12 volt motor with DCC mod

[auditdata]

Hi,

I have just bought a Thomas and converted it to DCC using a simple 4 wire Hornby decoder. On the box it says the model is 12 Volts

When I put it on the track it goes like the clappers! I am sure it will burn out at anything over 40%. There is no way to use CVs to change the motor profile. Is there a simple component to add to the wiring to reduce the voltage. Space is limited.

[96RAF]

Which DCC controller are you using.

It sounds like DC runaway which means you need to know the value of CV29 or if your controller cannot red-back CVs simply blind write a suitable new value to it that disables DC running.

[ColinB]

I am surprised about that, from what you are saying the DCC is working correctly so it just sounds like the motor just runs a lot faster. Hornby on their 0-4-0 locos added a resistor in series with the motor to slow it down. You don't say whether you bought the Thomas new or if it was secondhand. Sometimes Hornby will use the same looking motor in many models but with a higher speed. As motors are hard to come by people like me replace the motor with something that fits and works. I got caught out with the new Princess motor, it looks the same as the motor that powers the old design of Princess but as the gearing is different, runs at substantially higher speed.

[auditdata]

Thanks Colin, I think you have exposed the problem. I chopped out the capacitors and resitors to put the DCC decoder direct to the motor.

The resitor is in the bin so no idea of the value. Suggestions?

Clive

[auditdata]

JMRI with SPROG 3

Hornby R7274

[96RAF]

CV29 = 33

[auditdata]

I tried to send a screen shot of the CV's but as a newbie I think I am waiting for it to be moderated.

But by text cv's are

1,3

3,5

4,5

7,135

8,48

10,128,

17,193

18,240

29, 33

[Fishmanoz]

RAF is being a little subtle with his table so to be more direct:

• do you need reverse direction set? If not, you will need to subtract 1 from the value of CV29
• you do not have 128 speed steps enabled. Doing so will give you finer motor control. Add 2

So you need to write either 34 or 35 to CV29 (without or with reverse direction). That will give you finer motor control and don’t worry if you don’t have to turn it up very far to get the speed you want.

[ColinB]

Adding the resistor is a much easier way. Does anyone know the value? My Thomas is quite old so I don't think it has one. CVs are a wonderful thing until you decide to reset the decoder then hey presto they are all gone and don't get me started on CV29.

[P-Henny]

Does anyone know the value?

 

 

No I don't but I would think that as an approximation, if it was made around the same value as the DC resistance of the motor (Multimeter needed to measure it). Then that would approximately half the peak voltage across it. The resistor value I would estimate as needing a 2W power rating.