Loco circuit board with burnt out resistor

[Jamesn320]

Hi guys,

I have just fitted a ESU Loksound V4.0 into a Heljan Beyer Garratt (it’s a British loco but I was hoping somebody could help anyway). Important facts, the loco has two big motors and the total current draw is right at the limit of the chip.

Not long after installing, within the first hour, the loco was sat with just the sound on, not moving, when smoke was see coming from the smoke stack.  Power was shut off quickly and the loco inspected.  The decoder is fine but a 10 ohm resistor on the loco circuit board was burnout.  Following the copper tracks the resistor is in the gray motor feed.  Between the decoder and the two motors.  There is also a 10 ohm resistor, not burnt out, in the orange motor feed.

My question is does anybody know what purpose these serve?  And how it might have blown?
There is also a small capacitor in the orange feed after the resistor which acts as a suppressor to stop the train interfering with TV signals when run on DC, that’s a standard thing here in the UK.  Often I rip all that stuff output but as this loco has an 8 pin socket I left it.

I can easily replace the resistor I am just wondering I if should just remove the other one and bypass them both?

Cheers,

James

[peteski]

Are you sure those are resistors?  Most of today's Chinese made models (especially for the European market) include a Radio Frequency Interference (RFI) suppression circuit to suppress the electrical noise generated by the motor brushes arcing.

It usualy consists of 2 chokes (aka coils or inductors) wired in series with the motor leads and a small value capacitor across the motor leads.

Either way, those should never smoke unless there is something wrong with the motor creating an over-current condition (basically, a short or close to it).  ALso, the power source (decoder) would have to be capable of supplying so much current that those chokes would start heating up and smoking.

If you think that those are what I described then you can remove them (but that will make model violate European Radio Emission laws).    But if those smoked then whatever you are doing exceeds what the circuit was designed to handle.  I don't like to do that because later on something else in the circuit might fry.

I have been playing around with model trains (equipped with the RFI suppression circuits) for over 40 years and the only time I have seen a fried choke was when a motor was shorted (and that was in pre-DCC days so the power was coming directly from the track and the throttle.

[Jamesn320]

Hi Peteski,

Yep, resistors, 10 ohm.





I know what you mean about the 2 chokes and interestingly this loco doesn’t have any.

-James

[peteski]

Judging by the color of the body (light teal), and the chunky shape, I still say these are chokes.  Can you see copper wire winding inside the burned area?  Resistors of similar size and shape are usually  made from a carbon-coated ceramic rod.  Well, there are also wire wound resistors, but this is a bit small to be wire-wound.

Either way, if these are in series with the motor leads, then you can safely remove them. But I would still be a bit worried as to why it burned up in the first place.

[Jamesn320]

Hi Peteski,

I'm pleased I asked, yes inductors.  That would make sense.  The reason why I thought resistors is because of the symble on the board.

It’s the one in the top left.



Ware as an inductor should be this. (Again top left)



I'm guessing Heljan made a mistake there!

As they are inductors, as you say, I’ll have them out and bypass them.  As for the cause, now that’s another story.  Thanks for the help.

-James

[peteski]

If they used the symbol you indicated then it is indeed incorrect.  Is there also alphanumeric name of the component indicated?  Like C1 for capacitor, R1 for resistor, or L1 for an inductor (choke)?

If the color stripes are brown, black, black, silver then it is a 10µH 10% tolerance choke.
Here is one which looks like yours: http://techdelivers.com/10-micro-henry-inductor-india-100uH-Henry-Choke-coils