Page 18 - NMRA Roundhouse January-February 2018

Finding Short Circuits
Finding short circuits can be tricky, particularly if it is not easy to disconnect the circuit into smaller components.
Let us consider some track which uses printed circuit board ties. Let us suppose that one of the gaps cut in the copper has become bridged. This is shown as a red line on the drawing below.
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Assuming it is part of a modular layout, the first thing to do is to attach a meter set to ohms at one end. Remove the inter module jumpers, a pair at a time until the short disappears. This will tell you which module is faulty but it may leave a lot of potential ties that could be faulty.
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Testing the defective module with a meter set to ohms as shown above is most unlikely to tell you where the fault is as the measurement of resistance is probably not very accurate.
A better method is to attach a power supply as shown below and to regulate the current so that about 1⁄2 amps is flowing via the short. The current takes the path shown by the arrows.
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Set the meter to the 200m range (200 millivolts). Put the probes onto one rail about 2 inches apart. The resistance of Code 70 nickel silver rail is about 0.075 Ω per metre so a 2 inch length is about 0.004 Ω. If a current of 1⁄2 amp flows though this piece of rail, it will produce about 2 millivolts across every 2 inch length of rail. 1⁄2 amps is chosen as a current that will give useful readings but is unlikely to cause damage by heating.
A meter at the left hand position shown above would show a voltage of about 2 mV as a current of 1⁄2 amps is flowing through the rail. However a meter in the right hand position would read zero as no current is flowing here. By moving along the rail until the voltage disappears will give you the position of the short.
“Ah but...”, I hear you say. “My layout is DCC and I have droppers from the rail at intervals to reduce voltage drop.”
The droppers are shown in blue. They are a lower resistance than the rail, so the current will flow in the dropper in preference to the rail.
The main current flows are shown in the diagram. The dotted arrows show smaller currents flowing via the rail.
Now a meter at the positions shown would indicate;
• Position 1 would show a low voltage (probably less than a millivolt)
• Position 2 would show about +2 millivolts
• Position 3 would show about -2 millivolts (the current is
flowing the opposite way in the rail) • Position 4 would show zero.
The fault is at the position where the polarity of the current changes. If you happen to put the meter leads 1” either side of the fault, the reading would be zero.
This method can be used in complex trackwork as a means of finding which rails are carrying a current. If there is no current flowing, the voltage is zero.
So this was a quick run through what a cheap multimeter can do and how it can be used to find those irritating problems that beset most model railways. I hope that it will prove useful.
And if you are into on-line tutorials you could try this link though I do not vouch for its quality – it does address the subject though.
https://www.youtube.com/watch?v=TdUK6RPdIrA
You will find links to other videos, though NMRA British Region cannot accept responsibility for any guidance given or comments made (for example see my warning on page 2 above):
https://www.youtube.com/watch?v=lo8MWr3NuuM https://www.youtube.com/watch?v=ZBbgiBU96mM https://www.youtube.com/watch?v=EVFkKBFJsZg https://www.youtube.com/watch?v=zb7WHaL_dz8
sharing know-how
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Power Supply
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Power Supply
Power Supply
18 ROUNDHOUSE - February 2018