Capacitor Testing
Capacitors Testing Procedures For HID Lighting Transformers

Troubleshooting: What do I need to know about Capacitor Testing and Ballast Performance?

The proper test procedure is as follows:

  • Disconnect the capacitor from the circuit and discharge it by shorting the terminals or wires together.
  • Check the capacitor with an ohmmeter set to the highest resistance scale
  • If the meter indicates a very low resistance then gradually increases, the capacitor does not require replacement.
  • If the meter indicates a very high initial resistance that does not change, it is open and should be replaced.
  • If the meter indicates a very low resistance that does not increase, the capacitor is shorted and should be
  • The ohmmeter method of testing capacitors will only determine open or shorted capacitors. The capacitance
    value can be tested by many available portable TRUE RMS meters having that capability, though a test using
    a dedicated capacitance meter is more conclusive. The capacitance value will affect lamp performance of
    Constant Wattage ballasts in ways that cannot be determined by the ohmmeter method. A capacitor may look
    good visually, but should be tested for capacitance value or replaced.
  • The capacitor in a reactor or high reactance ballast circuits will only affect the ballast power factor and not
    ballast operation. Capacitor failure in these circuits will cause line supply current changes possibly causing
    circuit breakers to activate or fixture fuse failures.

What are Oil-Filled Capacitors?

Oil-Filled capacitors supplied today contain non-PCB oil and are a UL recognized component. Oil-filled capacitors
are only supplied with ballasts where the capacitor operating voltage cannot be satisfied by Dry Film Capacitors.
When required, the capacitor discharge resistor is connected across the capacitor terminals.

Additional precautions must be taken when an oil filled capacitor is installed. Underwriters Laboratories, Inc. (UL)
requires clearance of at least 3/8 inch above the terminals to allow for expansion of the capacitor in the event of

The maximum case temperature for oil-filled capacitors is 90°C.

What are Dry Metalized Film Capacitors?

Dry Metalized Film Capacitors are available to fill almost all needs for HID ballast applications. Advance's dry film
capacitors typically require only half the space used by oil-filled capacitors due to their not requiring additional
spacing for safety. Dry film capacitors have no exposed live voltage terminals, and thus do not require additional
clearance in the fixture. The compact, light weight, cylindrical non-conductive case and two insulated wires or
terminals reduce the required mounting space. Capacitor discharge resistors (when required) are installed within the
capacitor case. Dry film capacitors are UL recognized and contain no PCB material.

The maximum allowed dry film capacitor case temperature is 105°C.

What are Probable Causes of Shorted or Open Capacitors?

Normal capacitor end-of-life failure.
Overheated due to heat in the fixture or ambient temperature.
Capacitor mounted too close to ballast.
Incorrect voltage or capacitor value for ballast.
Mechanical damage such as over-tightened capacitor clamp.
What will cause reduced light output from my HID lighting?

As HID lamps age the light output decreases. First replace the lamps with those specified on the ballast label. If the
lamps have been replaced and the light output is still low, there are some tests to determine if the ballast is operating
properly. Measure the OCV and short circuit current of the ballast. The value of the OCV and short circuit current is
listed on the ballast label. Short circuit current is measured by shorting the output of the ballast at the lamp socket.
This will not damage the ballast as the ballast will current limit at the value shown on the label. If these two
measurements are as listed the ballast is operating normal. If short circuit current is low and the ballast shows no
signs of over temperature operation replace the capacitor. Ballasts that show signs of over temperature operation
should be changed.