :)
“We must conduct research and then accept the results. If they don't stand up to experimentation, Buddha's own words must be rejected.” ― Dalai Lama XIV

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Offline ntpntpntp

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Re: Advice on PWM Controllers
« Reply #1 on: November 17, 2017, 04:18:00 pm »
Not familiar with MedVend stuff.  As far as PWM goes, higher frequency is usually better. a lot of DCC decoders drive the motor at 16KHz or even 32KHz.   Lower frequencies can result in a more audible growl or whine.

16v AC is your typical model railway transformer or power pack aux output, as commonly input to controllers.  Just remember that rectified/smoothed 16v AC will be quite a bit higher than 12V DC. A controller designed to take AC will also be designed with internal voltage drops etc. in mind so the final output should be nearer the nominal 12V standard for our trains.  On the other hand a controller designed for 12V DC input is likely to pass most of any higher voltage input straight out to the track?   

Some say N gauge only needs around 9-10V anyway. Who regularly runs their locos at full tilt for long periods?   

Will the Gaugemaster work on DC? It might. Depends on the circuitry.  Quite possibly it rectifies the incoming AC anyway and would therefore work on DC. Certainly if it has a switch to change direction rather than a centre-off knob, as the direction switch is probably just a DPDT on the output.   Try it - it's not likely to damage it.
« Last Edit: November 17, 2017, 04:22:30 pm by ntpntpntp »
Nick.   2016 celebrating the 20th anniversary of "Königshafen" exhibition layout!

Offline PostModN66

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Re: Advice on PWM Controllers
« Reply #2 on: November 17, 2017, 04:27:27 pm »
Thanks ntpntpntp

In the meantime I have found a few threads such as this epic:

http://www.ngaugeforum.co.uk/SMFN/index.php?topic=30458.msg409771#msg409771

...which wants of dire consequences.....which I will bear in mind.   I had the idea that the MedVend ones were somehow better than others for N Gauge (as they are sold by the 2mmFS Society and on Z Gauge sites), but reading the instructions I am not sure why.  The frequency is not high like for DCC.

The instructions say, "choose the correct power supply voltage to avoid the overheating of engines" and for n recommends 9-12VDC.  I might try at first with a 9V battery.

...thinks - could I use a voltage regulator on the input like I have used for flashing tail lamps?  Or would a simple dropper resistor suffice?

Cheers Jon  :)
“We must conduct research and then accept the results. If they don't stand up to experimentation, Buddha's own words must be rejected.” ― Dalai Lama XIV

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Offline ntpntpntp

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Re: Advice on PWM Controllers
« Reply #3 on: November 17, 2017, 05:00:21 pm »

...thinks - could I use a voltage regulator on the input like I have used for flashing tail lamps?  Or would a simple dropper resistor suffice?

Cheers Jon  :)

Voltage regulator definitely. More stable and more efficient than just using a resistor.
Nick.   2016 celebrating the 20th anniversary of "Königshafen" exhibition layout!

Offline Nigel Cliffe

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Re: Advice on PWM Controllers
« Reply #4 on: November 17, 2017, 05:12:30 pm »

There are several different MedVend designs.   The one with selectable PWM is, I think, a panel mount.  Its not the same as the one which the 2mm Scale Association sold(*)

In terms of power supply, I'd use a Switched Mode power supply (they are cheap **), not the 12v from an old transformer controller.   If determined to use a transformer, then voltage regulator and decent smoothing capacitors to get the voltage tamed to something which meets what's needed. 

A 9v battery on the Medvend from the 2mm SA shop gives nice slow/medium speed control, but lacking top-end if running mainline trains.   


(* the supplier,  www.medvend.hu seem to have disappeared, which is a shame ).
(**  example, though many other sources
https://www.rapidonline.com/powerpax-uk-sw3113c-2-mini-desktop-smpsu-switch-mode-power-12-25w-12v-dc-1-2a-85-2943
)

Offline PostModN66

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Re: Advice on PWM Controllers
« Reply #5 on: November 17, 2017, 05:21:14 pm »
In terms of power supply, I'd use a Switched Mode power supply (they are cheap **), not the 12v from an old transformer controller.   If determined to use a transformer, then voltage regulator and decent smoothing capacitors to get the voltage tamed to something which meets what's needed. 

Thanks Nigel,

I have a big box in the loft of power supplies of various voltages that I have saved from computer monitors, printers, laptops etc.   Is it likely that these are "switched mode" and therefore suitable?

