Insufficient wiring : LUSENET : Wiring for DCC : One Thread

We just installed a Lenz Set 90 on our rather small club layout (only three stations). The layout has been in DC operation for over ten years, so I tried to preserve work by changing as little as possible.

The layout were already supplied by a 1,5 mm2 bus along the main line, so this was used as the DCC track bus.

All now works dandy except that on the station furthest from the booster, a short won't make the booster shut down. This is clearly in violation of your wiring principles, so I'm looking for possible solutions apart from rewiring the entire layout.

It seems the bus wire isn't the problem, as the resistance from the control panel through a shorted track and back is already 55-60 ohms according to our cheap digital ohmmeter. This makes me fear that station will have to be rewired. Are there other solutions, or am I just being lazy?

-- Jan Frelin (, August 07, 2003



You were smart to short the track and test shutting down the booster. Many manufacturers recommend this test in their manuals. It is a good, simple test that will quickly tell you that you may have problems. Sometimes things do work well. Other times, the locos will not respond to commands being sent them. A wiring problem is sometimes found to be the culprit.

I assume that 1,5 mm2 is the cross sectional area of the wire. For the US folks, that would be about 17 AWG. If your layout is truly small, you may be able to use this. I know of one large layout that was wired with wire this small. It not only failed the short test, but it also had the erratic operation problem.

Your message indicates that you have another problem. Even with wire as small as you have, you should not be reading 55-60 ohms. Many digital meters won't read zero, but you should still be reading just a few ohms if you have truly shorted the track for this size wire.

You sound like you have a pretty good idea of what you are doing, so when you say that you shorted the track, I trust that you have a definite short across the track. Make sure you are not reading a locomotive. Make sure that whatever you are shorting the track with is doing a good job. Sometimes you can lay a piece of metal on the track but it does not make good contact. Take a meter reading on the track right near the shorting device to be sure.

If you have a cheap meter you have doubts about, test a resistor to verify the meter is working reasonably well. Don't forget to consider the tolerance of the resistor and that your meter isn't perfect, either. So if you test a 5% resistor and your meter has a 5% accuracy, a 100 ohm resistor is going to read between 90 and 110 ohms.

Once you are reasonably sure your meter is working good enough, you need to go find your problem. The fact that you can't shut down the booster suggests to me that the meter reading you are getting is right.

Check block switches and connections. The contacts in old switches can be corroded. Are wires twisted together or soldered? The wiring you are using is a little on the small side, but it definitely should not be causing such a high resistance. Something else is wrong. You will need to identify the problem or rewiring won't help.

After you identify the problem, try the short test again. You might be okay.

Note: If this layout has been around a while and it has twisted together wire connections, I suggest you rewire your buses with something a little heavier. Twisted connections could be corroded and causing the resistance you are experiencing. A quality, liquid flux DESIGNED FOR ELECTRONICS (see my soldering section of my web page) can be used to solder existing corroded connections, but why bother? At that point, it won't take much longer to rewire with something heavier. You would hate to solder existing wiring only to find out you still can't trip the booster.

-- Allan Gartner (, August 07, 2003.

Thanks Allan,

Do you have a target resistance we should be looking for, or a minimally acceptable one? I did some simple calculations, and obviously 55-60 ohms is way high... I'm thinking we need to get down to 2-3 ohms from the booster end.

-- Jan Frelin (, August 08, 2003.

Yes, 3 is about the worst you should accept. To trip a 3.5A booster putting out 14V, You need a resistance lower than 4 ohms or it won't trip. Again, the short test is the ultimate decision maker for you.

Since you are converting an existing DC layout, you certainly have a set of switches if not two. Quality switches in good condition will have neglible resistance. If they have some corrosion, this could be your problem. Screw connections or twisted wires also could be the problem.

Note on screw connections: If the wire and screw were shiny when the wire was screwed down, it is probably making good contact. If the wires and screws were not in optimum condition or, if years later, the wire was loosened for any reason and then retightened, then you probably have created a problem. The wire does not frequently retighten right on the non-corroded surfaces of the wire and screw after being loosened.

-- Allan Gartner (, August 08, 2003.

We went through the layout on club night yesterday and measured resistance from the DCC distribution point throughout the layout ast different points with a different instrument. Measuring was done by disconnecting the booster and shorting the rails (with a large washer) at various points

We came up with values that were less scary than the last one resistance througout the layout varying between 1.1 ohms and 3.5 ohms. (55 ohms must have been an artifact of bad measuring).

Since these new values were closer to the acceptable range, we then choose a sledgehammer route of increasing the power output on the booster to 22 volts - and it worked!

Pondering the effects of shorting out our tiny H0n3 locos through the current collectors with 5 amps, we now off buying automotive rear lights...

-- Jan Frelin (, August 14, 2003.


Before you smoke your HOn3, you need to back off the 22V. That works, but that is not a great solution. If you must stay at 22V, do two things. 1. Make sure all the decoders you use can handle that. (Most can. Some can't) 2. Reduce your Vmax to keep your little motors from getting too much voltage.

Your 3.5 ohms is too close to unacceptable. Given tolerances in your meter, you may be over. Your booster has tolerances, too. So 3.5 is probably too high.

I know you are trying to avoid rewiring the layout. Perhaps there are creative ways to avoid it. Can you run heavier bus wires to the areas of the layout that have the high resistance readings and won't trip the booster? I don't know how many problem points you have are, but if you don't have a lot, this may solve your problem without a lot of work.

-- Allan Gartner (, August 14, 2003.

You seem to imply that the decoders will let through the maximum voltage (regardless of value) at maximum speed. I didn't realise this.

The rear lights we found are 24V, 21 W lights. If I'm right, that would mean they let through 0.875 amps when lit. This is sufficient for about 2 engines with some margin, and would presumably be sufficient to deal with fire safety issues.

Thanks for all the help, it seems we still have some thinking to do.

-- Jan Frelin (, August 15, 2003.

Yes, decoders do not have voltage regulators in them. That would make them bigger, more expensive, and run hotter. Besides, if the booster is not putting out too much, why build unnecessary circuitry into a decoder.

Note that by lowering Vmax or lower values in your speed tables, you would still be hitting the motor with the high booster voltage. By lowering these values, you are just adjusting the maximum pulse width the decoder is sending to the motor so that the APPARENT voltage the motor sees is lower.

I definitely suggest you find a way to lower your resistance and go with a lower booster output voltage. Sooner or later, someone will put a locomotive on the track that didn't get Vmax adjusted. If that locomotive gets run too fast, you will ruin the motor.

Light bulbs are not magic. I just pick one that inexpensive, doesn't light when I have a "normal" locomotive load, and works when there is a short. So if you find a bulb that works for you, go with it.

Good luck!

-- Allan Gartner (, August 15, 2003.

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