Re: programing a decoder thats not in a loco

Mark Gurries


I understand you point.  However if I was to do motor tuning, I would do it with the decoder installed so that the tuning is done working with the actual target motor.   Using a different motor to tune will not give you the results you want and you will have to re-tune again when you installed the decoder..

A better way to say this is that programming with a resistor for a motor load is fine but one should avoid doing any type of motor tuning until the decoder is installed a connected to the target motor.

As stated your "No No" falsely “implies" there is some bad electrical reason that can damage the decoder which I know you know is not true.


From a pure resistance value point of view, the resistance value must allow enough current to pass the minimum programming acknowledge pulse current of 60ma but high enough so that it does not exceed the decoder current rating.

Stated another way, for a 1Amp decoder running on 12V, any resistance value between 12 and 200 ohms will work.

HOWEVER from a safest and practical thermal standpoint, I would ONLY use a 200 Ohms 1 Watt resistor* because going any lower in resistance value will cause the resistor to get hotter and require a physically larger resistor with a higher watt rating as well as increase the cost with nothing gained.  Using a resistor with a less than a 1 watt rating will potentially cause the resistor to burn up IF you decide to test the motor control after the programming is complete.   

EXCEPTION:  In cases where one is using a full decoder as a FUNCTION ONLY decoder to control lights such as installed in a caboose where there is no motor, then there is the option of using a much smaller resistor, such as a 1/4 watt rating, provided you follow this one RULE.    RULE: You enable the speed table and turn all the speed steps to zero.  That way if you attempt to run the decoder like a engine with speed commands, the motor output will remain at 0 and do nothing.  The only time the resistor will be used is on the programming track where the resistor will never generate any heat of any consequence when it is used to generate the acknowledge current pulse.

Hope this helps clarifies things for everyone.

On Nov 17, 2014, at 1:17 PM, len.jask@... [WiringForDCC] <WiringForDCC@...> wrote:

I use mostly ZIMO decoders and the no-no was through their USA distributor. A lot has to do with speed testing and back EMF that the decoder is looking for.  A resistor can't provide back EMF. I use an old chassis and have it set on test rollers. Since adjusting speed is part of the decoder programming why use a resistor? My logic.

Len Jaskiewicz

Best Regards,

Mark Gurries
Electrical Engineer
DCC Website & NMRA DCC Clinics:

Join to automatically receive all group messages.