In fact I have one bought for powering guitar effects pedals which are 9v (as an alternative to a PP3 Battery), which would be perfect if indeed "switched mode"

Cheers Jon  :)
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Offline ntpntpntp

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Re: Advice on PWM Controllers
« Reply #6 on: November 17, 2017, 06:45:22 pm »
In fact I have one bought for powering guitar effects pedals which are 9v (as an alternative to a PP3 Battery), which would be perfect if indeed "switched mode"

Cheers Jon  :)

Check the current capability? Some of the effects pedal PSU's I've seen are quite low, eg. 100mA.  I'd look for  something 500mA + to power a controller. Most modern N gauge locos draw a lot less than that, but older ones can need a bit more.
Nick.   2016 celebrating the 20th anniversary of "Königshafen" exhibition layout!

Offline PostModN66

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Re: Advice on PWM Controllers
« Reply #7 on: November 18, 2017, 01:07:41 pm »
Found a 9v power supply rated at 500mA, and it even says "switching" on it so I guess it is OK.

Here is a video showing the first trial run.  I used a 158 chassis as 1) I know I can get a replacement motor if I burn it out and 2) no lights to blow.  Here is a short video...

http://youtu.be/llTkpUIEXu0

Any thoughts?

Cheers Jon  :)

PS: The video was the first one I have taken on my iPhone and it took hours to upload to YouTube.  Any thoughts on why that is would also be appreciated!
“We must conduct research and then accept the results. If they don't stand up to experimentation, Buddha's own words must be rejected.” ― Dalai Lama XIV

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Offline keithfre

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Re: Advice on PWM Controllers
« Reply #8 on: November 18, 2017, 01:13:51 pm »

Offline PostModN66

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Re: Advice on PWM Controllers
« Reply #9 on: November 18, 2017, 01:18:45 pm »
http://youtu.be/llTkpUIEXu0

"This video is unavailable".


I think there was one more click needed - hopefully OK now. Cheers Jon  :)
“We must conduct research and then accept the results. If they don't stand up to experimentation, Buddha's own words must be rejected.” ― Dalai Lama XIV

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Offline ntpntpntp

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Re: Advice on PWM Controllers
« Reply #10 on: November 18, 2017, 01:35:43 pm »
That looked pretty reasonable and smooth, and actually that top speed looks ok to me.  I don't run my models much master than that.      The buzz is typical.  Presumably you can turn the controller all the way off and it goes silent?  The buzz wouldn't worry me particularly, though if you have higher frequency options use those in preference.
« Last Edit: November 18, 2017, 01:40:03 pm by ntpntpntp »
Nick.   2016 celebrating the 20th anniversary of "Königshafen" exhibition layout!

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Re: Advice on PWM Controllers
« Reply #11 on: November 18, 2017, 01:39:42 pm »
That looked pretty reasonable and smooth, and actually that top speed looks ok to me.  I don't run my models much master than that.      The buzz is typical.  Presumably you can turn the controller all the way off?  It wouldn't worry me particularly, though if you have higher frequency options use those in preference.

Yes, I was just deliberately trying to illustrate the sound by selecting almost off, though it didn't come through on the video.  Would the higher frequency be less potentially damaging?   The higher pitched wine sounds more sinister   :o

As you can see in that bit I inadvertently achieved a crawl speed, though I don't think you can achieve it at will.

Cheers Jon  :)
“We must conduct research and then accept the results. If they don't stand up to experimentation, Buddha's own words must be rejected.” ― Dalai Lama XIV

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Offline ntpntpntp

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Re: Advice on PWM Controllers
« Reply #12 on: November 18, 2017, 01:50:55 pm »
As you can see in that bit I inadvertently achieved a crawl speed, though I don't think you can achieve it at will.

Really? That's kind of the whole point, using PWM to give brief pulses that give the motor regular "kick"s to overcome the "sticktion" of the motor but keep the speed slow and even.   The behaviour very much depends on the model of course.

I think that thread you found goes into the details of the benefits and drawbacks of PWM frequency etc. 

Personally, I still use my venerable KPC "exhibition grade" switchable feedback controllers from the 80s.  I've not measured the pulse frequency, but I don't think it's particularly high. Must put a 'scope on one some time.  IIRC "exhibition grade" was something about keeping the maximum voltage a bit lower and thus improving the slow speed control over the range of the knob.  I do find some motor types (Particularly Kato) are happier with the feedback circuit switched out, and it's mostly the older models that respond better with feedback on.
Nick.   2016 celebrating the 20th anniversary of "Königshafen" exhibition layout!

 

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