Volts, Amps, Watts, VoltAmps


I'm offering some fundamentals here. As you have probably discerned, this is not a "teaching site" where I attempt to give fundamentals from the ground up. The site exists to mainly provide information that is not otherwise on the Internet, or to provide corrections to erroneous information.

But I will offer some fundamentals here, since often a poor job is done on the forums, or really misleading, (I just read yet another post where people think LEDs work on voltage only)


You need to understand Volts and Amps first. Electricity "moves" as electrons through a circuit.

And for electricty to flow, there must be a "complete circuit", like running a lap of a race course, the electrons start at a source of power, and end back up there.

The electrons start from the positive terminal, and return to the negative terminal of a DC power source, like a battery or switching power supply.

(we will only comment that AC, alternating current, flows back and forth, as the power supply changes polarity (the plus and minus) several times a second.

Volts / Amps

Volts and Amps (Amperes) are the 2 basic measurements that relate to flow of electrons.

Volts is like water pressure, the force of the electrons.

Amps is like gallons per minute, the number of electrons.

You can measure these, almost exactly like you measure the flow of water:

For voltage, you can touch 2 probes of a meter anywhere there is voltage. This is just like attaching a pressure gauge to a pipe, you can put it anywhere, and the water does not flow through it, it just measures pressure present. The voltmeter does the same, measuring the "potential" pressure of the electrons, voltage.

For current, it's a way different story. You really want to "count" all the electrons in a wire, just like measuring how many cubic feet of water are moving in a pipe. This means your measuring device needs to go "in line" in the pipe (or electrical wire). This means you "break" the circuit, and insert your meter so that all the electrons go through your meter.


OK, so how do I use this knowledge:




In our trains, in motors, the higher the voltage the faster a motor will spin

The more amps the motor draws, the more load it can handle.

Normally in our scale, we need to know the amperage the system can supply, regardless of the voltage (speed).

So most better quality systems are rated like 0-24 volts, and 10 amps.

But, like many other things in advertising, there are ways to make things look better than they are, and VoltAmps or VA is commonly used in lower quality/power systems.

VoltAmps is volts times amps, but there is a "trick", since you have variable DC, it turns out that VoltAmps can be at only ONE combination of Volts and Amps.

What happens is that in most systems rated this way, as the load (amps) increases, the power supply is UNABLE to hold the maximum voltage.

The net result to you is that normally when you need the power (amps), the voltage (speed) drops and your train does not make it up the hill.

MRC power packs are famous for this, and it was quite a learning experience trying to get a 60 VA power pack to run an LGB track cleaning loco.

So, it is NOT Watts, and is sort of a maximum power, but almost always NOT at max voltage (where we often need it).

so if you had a 48 VA supply, and it is rated to a max of 24 volts, most people think they will get 2 amps at 24 volts (volts times amps = 24 x 2 = 48 VA)... but this is almost NEVER true.

In most cases, the voltage sags greatly and you get in sort of a vicious circle. When you have a load (long train or a grade) and you increase the speed (voltage) then often the increased current draw will cause the output voltage to "sag", whereupon you add more throttle, but the increased current draw will again cause more voltage sag.

I had exactly this situation trying to run an LGB track cleaning loco from a 60 VA supply, which I thought (at the time) should source close to 3 amps. It might have been able to put out the amps, but the voltage sagged like crazy, and was not enough to make the cleaning wheels spin. Electric motors draw the most current when stalled, and so this just further compounded the problem.

I went out and bought a true 10 amp supply and gave my MRC 6200 away.

Weather Underground PWS KCACARLS78