Because all Boys (and some girls) love Trains



This simple design will provide you a positive indication when you’re approaching the maximum current capability of your throttle (you really don’t want to exceed the throttle’s capability…trust me). You only need to add an 0.1 ohm resistor in series with the throttle output (but located before the direction control), so you lose only a minute amount of throttle voltage. You can see examples of this resistor’s location on any of the VST throttle. Sadly, this circuit is not usable with commercially-made throttles (eg, MRC) unless you’re willing to get inside the case and make the modification (not recommended if you’re still under warranty…and unless you really know what you’re doing).



Construction is straightforward and forgiving…although I do suggest you socket the IC and make all solder connections before inserting said IC. If you can’t lay your hands on an LM358, you can easily use an LM324 (the quad version); just note the change in power and ground pins, and connect unused pins to ground. Make your Vcc (DC power for the circuit) connections at the INPUT to the regulator in the VSTs and to a fixed and filtered DC voltage point (10-20 volts) in other designs. Keep in mind that this is NOT a protection circuit — it’s merely an indicator; designs using an IC regulator (such as the VSTs) enjoy the protection built into the IC. Commercial throttles will also have built-in protection. The “current limit” (see below) should be set just below the rated current of your throttle so that the indicator tells you that you’re approaching the maximum rating.

When you’ve gotten everything connected together, you’re ready to make the current threshold adjustment: connect your DVM or VOM to the “Vref” point in the circuit (the red dot) and set the “Current Limit Adjustment” pot to one volt per one amp of limit (eg, for a 1.5 amp limit, set Vref=1.5V; for a 3 volt limit, make Vref=3V). For current limits above 5 amps (ie, Vref>5V), I’d suggest deleting the 4.7K ohm resistor attached to the pot. To make more precise settings, you can add resistors on both sides of the pot.

Once you set Vref, I’d suggest checking the circuit by setting the throttle voltage to 6 volts or so and connecting known loads to the output (no trains, just resistors); check by applying a current load 10-20% below the limit and verify the indicator is not illuminated; then apply current load 10-20% above the limit and verify that the indicator IS illuminated. Make minor tweaks to Vref as needed to compensate for parts tolerances. Sorry, but you’ll need to use Ohm’s Law to find these resistor values and don’t forget to ensure an adequate power rating for your load resistors.

One might ask, “When would I expect to see an overload condition?”…and it would be a good question. Here are several possibilities:

  1. You’ve just added a 4-loco consist to your road.
  2. A long consist starts climbing a steep grade.
  3. You’ve just added 36 freight cars to a lengthy mixed train.
  4. A loco stalls on the track.
  5. A cast-metal scale caribou grazing on a hill falls onto the track.
  6. That slow-moving freight clears the yard and you give a big boost to the throttle setting.

If you do get an overload indication, stop the trains and fix it; you really don’t want to operate this close to the limit of your throttle.