Hello,
So my brother blew a DCC board recently and I think it is because the motor is pulling too much current. I was wondering as I ponder how I'm going to correct the issue...can I use a resister to keep the motor from sucking too much juice, and, if so, what resister would I need?
If the motor draws too much current, it is time to REMOTOR.
A resistor is not going to help you.
What loco and what decoder? Perhaps the motor was not proper isolated. Most HO decoders can handle 1.5 amps continuously which is a current level that is rare among anything of fairly recent manufacture.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
A Backmann Spectrum...one of the older ones. I think it is an SD-38. I installed the original decoder, and it was WELL isolated from the current. I bent the contacts down on the trucks so that they do not make contact with the frame at all. THEN....I coated the frame in with nail polish, witch is non-conductive when it is dry. THEN....once I had secured the orange and grey wires to the motor, I wrapped the motor contacts in a thin strip of electric tape. THEN...I ran wires from the trucks to the decoder. Unless one of the wires got rubbed raw in a spot that just happened to be missing nail polish...and one of the motor contacts just happend to be poking through the tape...it couldn't have been due to a failure in isolation of the motor from the current. Unless I'm overlooking something here, which does happen often! We used a digitrax 123D which has a fairly low resistance to overcurrent. I'd say it can handle no more than 1.2 amps, though the manufacture does claim a higher max current than that.
If the motor is pulling too much current it is either bad, or being dragged down by something. Motors draw the most when they are trying to turn but can't.
Dave
Lackawanna Route of the Phoebe Snow
How long did it take to blow the decoder? If it was right away you probably don have an isolated motor.
Before replacing the decoder hook up the loco without a decoder. Then put the loco on a DC track and measure the current it draws when holding it nearly still. You'll need a current meter to do so.
Springfield PA
KevinsHope THEN....I coated the frame in with nail polish, witch is non-conductive when it is dry.
THEN....I coated the frame in with nail polish, witch is non-conductive when it is dry.
I would have used electrical tape, not nail polish to isolate the motor from the frame.
Rich
Alton Junction
I did use electric tape on the motor itself. I wasn't interested in electric taping the entire motor housing...but I wanted it to have a non-conductive coating...just in case the contacts made accidental contact with the frame. I think I put electric tape on the frame in the areas that the contact on the trucks rub against...but I bent the contacts down, so there should be no current at all in the frame either way. The nail polish was the fail safe for the fail safe.
And....to answer the other question here...it ran on the track for a while before it burnt.
So...back to the topic...why can't I use a resister to prevent the motor from drawing too much current?
KevinsHope So...back to the topic...why can't I use a resister to prevent the motor from drawing too much current?
The motor in that loco does not draw enough current at a stall to fry a decoder. and in all my experience I have not seen a loco stall the motor under normal conditions. Even if you had to many cars on a grade the wheels would spin, even with a traction tire the loco would hop. I have seen Bachmann locos with DCC on board with capacitors and what looks to be like a large size resistor to the motors on both terminals but that has somthing to do with radio or tv interferaces. No other decoder manufacture has ever recomender a resistor to the motor to my knoledge. and I read up as much as I can on these subjects. You had a short and that is what I beleive fried your decoder. You may want to look your installation over, and check things over with an ohm meter.
Short answer: A resistor in series with the motor will drop the voltage to the motor, and not the current. The insertion of the resistor affects the speed of the locomotive, not the current draw.
Longer answer: A motor is not a constant resistance. The only time a motor is a constant resistance is when it is stalled (not turning). Then the only resistance is the armature windings (usually around 10-20 ohms). A motor draws the current it needs to continue turning under the load it has, assuming the current is available from the power supply. The voltage to the motor determines how fast the motor turns, not the current draw. Thus, the current draw of a motor is related to the load, and not to the supply voltage. The more load, the more current the motor will draw until it finally stalls.
Most motors will burn up fairly quicky when stalled (Tortoise switch machines are a major exception that use a motor designed to be stalled). It is the only way to get enough torque from such a small package.
Because the current draw of a motor is related to the load, measuring the current draw of the motor with and without the mechanism attached can give a good idea of how free turning the mechanism is (or isn't).
Most model locomotives HO and smaller have motors powerful enough to spin the drive wheels in a slipping mode without stalling the motor. Weighting the locomotive so that the drivers slip before the motor stalls is a way to protect the motor. So you have a several different current ratings to consider:
DCC decoder installers insist on rating the decoders for the stall current of the locomotive. This means their installation will fail safe - the motor will burn up before the decoder does, protecting their decoder installation work.
I prefer a different tack. I would rather replace a decoder than a motor. So I would rate the decoder based on slip current. Then, in the rare event the motor did get stalled (should never happen under normal circumstances), the decoder goes first, hopefully de-powering and saving the motor.
In your case, you are trying to fix the symptons rather than cure the cause with the resistor idea (which won't work). Either your decoder isn't rated high enough for the motor current (highly unlikely unless you have an old open frame motor), or you have a mechanism which has problems. A binding or "sticky" mechanism can cause a motor to draw too much current, and even stall it. Measure the current draw with the engine running by itself. With a can motor, this needs to be less than 0.4 amps (and preferably 0.25 amps or less). Anything more is a mechanism issue.
hope this helps
Fred W
Thanks Fred.