High decoder voltage.

[Potrail2378]

Hi, I have converted several loco's to DCC but I'm stumped with my latest venture. I couldn't get any LEDs to work, and when I touched the 1Kohm resistor, it was red hot. I measured the voltage and it was 42vdc! I left it for an hour or so and checked the voltage again, this time it was 26vdc. Is my standard Hornby decoder beyond repair?

Thanks in advance. 

[Rog RJ]

What controller are you using?  What is the voltage on the track? (use the AC range on your meter)

[96RAF]

Hornby decoders are rated for max 27v so it is marginal if you have permamently damaged it or not.Possibly if its still working the life has been reduced Somewhat.

 

I wonder how you managed to get that potential out of a decoder as the internal gubbins should regulate it down to 12v. As Rog asked - it makes me wonder what the track voltage was.

[ColinB]

I have great difficulty understanding how you got 42 volts DC, unless you got some capacitor voltage doubling effect. I think the max voltage for DCC is 18 volts, but I am not certain on that. The first question is did you smell smoke and the second is does the loco sit on the track without causing a short ( the controller trips out). If neither of these two has happened you might be lucky, normally from my experience and I have had a few die on me, you normally get those two effects, normally the bridge rectifier diodes fry themselves and eventually cause a short circuit. First thing to do is go back to dc ( I bet you didn't use a socket, that is why I always suggest using one) and see if the loco still works without the decoder. To me it sounds like something is wrong with your DCC controller, but I find that hard to believe.

[Chrissaf]

Nobody has yet asked the most important question. Which is, across what two points was this 42 Volt DC and 26 Volt DC actually measured?

 

Across the resistor

Across the LED

Between the Blue wire and somewhere else.

Across the wheels

 

The list goes on.

No suggestive deductions can even be really attempted until one knows the answer to the above, else it is just all speculation.

[ColinB]

I can't see anywhere on a decoder that you would get 42 volts. DCC is about 18 volts if I remember, I did check the specification but I couldn't find it. Even if it was measured peak to peak that makes it 21 volts plus or minus, which is way out. As I say the only way I could see you would get that voltage is if you measured some noise spike. 

[Potrail2378]

It's a Hornby Select, it's measuring 15.5 vac across the rails. I'm now getting a constant 26vdc across the blue wire and the green wire. There is no smell of smoke, and yes, I am using a socket. I have tried three different decoders, all giving the same results. 

[ColinB]

I hope you haven't blown up all three. Ok, do a continuity check between the green wire and the track with no power connected. Check that the green wire is actually connected to something, also check the resistance value of that resistor yhat was very hot. Does it still work on DC ?

[Fishmanoz]

Remember the green wire is switched -ve. When switched on (via F1), you get -12 volts v the always on blue positive.  So with it switched off via F1 or without track/decoder input voltage, it will be floating and what you read versus anything else will be anybody's guess.

 

So power your decoder, switch the green on and measure v blue.  You should get - 12 volts.

[Potrail2378]

Thanks for the help, there must have been a short somewhere - maybe the green wire to a rail? Anyway, it's sorted now. I have put directional lights in now, and am waiting for my sound decoder to arrive. 

[96RAF]

Are you saying it could have been operator error and not a decoder problem.

[ColinB]

He didn't say, he also didn't say whether he blew up the other 3 decoders. If the green wire was shorted to track, they would all be dead.