In model railroad application, the power is from the rails, the contact between wheel and rail is important. If the contact is not always solid, the power may be opened, then LED will be turned off or blinks and model stops to move.

The capacitor is a component which has the ability or “capacity” to store energy and can act as an Uninterruptible Power Supply in short period in case that the contact between the wheel and rail is opened and model lose the power from rail.

The unit of capacitor is Farad (F), the larger the capacitance, the more energy can be stored and can supply the model for longer period.

Standard Units of Capacitance

  • Microfarad  (μF)   1μF = 1/1,000,000 = 0.000001 = 10^-6 F
  • Nanofarad  (nF)   1nF = 1/1,000,000,000 = 0.000000001 = 10^-9 F
  • Picofarad  (pF)   1pF = 1/1,000,000,000,000 = 0.000000000001 = 10^-12 F
Pico-Farad (pF) Nano-Farad (nF) Micro-Farad (μF) Farads (F)
1,000 1.0 0.001  
10,000 10.0 0.01  
1,000,000 1,000 1.0  
  10,000 10.0  
  100,000 100  
  1,000,000 1,000 0.001
    10,000 0.01
    100,000 0.1
    1,000,000 1.0

There are a lot of capacitor types, eg. Electrolytic Capacitors, SMD Ceramic capacitor, Tanta Capacitor. As choosing/using capacitors, please note two important factors, polarity and voltage rating. In case of HO scale model railroad, the voltage is 18V, capacitor with higher than 25V voltage rating is a safe selection. To assemble the polar capacitor (eg. electrolytic capacitor), please ensure to apply the positive voltage to positive lead of capacitor, otherwise will damage the capacitor.

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