Large Scale Central

Testing for Amps? Testing a power pack?

A Question. How can I test for amps on my track? Is it possible that my Crest switchable supply is dying? The Crest has a 23v 12amp output. I have it connected to a Revo base unit that puts out PWM which was messing with my older sound system and lights so I bought a converter to put it back to Linear. I have tested by bypassing the converter and going to PWM and the trains still act sluggish.

The biggest power user is my track cleaning rig I send out before running any train. It has a USAT S4, a NW2, a custom rail broom and a custom car dragging 3 cleaning pads. This train has been slowing down in odd spots. At the far end of the RR, the furthest away from the power pack there is a drop in voltage by maybe one volt. I have several power feed points spread over the 800’ of track.
My other trains are also slowing but they do not pull as much power as the 5 motor Mow train.

There is a meter on the Crest unit but it stopped working years ago.

Without going too deep into the electronic aspect what is the best way for me to test my power pack to make sure it is still putting out enough amps?
I have a basic multi meter.
Is there a simple diagram I can use? I don’t want to blow anything up.

If your multimeter has an amp setting you can use it.

or you can upgrade your test kit

Thanks Bob.
I watched that video this morning when I was first researching this and still had questions. In the case of testing the track with a train on it and running I would attach the multi meter leads to the track or using jumpers attach them to the power pack terminals?
The video also says to connect the wires in series. I’m still trying to wrap my head around the series vs parallel thing.

It would be a lot easier to check with one of those clamp on units.

your meter needs to be hooked between one feed wire and set to amps.

He can also go down his line to find were he’s losing power I believe…

Be SURE that you use a range on the meter at least as high as the expected draw or you’ll blow a fuse on the meter (or possibly damage it). In your case that would be at least 12 amps. Most multimeters don’t go that high.


Sometimes a water analogy can be useful to get your head around parallel and series, voltage and current.

Think of your powerpack as being a water tower. Voltage is analogous, sort of, to the height of the water above the outflow pipe (the wire). The water flows through the pipe into the ground (the pipe is the positive terminal, ground the negative). This is an analogy so don’t be surprised if it breaks down when we start to talk about pulsed vs AC vs DC power. Just pretend for now.

So you can measure the voltage at the source, or indeed any way along the wires. Kind of the height above the ground in this analogy.

The electrical current is similar to the water flow (current in a stream). You can’t measure that by measuring “across” the wires lilke you do for voltage. Instead you need to measure the flow within the pipe, or wire for electricity. So you want to break the flow and insert your measurement into the flow of water as it goes by. That would be “in series” in this case. Disconnect the plus terminal, put the positive probe of your multimeter there, then put the other probe connected to the wire you disconnected. This means that the “current” will flow through the multimeter and it can measure it.

Note that this only works when the circuit is complete and the “water” is flowing. So this will happen only when you are running a locootive.

As Steve said, be careful not to put more amperage through than your multimeter can handle. Not sure how you will be able to figure out how much before hand. Start with low voltage on the power pack and slow ramp it up, keeping an eye on the amperage as you go.

Anyway, hope this helps give you more intuition.

Good luck!

edited to add a wikipedia article:

A meter with 10 amp capability is fine. You don’t need the meter to read the full “capable” output of the throttle just so long as the load (i.e., the trains on the track and coach lighting) don’t exceed 10 amps…, and in all probability they won’t. You would need to be running like four engines of more at once to reach 10 amps.

Lets keep this simple. Disconnect the positive wire that goes from the throttle to the track. Attach the meter’s positive probe to the throttle where you just pulled the wire from. Then put the wire that you pulled off on to the meter’s negative probe. Set the meter to 10 amps and start your trains. They should run and the meter should display the current being drawn in amps.

Thanks for all the info.
I connected my meter up, it is an older one that is good for up to 20a.
The findings were interesting and confusing. I set up the individual pieces of the MOW Railbroom train on my test rollers and put 12 volts to the track. Here are the results.

USAT NW2 .59 A
USAT S4 .47A
Railbroom 1.37A
I also tested a Stainz .33A

When I turned on the sound in the 2 diesels the amp draw nearly doubled but I do not run the sound when they are working the railbroom.

This is the odd part. I thought when this train went all together outdoors that each of the amp draws would be added together but that was not the case. I sent the train a round and saw the amp meter read between 1.5 and 2.2 Considering that my Crest switchable transformer is good for up to 12amp I would say that running 5 motors is well within its capacity.

I checked the voltage on the track closest to the power and it was about 12v. I went to the furthest reaches of the RR and it dropped a lot more than the last time I checked it down to 9. I think I found my problem. I have several jumper lines that attach to the track in key spots and then they leap frog to the next spot. When it gets a bit warmer I will have to clean them up and check the rail joints because I think a few of those could also be improved. I will have to make up a pole with the volt meter attached so I can walk along and touch it on both sides of the joints.

When the train was at the furthest point that is when it was pulling the 2.2A .

Does this make sense that the 5 motor train was only pulling up to 2.2A ?

My Railbroom MOW setup

If the voltage drops on a section of the layout, the amp draw goes up there. You found that out!

And, you correctly surmise that one or more of your jumpers or rail joints are the culprit.

I need to do similar work this spring. I run battery in all my locos, but use the track as a constant 12V buss for building lights at night. I have a few out and a few that intermittently light.

With 5 motors it wasn’t working very hard, so yes.