What’s a stall current?
About two weeks ago I had a problem with a Tsunami2 decoder I had installed some months ago. The loco would respond to all my commands except it wouldn’t move. After exhausting everything I could think of I called Justin at Soundtraxx to see if he had any suggestions (by the way, Justin has moved on from Soundtraxx as of last week so George is now taking the calls until a relpacement tech support guy is hired). Justin’s first question was what is the stall current of the locomotive? Now I happened to know that the stall current of the locomotive was under 0.5 amps which is well within the capabilities of the decoder.
So as the title indicates what is a stall current and why are they important enough for Justin to ask me this question? Stall currents used to be a big concern when choosing a DCC decoder but have largely receded into the background due to a couple important technological advances. First let’s consider what a stall current is, why it is important, how to measure it, and how to pick a decoder once you know what it is.
A stall current is the current drawn by a motor that is physically stalled. This can happen if the locomotive is attempting to pull more cars than it is physically capable of pulling. It also can happen if a mechanism jams and the motor is forced to stop turning. Consequently there are a couple different types of stall current measurements. First there is a stall where the locomotives just sits there and spins its wheels. The motor usually spins up to its maximum RPM drawing the maximum current of the turning armature. The second and worst case scenario is a locked rotor stall where the armature literally becomes locked in one position–this is usually the killer stall current.
So what happens when a motor stalls? Well as I described above, the motor draws a larger than normal current, possibly exceeding the maximum current the decoder is capable of providing. When this happens you can quickly burn out the power components in the decoder, although most decoders now will shut down before burning up. This is similar to what happens if you select a decoder with a maximum current output less than what the motor draws under normal operations. In some case you may see repeated thermal shutdowns and finally a burn out when one or more components fail.
You can measure locked rotor current by hooking up a volt ohm meter in series with one of the track feeders. Place the loco on the track, grasp the flywheel so the motor can’t turn, turn on the power, and read the amperage measurement. As shown in the included photo this older Athearn motor has a stall current of about 0.9 amps. Similarly you can measure the slipping current by just holding the loco in place and letting the wheels slip. You can also find the stall current for most locomotives produced in the last 20 or so years in product reviews in Model Railroader.
Decoder manufacturers always advertise the maximum current rating for their decoders and you should always select a decoder with a rating greater than the locomotive stall current. Fortunately advances in electrical component design have translated into decoders with larger current capabilities than in past years. Most now are rated at 1 amp nominal with a catastrophic failure value of 1.5-2 amps. In many cases however these values also include the current drawn by all your functions in addition to what the motor is pulling. So you need to consider the lights, smoke generators, or other wired devices in your locomotive.
Most HO scale locomotive motors made today have a stall current between 0.5-0.7 amps so a decoder rated at 1 amp should not have any problems. However older models are a concern, especially those with open frame motors, which can draw over 1 amp. Also some of the very small sound decoders are only rated at about 0.7 amps so care should be taken when using them.
Decoders are also sensitive to heat buildup and I have seen some folks attach their decoders to metal plates to help dissipate heat. If you have a decoder that is repeatedly shutting down, then suspect heat buildup as the culprit. One source of heat often not considered is the motor itself. Try placing a heat sensitive decoder in an area where it can get adequate ventilation such as over a truck gear tower instead of over the motor. Hopefully these tips will help you with a trouble free installation.
But what about my Tsunami2 problem? I pulled the decoder and hooked it up to my ESU Decoder Tester and it worked fine. So I reinstalled it in the loco and it worked fine. At this point I will be keeping an eye on it and marking this one up to gremlins. Some days the electrons just do what they want to do and ignore the laws of physics!