Protecting against short circuits

here are four means of protecting fault blocks for DCC that I am aware of:

• 12V Incandescent Lamps. Automotive turn signal lamps are a good type to try. These have to be chosen so that the maximum usual current in the section protected by the lamp does not light the lamp.

  When a short occurs, the lamp heats up and lights. The resistance of the lamp goes up when this occurs, limiting the current. Once the short is gone, the lamp cools and allows normal operation to resume automatically. The lamp's light also provides a positive indication of trouble, if the lamp is placed where it can be seen.
   

• Fuses. You must test to see if the fuse you select will respond quickly enough to keep the DCC system from going off-line first. (To protect the fuse, of course...) Sized correctly, these may do the job, but replacements will cost you $.
   
• Circuit Breakers. These have a respectable initial cost, and again you must test to see if the model you select will respond quickly enough to keep the DCC system from going off-line first.
   
• Raychem's "PolySwitch Resettable Fuse." This device offers the possibility of a compact and accurate means of protecting against shorts. It has a "hold current" which it is guaranteed to sustain, and a "trip current" at which it is guaranteed to trip. After tripping, it resets automatically once the short has been cleared. The RXE and RUE series have trip currents which are twice the hold currents. Models in these series can hold up to 3.75 and 9 amps respectively, which should be adequate for most command control systems I can imagine. Cost averages $1.00 each from Digi-Key.
   

The following circuit uses two 1N4148 diodes to protect parallel port against higher than +5V signals and also against wrong polarity signals (power on the circuit is accidentally at wrong polarity.

          Diode          
          1N4148  4.7K   
parallel  >-|>|-+--\/\/\/--
port data       |          
pin             +-|<|-+    
            1N4148    |    
parallel  >-----------+
port ground           |
                    Ground

Adding even more safety idea: Replace the 1N4148 diode connected to ground with 5.1V zener diode. That diode will then protect against over-voltage spikes and negative voltage at the same time.

Another way to protect against over-voltage is to put a regular diode with the cathode on the signal line and the anode on the power supply line. Keep in mind that this will cause the power supply to recieve the over-voltage and may result in damage to other components on the board.

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:

• http://www.microlamps.com/data/Search/336.htm 5V @ .02 amps = 250 ohms resistance when lit.+
• http://www.microlamps.com/data/Search/717.htm 5v @ .017a = 294 ohms.
• http://www.microlamps.com/showpart.aspx?modelid=186&cat=57&subcat=28 +
• http://www.microlamps.com/showpart.aspx?modelid=187&cat=57&subcat=28 5v @ .021 amps = 238 ohms resistance when lit.+

Also look at: tranzorb a brand of SILICON AVALANCHE DIODE +

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