J94 motor and decoder failure

[Michael_A]

I have a brand new J94 to which I fitted a Hattons 8 pin decoder. (Wired as not DCC ready....)

 

All was well for several hours, then I decided to try and adjust the speed settings. I set the max speed to 200 and the mid to 100, placed it back on the running track. I then got the dreaded puff of smoke and the decoder shorted. (Error on the Elite.....)

 

So I replaced the decoder. That went up in smoke as well, and I also noticed the motor smoking. I removed the decoder, reconverted to DC and sure enough when tested on DC the motor had failed and crackled and smoked when I tried to operate it.

 

I did check and there was nothing wrong with the wiring. So question, did the motor blow the decoder, or is it possible for a faulty decoder to destroy the motor?

 

Fortunately the motor is only 4.50 from peters spares, but I'm 20 quid down now on decoders!

 

 

 

[96RAF]

Unusual as most decoders have an overload protection circuit. Your Hattons one may/may not have this self protection feature thus when the motor shorted the decoder couldn't handle the load and burnt out. Have a look at the decoder spec sheet.

Rob

[Michael_A]

Unusual as most decoders have an overload protection circuit. Your Hattons one may/may not have this self protection feature thus when the motor shorted the decoder couldn't handle the load and burnt out. Have a look at the decoder spec sheet.

Rob

 

I suspect you are right looking at the service sheet. This was particularly amusing:

 

"(4) It has got a big black hole in the heat-shrink! The only way this can happen is by incorrect wiring, overload or similar. You killed it, sorry!"

 

But that doesn't really answer my question. Which came first: motor killed decoder, or decoder killed motor? I guess there may be no way of knowing, but theoretically the AC track voltage can find it's way to the motor if the right components fail in the right way.

 

[Howbi]

Michael...........the Hattons decoder instructions for CV6 mid-speed say.......range 0-255, keep CV6 1/4 - 1/3 of CV5 but you set to 1/2 so maybe that was the problem...........HB

[Michael_A]

Michael...........the Hattons decoder instructions for CV6 mid-speed say.......range 0-255, keep CV6 1/4 - 1/3 of CV5 but you set to 1/2 so maybe that was the problem...........HB

I doubt it has anything to do with the CV value. I have regularly ignored that, which is any case only guidance. Even if the decoder did fail first, my question really is can a failed decoder blow a motor?

 

[Howbi]

Very likely if the full AC track current hit the DC motor........the DCC modified AC can peak at about 28volts so not surprising that this little motor could not withstand that..........HB

[Fishmanoz]

Have you made sure both motor connections are isolated from the chassis?  Not doing this will ensure you blow the decoder immediately. 

[Michael_A]

Have you made sure both motor connections are isolated from the chassis?  Not doing this will ensure you blow the decoder immediately. 

 

Reply:

 

Yes I checked and double checked: 'I did check and there was nothing wrong with the wiring'. 

 

Very easy little loco to wire, and motor connections well isolated and away from any metal chassis and impossible to get near the pick ups. Bit surprised Hornby didn't DCC ready it, though if fitting a socket would  probably have limited it to a 4 pin decoder due to space limitations (Plastic chassis, huge metal weight. Which doesn't make a lot of sense really when you think about it.....).

 

[Michael_A]

Very likely if the full AC track current hit the DC motor........the DCC modified AC can peak at about 28volts so not surprising that this little motor could not withstand that..........HB

 

Reply:

 

Never had it before. Worrying. I was kind of wondering if anyone else had experienced the same thing.

 

[96RAF]

Looking at a typical decoder block schematic (credit Brian Lambert website) if the motor failed there would have to be cascading failure of the decoder self protection circuits to kill the decoder and it would also be very unlikely to see full track voltage getting through to the motor as a prime causal factor in the motor failure.

Rob

 

/media/tinymce_upload/f762dd38fde13356cdae6d50d9972b4f.PNG

[Michael_A]

Looking at a typical decoder block schematic (credit Brian Lambert website) if the motor failed there would have to be cascading failure of the decoder self protection circuits to kill the decoder and it would also be very unlikely to see full track voltage getting through to the motor as a prime causal factor in the motor failure.

Rob

 

/media/tinymce_upload/f762dd38fde13356cdae6d50d9972b4f.PNG

Reply:

 

Thanks

 

Two diodes would have to fail s/c, and just the two feeding the internal decoder power suppy, and the motor driver trasistors or Mosfets would also both have to be s/c. Too much of a coincidence. Think I'm going with motor failed first.

 

Lesson learnt: next time after a decoder failure check loco runs ok on DC agan frst.............

[Michael_A]

Too much of a coincidence that the right 2 diodes and the motor drivers all went s/c. I'm going with motor failed first.

 

Leson learnt. Next time a  decoder fails, check all ok on DC first.

[96RAF]

Just for info to those who are interested the motor control processor in that block diagram is normally an dual mosfet H-bridge set up, with each side directionally feeding an output transistor to the motor.

Rob