Resettable fuses

[ColinB]

I recently had an instance where a motor suppression capacitor on my Hornby class 66 went short circuit. I know for DCC operation I should have removed it, but I didn't know it was there, it was burried next to the motor on the bogie. It subsequently blew up two TTS decoders, first one while it was running and then its replacement at which point I then started checking the loco. The TTS decoders have no short circuit protection, so if your motor goes short circuit, they instantly blow up. When I worked in electronics, I used to use resetable fuses to protect exactly this event, I wondered if anyone has any knowledge of whether they would work in protecting the TTS decoder from a motor short. In my case I am returning the two TTS decoders to Hornby under their replacement plan, as someone kindly pointed out to me, but I would rather protect against it happening again.

[morairamike]

Resettable fuses i think maybe you are talking about circuit breakers. The problem i see with using them is that they take time to react as they operate by current being drawn heating a coil that trips them. Thus they alliw transient over current. I expect the TTS decoder to be destroyed almost instantly.

[Flashbang]

A quality decoder whether sound or non sound will have built in overload/short circuit protection. The TTS decoders are of course budget devices and you say they do not have such!  I have no definitive knowledge of what is on a TTS board so I'm unable to confirm.

Thermal devices would be far too slow to protect the electronics, which is exactly what morairmike has said.

[Chrissaf]

The resettable fuses that Colin is talking about do not have coils and moving contacts. They are positive coefficient thermistors and are totally electronic with no moving parts an example is shown in the image below. They are frequently mistaken for being capacitors.

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Typically, a normal thermistor has a negative coefficient. It is a variable resistor that changes its resistance based upon the current flowing through it and thus the amount of heat generated. With a negative coefficient curve, the resistance is high with low current, but gets lower with increasing current.

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A 'positive coefficient thermistor' functions the other way, the resistance starts low and increases with current and heat generated. They are very common in electronics as 'resettable fuses'. They are already used in some Hornby controllers. The HM2000 for example and the Elite are two that I know of. Despite reacting to heat, they do in fact trigger very quickly to increases in current flow, which makes them ideal as electronic fuses.

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The main issue is that you cannot use just any old resettable fuse at random. Their operation curve has to be closely matched to the circuit conditions into which they are going to be placed. Selecting the right specification product is a 'black art'.

 

[ColinB]

Great thanks Chrissaf, I have used them but I didn't realise that exactly what they were, I knew they reacted fast enough to protect electronics. There are a list of current ranges so I was going to pick one to suit the application. As for quality decoders having short circuit protection, I am not so sure about that. On all the decoders I have used it is never mentioned, the only exception was an old Backmann where it did state all outputs are short circuit protected. Perhaps someone can enlighten me as to which ones have it, I have been looking for ones with it for ages.

[96RAF]

TTS decoders are indeed self protecting but only on the motor circuit. The function outputs are not protected. This is clearly stated in the blurb. Maybe there is a zenner diode, choke or other smart device in the motor circuit that limits current draw. Outwith my expertise.

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I would not expect a track short to kill a decoder, maybe reset it to address3, but this is common with many decoders not just TTS.

[ColinB]

The first one is definitely dead, and it gets very hot, the second one goes backward at full speed with no control and no sound, so that is damaged. So if it is protected why did it blow, and that definitely was the only fault with the loco. It wasn't a track short it was a motor short. I will try the reset, but I don't hold much hope.

[96RAF]

It is well known that if you apply an externsl voltage to the motor terminals the decoder will be damaged.

 

It is possible there was some form of short that routed a wild voltage back into the motor control H-Bridge circuit causing it to blow. This can easily happen when an unsecured and unprotected decoder has fallen off a loco under test and touched the track or wheels. Been there - done that - paid the price.

 

The motor only responding in one direction and without speed control also points to H-bridge damage.

[Chrissaf]

I concur with the H-Bridge damage analysis regarding one direction only with no speed control.

[96RAF]

Refer also to Colin’s other post on the 66TTS failures due to a motor cap short.

[ColinB]

It is weird, now I have cleared the fault I put the original decoder back in and it does exactly what the second one does, goes backward at top speed, so the shorted cap did exactly the same damage. I got a returns number from Hornby so I will let them sort it out. I am intriged from the data it should be ok with a motor short and I am 100% sure it didn't touch any of the track inputs, as you say, been there killed modules so I am always very careful.I even checked all the lights had resistors on them, in case that caused an issue.

[ColinB]

I think I might have figured it, it probably is short circuited protected, but only for a certain time, then probably it overheats. It doesn't mention a time, which probably explains why the sound worked on the second one for about 10 seconds before it died.