Re: Updated album Car lighting keep alive cap #photo-notice

Don Weigt

The values George gave will work fine.
The bridge rectifier is about as low a rating as you can get, so it will be small and inexpensive.
The 270 Ohm resistor will limit the inrush current to about 0.06 Amperes, so many lighted cars in a train will work without overloading most boosters. The peak inrush current and voltage across the 270 Ohm resistor momentarily creates a 1 Watt power level, but it will drop off quickly to about 1/100th Watt, so a 1/4 Watt resistor, which is much smaller, can be used in place of the 1/2W.
Same for the 4.7k Ohm resistor. If the cap reaches 16 V DC, which is highly unlikely, the current will still be less than 4 milliAmps (0.004 Amperes) That gives a steady state power level less than 0.064 Watts. A quarter Watt resistor would be fine here, too.

The 470 uF capacitor will indeed keep the lights on for a long time used with that 4.7k resistor. The time constant (time to drop to 37% of the initial voltage)  of the resistor and capacitor is about 2.2 seconds, and many of these capacitors have a higher capacitance than their nominal value, tolerances being something like +50% or +100% and -20%. Given the working voltage for HO and smaller gauges should be less than 16V, a 25V capacitor of about half the size could be used in place of the 35V one. The 35V part will have a longer life, all else being equal, but both will probably last for the lifetime of the cars.

Don Weigt

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