Monday, October 25, 2010

Zap !

Terry asks: Maybe you could throw some light on this.

I have been trying to convert my Bachmann class 04 diesel to DCC. I found instructions on the Bromsgrove Models website complete with pictures. The class 04 has a split-chassis, this being a large metal block which fills most of the loco. The appropriate leads from the decoder were soldered to the tags on the motor which were then insulated from the metal chassis with insulation tape.(The website advises heat-shrink tubing of which I have none). The other two leads from the decoder were attached to the metal chassis, one via a screw and the other via a plastic nut with which the chassis can be screwed together whilst maintaining insulation between the two halves of the chassis. Not the most secure joining of wires to the chassis!

Anyway, I stuck the decoder on the chassis where the cab will be. The decoder is insulated from the chassis with more electrical tape. I placed the chassis on the track, turned on my Bachmann E-Z controller and, amazingly, the loco chassis moved, although it wasn't as smooth as previously. Now here's the problem: I went to pick up the chassis by putting my fingers and thumb either side and 'ZAP!' a severe jolt was sent up my left arm! Any ideas as to why this happened?

Strangely, the decoder continued to work afterwards so that hasn't been fried. I wouldn't have expected 16volts AC to be powerful enough to give me a shock. What do you make of this?

Interesting. I can't comment on the DCC angle other than to say that as I understand it, the power through the track is 16V AC rather than the more conventional DC. A similar thing happens when you wire a Relco or other high frequency track cleaner in.

I know this because I once tried to clean the track on a fiends 009 layout and got the same mighty jolt. Something to do with oxidising my fingertips apparently although I can't claim the know the science behind it.

All I can suggest is learn from your shock and be careful not to bridge the opposing sides of a split chassis. I'm surprised that the chip survived the inadvertent short circuiting.

Can anyone else help ?


Zabdiel said...

I would expect the chip to be fine - the short circuit went through Terry's hand bypassing the chip. Even small electric shocks can be painful.

Michael Campbell said...

I use a high-frequency track cleaner and that gives a similar zap if you manage to bridge the rails with your fingers!

The system may work at "16v ac" but that is the RMS voltage - the peak-to-peak would be about 20V (like a CDU gives). As they saying goes, "it's the volts that jolt - the current that kills". At about an Amp or so, you should survive.

Anonymous said...

Thank you chaps. I now have the solution to prevent further shocks to me - I'll ask the wife to pick it up instead!