Looking for plans for transistorized throttle project from MR

[OldEastRR]

It was one of those "you can do this with parts from Radio Shack" articles that MR ran once in a while back in the day. I have the throttle itself -- a rheostat and a reversing switch in a simple box -- but the wires have been cut and I don't have the original plans on how to wire it to a DC power source (which I also have). Does anybody remember the article, or the general plans about how to wire up such a throttle?

[woodone]

Is this the one that had pulse width modulation?
I think that I still have that one. If you know when MR ran this article , I have MR’s from 1975 to about 2000 hard copies.

[jdcolombo]

Try January, 1986; December, 1986; and/or May 1987.  MR ran a bunch of DIY throttle articles in those two years.

I think the December, 1986 article is the one Old East is looking for, but I no longer have back issues of MR.

John C.

[peteski]

If it is really just a rheostat and DPDT switch, then it should be simple enough to decipher, even without schematics.  Maybe post few clear photos of  all the connections and the combined brain-power of TRW should be able to assist.

Edit:    So the info in the initial post states it is a simple rheostat-and-switch throttle, but the subject line states "transistorized".  My brain does not handle stuff like that very well. 

[jbonkowski]

Here is a snap from the Dec 1986 issue.

Jim

[OldEastRR]

Yes, it does have a transistor & potentiometer, no rheostat. I need to open it up and see if it resembles any of the listed MR projects. it does have a 4-wire cable, tho.

[jbonkowski]

Quick electronics facts:

Potentiometers and rheostats are both variable resistors.

Potentiometers are used to vary voltage and rheostats are used to vary current.

Potentiometers are three terminal devices (used here). Rheostats are two terminal devices.

They are very similar, so the names are often used interchangeably.

Jim

[peteski]

Quick electronics facts:

Potentiometers and rheostats are both variable resistors.

Potentiometers are used to vary voltage and rheostats are used to vary current.

Potentiometers are three terminal devices (used here). Rheostats are two terminal devices.

They are very similar, so the names are often used interchangeably.

Jim

It is all in the semantics.  Varying current (by changing the circuit's resistance) also automatically varies voltage (Ohm's Law).  Potentiometer has 3 terminals. If only 2 are used, then it becomes variable resistor. Just like rheostat.

To me rheostat is an old term, and usually describes a variable resistor with higher power rating than a potentiometer (like several Watts).

In either case OldEastRR, if you take and post some good closeup photos, it should be easy for anyone with basic electronic knowledge to decipher the circuit.

[Point353]

Quick electronics facts:
Potentiometers and rheostats are both variable resistors.
Potentiometers are used to vary voltage and rheostats are used to vary current.
Potentiometers are three terminal devices (used here). Rheostats are two terminal devices.
They are very similar, so the names are often used interchangeably.

I seem to recall that my old MRC power pack used a rheostat to vary the output voltage.
If, as you say, "rheostats are used to vary current", then how did the rheostat in the power pack vary the output voltage?

[nickelplate759]

I seem to recall that my old MRC power pack used a rheostat to vary the output voltage.
If, as you say, "rheostats are used to vary current", then how did the rheostat in the power pack vary the output voltage?

Ohm's Law, expressed as V=IR

The rheostat changes R, so V has to change as well.    The actual V you get depends not only on the power pack dial setting but also I - which is how much current your motor draws.  If I is constant (hint, in a model train motor it's not, but  close enough) then turning the rheostat linearly varies V.

I actually have an old MRC power pack that uses a variable transformer rather than rheostat.  In theory it's insensitive to fluctuations in I

[peteski]

I actually have an old MRC power pack that uses a variable transformer rather than rheostat.  In theory it's insensitive to fluctuations in I

Yes, that was a very good design from the days before transistorized throttles.  The full current was available at any throttle setting with the load (locomotive) not affecting the voltage.  I think some large scale throttles (Lionel?) use this type of design.

The PIKO power pack I used with my TT gauge model trains (when I lived in Poland) also used the variable transformer design.



In this photo you can see part of the circular area of stripped enamel insulation on top of the winding.  The speed control knob had a metal wiper which swept that area, resulting in variable voltage.  Also visible is the old selenium rectifier.  As I said, this was before solid state devices were used in the model train throttles.

[OldEastRR]

Finding this cab and thinking about using it re-activated the memories -- it's not from an MR plan. The thing was built for me by an electronix whiz who got the plan from somewhere. Maybe he whipped it up himself. The hand unit is just a pot (3 terminals, all used) and a SPST. The guts of the thing which was at the base end of the cable tethered to the cab has a power transistor w/ big heat sink, a rectifier diode (square block with four thick leads), a relay (!), and a few resistors all wired to a board. I think this was fed by raw AC power from the huge transformer (formerly from an MRC power pack). It's been separated into pieces and I vaguely remember I had a hand-drawn sketch of how to put it all together but that's long gone.

All I wanted was a simple DC pack for an alternate power source to my DCC layout -- with a DPDT to switch between DC or DCC -- so I think I'll look for some toy train power pack to do the trick. Thanks for the help anyway.

[Point353]

Ohm's Law, expressed as V=IR

The rheostat changes R, so V has to change as well.    The actual V you get depends not only on the power pack dial setting but also I - which is how much current your motor draws.  If I is constant (hint, in a model train motor it's not, but  close enough) then turning the rheostat linearly varies V.

How much current does a voltmeter draw?
Should the output voltage from a rheostat based power pack be different when observing the output voltage with a meter alone as compared to when a loco/motor is also connected to it?

[MarkInLA]

Back in the 1950s when I was about 8-9 years old there was an article in either MR or RMC on 'how to build a transistorized throttle'..
I didn't even have a soldering gun (nor did my father) so I followed all the instructions to the letter and used a tube of liquid solder..
IT WORKED ! My few HO locos had 'momentum' for the first time when starting and stopping !! Man, what a thrill that was !
Maybe OPer could hunt down this Mag issue as well..all old school components....M

[peteski]

Finding this cab and thinking about using it re-activated the memories -- it's not from an MR plan. The thing was built for me by an electronix whiz who got the plan from somewhere. Maybe he whipped it up himself. The hand unit is just a pot (3 terminals, all used) and a SPST. The guts of the thing which was at the base end of the cable tethered to the cab has a power transistor w/ big heat sink, a rectifier diode (square block with four thick leads), a relay (!), and a few resistors all wired to a board. I think this was fed by raw AC power from the huge transformer (formerly from an MRC power pack). It's been separated into pieces and I vaguely remember I had a hand-drawn sketch of how to put it all together but that's long gone.

All I wanted was a simple DC pack for an alternate power source to my DCC layout -- with a DPDT to switch between DC or DCC -- so I think I'll look for some toy train power pack to do the trick. Thanks for the help anyway.

You provided lots of clues here.  The 4-terminal diode-rectifier is a "bridge rectifier".  Assuming the cord has 4 wires, I suspect that 2 of those 4 wires attached the bridge rectifier will be where you would hook up the voltage source.  Either AC or DC should work (the bridge rectifier will take care of the polarity).  Any 12V wall-wart should work.  Then the other 2 wires from the handheld throttle should be the output to the track.  I'm also assuming that the switch (for changing direction) is a DPDT switch (6-terminals).  The pair of the track output wires should be connected to that switch.  That should give you that DC throttle you are seeking.