Oh, the things one gets to learn on the railroad!
Okay, so I got a nice digital readout multimeter on sale. I can see how to test for continuity for motor isolation and identifying wheel pickups, but what multimeter settings do I use to test the stall current (I can see how to hook up the meter to the track etc., just am not familiar with electrical terms enough to set the meter.)
Is there ever any danger of damaging a loco’s circuitry using the multimeter incorrectly?
probably not.
Depending on the MM type, you will have either two or three plugs on the lower part of the face, two red and a black (the common). You’ll have to read the labeling for specifics, but the one I have ATM has a capacity of up to 400 mA on one of the red leads. the other one is 20.
Before testing the stall current you need to find where on the dial the powerpack is putting out 15V, or whatever the output of the DCC system you want is. This simplifies things, as you don’t need to then do the conversion of “well, it pulls .75 A at 12V… but what does it pull at 15V”. Ohm’s law is not a strong point of mine…
In order to test the Stall current (or the slip current):
Hook the Multimeter in series with one of the leads from a power pack (lets say the lead that would go to the left rail)
Attach the other lead of the multimeter to the left rail of a test track (alligator clips REALLY help here)
the other wire from the power pack gets connected to the right rail.
Place the loco on the track, and press down so that the drivers are slipping. This is the slip current (a “close enough” to the stall current). Keep pressing down til the wheels stop to get the stall current. DO NOT STALL THE LOCO FOR MORE THAN 1 - 2 SECONDS.
Most (newer) locos have a stall of less than .5 Amps. Older locos have higher stall currents, some to the effect of 1 Amp or more.
Here are some links I found when I did a Google search for multimeter tutorial. It is not possible to burn out the engine with the meter but it is possible to burn out the meter if incorrectly used.
http://www.autoshop101.com/forms/h4.pdf
http://www.electronictoolbox.com/How_to_use_a_Multi_Meter.html
http://mechatronics.mech.northwestern.edu/design_ref/tools/multimeter.html
Download them to your computer. Take your time and study how you use each function.
rich
Unless you’re installing a decoder into a really old engine with a Pittman open frame motor you don’t need to worry about stall current. How likely are you to ever go up to an engine that is on your track and push down on it so hard that the wheels can’t slip and the motor stalls? Under normal operation that will never happen.
But if you do want to measure the stall current, here’s how:
Put one lead from your power pack to the track just like normal. In place of the other lead, connect your meter between the power pack and track. The meter’s red lead should be in the socket marked Amps (my Cen-Tech is marked 10ADC) and the black lead in the COMMON socket.
Turn your throttle to full output, set the meter dial to the DC Amps range, put the locomotive on the track, and press down on it until the wheels stop spinning. That will be your stall current.
If you make the mistake of using the ma jack and exceeding the current limit, there is likely a small fuse inside that will blow. Most inexpensive meters do NOT have any protection on the high current jack, so respect the limit. Even my cheapy $5 meter is rated for 10 amps on that line, so it’s plenty to test any HO loco you’d ever come across. Applying voltage when in the Ohms setting might cause some damage too. ALWAYS make sure you have the correct setting selecting BEFORE touching the leads to the circuit under test.
–Randy
Thanks, Cacole. I kind of had this in the back of my mind…because for all the warnings on the web and forums and decoder install directions about testing the stall current, I could not find any information on the web about an engine higher than 1 amp stall unless it was very very old.
If I might add a couple points…
The stall current is measured with a DC supply, NOT a DCC supply. That may have been obvious, but maybe not. I’m not sure how “good” of a DC supply it has to be. I suspect some inexpensive DC power packs might supply rather choppy DC where the voltage rises and falls at 60 hz. I can’t really say I’ve run into a loco which pulls too much for a decoder rated for 1 amp, including a 1997 vintage Athearn. I have heard of some motors in brass engines which may have caused a problem, but they may not have been properly isolated from the frame either.
The reason stall current is of interest is because thats the condition under which the motor pulls the most current. As soon as it starts to spin, it creates back-EMF, which is a volatage across the motor which opposes the supply voltage and therefore the reduces the current. Kind of academic, I know, and I’m really a software guy, so a motor guy may clarify a bit…
Jim
Cacole,
Take an old proto west can type motor that someone has used to re-power an athearn and also packed the shell with with lead weights. Put in a decoder with a 1.5 amp limit. Decide to have a pulling contest up a 2 percent grade. You don’t even have to listen carefully to hear the decoder POP half way up the grade.
Actually, I was surprised any Proto West motor drew that much juice. That was one of the decoders I turned into NCE. [:D]
Tilden
Tilden
Unless the can motor is quite large, I don’t think the motor would/will survive very long drawing 1.5 amps - that’s close to 20 watts of heat to dissipate. The fine wire in the windings has to conduct that current without melting the insulation, and the brushes have to do likewise. Open frame motors with the same windings can carry more current because the windings are in the “open” and have more cooling air than in a can motor.
Likewise, the decoder rating is primarily built around heat issues, not actual current. If the decoder has smaller component density (larger physical size) and is not tightly wrapped in mounting tape, it can get rid of more heat, and therefore take a higher current before it “pops”. Higher rated components (bigger physical size) and electrical efficiency are the other paths to higher decoder current capacity.
Ideally, a locomotive is weighted so that drive wheel slip occurs before the motor reaches its maximum safe current rating. Weighting more than that, or using traction tires, prevents the safe slipping, and allows a motor to overheat under excessive load. In our locomotives, the need for high torque and power means the stall current of the motor exceeds the maximum safe current.
The bottom line of the above is that if the locomotive is safely weighted, the curre
Two small items to protect your multi-meter. Always put the knobs, switches in the AC position at the highest voltage when not using. This will prevent destroying the meter by connecting to a high voltage when it it not set ot do so. Do not leave it set in the resistance setting. there are batteries in the meter that power this setting. You will put an unnessary drain on the batteries by leaving it this setting.
Always a nice tool to have on hand. You do not realy need an expensive one, just one that meets your needs.