Future Kit PWM DC Motor Speed Control Model FA804 12vdc, 20w.

[mmagliaro]

Max,

I operate with DCC and use the old MRC powerpacks only for testing non-equipped locomotives. I wanted to use a PWM control because the old powerpack rheostat output produces a high start speed and cannot be turned down enough to test run at a crawl speed. The voltage remaining on the rails at stop is not a problem since it is not enough to move the loco, but Is the hum created by the PWM harmful to the motor?

I have gotten the 150 pF capacitors and will install them if your test is successful.  If it is necessary to replace the pot and some other parts, it may be better fot mè to just buy another controller. It looks like many inexpensive models are available.

I totally agree.  I would not bother replacing multiple components.  There are plenty of PWM controllers on eBay that operate at higher frequencies that won't cause the hum problem.    I doubt that the motor sitting there humming at 264 Hz at throttle off will cause a problem.  The amount of power being dissipated in the windings is extremely low, so there should be no heat or brush wear.

Now that I understand what you are trying to do, I still think it is important to use a higher frequency PWM generator than the one you have.  DCC decoders use a much higher frequency PWM to control the motor, and I assume you want to do a comparable test.

UPDATE: KIT ARRIVED IN THE MAIL - so I can start messin' with it.

I am going to test that circuit with various components, and put its output on a scope, when it arrives.  But meanwhile, I might be able to just cobble the same thing together on a breadboard.  I have 555 timers, and the other parts, lying around here.  I just want to make sure that the circuit actually oscillates and produces a nice square wave at the higher frequency,  and looking at it on a scope is the only way to know for sure.

[mmagliaro]

DON'T CHANGE C2 !!!

I got the kit.  With the included parts, the frequency is 250 Hz, and a motor in a conventionally powered DC loco (I was using an Intermountain F unit) makes a constant vibrating whine at throttle off.  As the throttle goes up and the duty cycle increases,
the perceived audio tone of the hum rises.

now... ONLY change C1.  If you look at the schematic, you can see that only C1 is in the R/C paths that affect the timing
of the 555 chip.  C2 needs to stay at .01 uF.   Pin 5 can be used to affect the timing of the chip by applying a voltage there (through a resistor).  But when you aren't using that, a cap connected to ground acts as a small filter on the chip's internal voltage.  It keeps the timing even and gets rid of noise.

Changing C1 to 150 pF raises the frequency to 16.6 kHz in my test sample.  And my 150 pF cap actually meausures 180 pF (little ceramic caps like this are not usually better than +- 20% anyway.)

The hum goes away, and the control of the engine feels better.

HOWEVER... the average voltage on the track at zero throttle rises from about 1.5 to 2.5v.  My test loco still does not move, but
this may be an issue for you.  This circuit just isn't designed well enough to get closer to 0% duty cycle, especially at higher
frequencies.  Now... by altering R1, R2, R3 to lower values, you can get closer, and that zero throttle average should drop
(I'd try 1/10th of what's in there, so R1=1k, R2=470, R3 = 3.3k).  But be aware that will also affect the frequency.
The sheet that comes with the kit has the equation to compute how changing those will affect the frequency.  It is significant.
If you cut them all by 1/10th, the C1 value should probably go to more like .001uF or .002uF.   150 pF will be too small and
the frequency will go too high.

FYI, I tried it with a 500 pF C1. The frequency was about 3.5 kHz.  It had no hum at zero throttle, but definitely had an annoying
motor whine once the engine started moving.

There are better cheap PWM modules out there.  I think if I were going to be using one of these for testing, I'd look for something with a high frequency and better zero cut-off out of the box.

[Trnsrus]

DON'T CHANGE C2 !!!

...Changing C1 to 150 pF raises the frequency to 16.6 kHz in my test sample.  And my 150 pF cap actually meausures 180 pF (little ceramic caps like this are not usually better than +- 20% anyway.)

The hum goes away, and the control of the engine feels better.

HOWEVER... the average voltage on the track at zero throttle rises from about 1.5 to 2.5v.  My test loco still does not move, but this may be an issue for you...

I replaced C1 with a 150 pF capacitor which eliminated the motor hum in the N scale dc test loco. However, the track voltage at zero throttle now measures 3.9 vdc. and locos do not come to a standstill. Since I don't have a supply of resistors or other parts (besides the remainder of a pack of 25 150 pF caps!) I think I'm better off buying a better suited PWM control than trying to tweak this one. I welcome recommendations for currently available PWM controls proven suitable for N scale use. Thanks to all who have contributed to this discussion.

[Sumner]

https://www.therailwire.net/forum/index.php?topic=58728.0

Sumner

[Trnsrus]

https://www.therailwire.net/forum/index.php?topic=58728.0

Sumner

Thanks for the suggestion for an alternative PWM control; I've ordered a pair from Amazon. I was reluctant to give up on the Future Kit version because I had already shoehorned it and a mini dpdt toggle into a tiny project box. The controller was factory assembled and I foolishly failed to test it before starting the project.