My MRC Ampack is now cranking along with the
finest 1960s technology available. This is the finished
First, though, I had to figure out why it wouldn't work.
Everything seemed fine until I soldered in the TIP-120,
and all of a sudden I couldn't get anything but full power.
Turns out I had the protector diode backwards.
Finding that out was a minor lesson in itself, but now
everything is fine.
I did reverse the rectifier polarity and ground the center
tap and case. The transistor collector is grounded to the tab, which
is bolted to a homemade aluminum
heatsink, and bolted to the case, with compound at
both interfaces. I suppose I could have sprung for the
insulated mount, but since I am planning to replace the
cord with a grounded one, in order to ground the case,
I figured this would be fine.
Control is good. I dug out my bad actors and tried
them out. The half-wave pulse power is very helpful,
and it seems to help my very worst
locomotives the most. My war-weary Life-Like
Dockside, with a bent motor shaft, went from a
noisy 12 SMPH starting speed to a (still noisy)
There is a slight surge when switching from pulse to
full-wave, but it's not a real problem.
Control was equally good with my remotored
Teakettle, whose tape-player motor seems to
draw very low current. With pulse power, this
crept along at something between 1-2 SMPH.
Motor heating did not seem like a problem.
The transistor got warm after all the testing at
low throttle settings, but not too hot to touch.
The circuit does bring output to zero, but there is
indeed some wasted space at the low end of
throttle range, so I might add some diodes to my
next attempt, or add them later to this one,
as discussed in the other thread.
After some testing with locomotives, I loaded the
throttle with a 10k resistor, then the ~10 ohm
variable resistor salvaged from the Ampack, and
determined that the output was about 0-15 VDC
at no load, dropping to 0-12VDC at 1.2 A. This
I'd recommend this project to any novice. It was
fun and easy, and the cost was low, even
using Rat$hack components.
Next project: replacing the selenium rectifiers
and adding pulse power to my ancient KF
President, a box of track and a solid-state Ampack.
14 years ago