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Protecting against short circuits


Here are a few means of protecting fault blocks for DCC:

My father often put a standard incandecent lamp in series with the supply. He would select a bulb that was rated at about the maximum  voltage and amperage the supply was to deliver and the bulb would light ONLY if the supply was shorted. A cold lamp fillament has a resistance much lower than when glowing. Manufacturers are required to make it at least a 4 to 1 difference, but most are 10 to 1 or so. Bulbs can also be operated at below the rated voltage to change the resistance offered. As shown in the chart below, if a bulb is opperated at half its rated voltage, it will still consume about 69% of its rated current, produce less than 1/10 its normal illumination and last about 4000 times as long. So a 10V, 0.017A bulb in series with a 5V signal, will, when shorted to ground, supply about 450 ohms of resistance limiting the current flow to 0.011A and when not shorted will have a resistance of about 45 ohms.

See also:

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