Ballast OCV Testing
HID Ballast Transformer Testing / True RMS Meter

Are there any precautions that should be taken when measuring Open Circuit Voltage (OCV)?

Any fixture that includes a ballast with an ignitor will have high voltage pulses at the lamp socket, when power is applied
to the ballast and the lamp is not operating, that can damage a voltmeter used to measure OCV. The ignitor should be
disabled or the meter protected by a high voltage capacitor (approximately .0027 uF, 2,000 V) across the input. Metering
could also be connected after the lamps starts (at which time the ignitor shuts off).

Troubleshooting: What do I need to know about measuring Open Circuit Voltage?

To determine if the ballast is supplying proper starting voltage to the lamp, an open circuit voltage test is required. The
proper test procedure is as follows:

Measure input voltage (V1) to verify rated input voltage is being applied to the ballast.

If the ballast has an ignitor (HPS, low wattage MH (35 –150W) or pulse start), the ignitor must be disconnected or
disabled with a capacitor (1000 pF or larger) across the voltmeter input to protect the meter from the high voltage ignitor
pulse. Some ballasts have an integral or built in ignitor. If you are not sure if an ignitor is used put a capacitor across the
meter for all open circuit voltage measurements.

With the lamp out of the socket and the voltage applied to the ballast or the proper tap of the ballast with multiple voltage
inputs, read the voltage (V2) between the lamp socket center pin and shell. Some lamp socket shells are split. Make sure
connection is being made to the active part. The reading must be within test limits shown in the table. Open circuit
voltage must be measured with a TRUE RMS voltmeter to provide an accurate reading.

Constant wattage ballasts have a capacitor in series with the lamp. If the capacitor is open there will be no open circuit
voltage. Measure the voltage on both sides of the capacitor. If the voltage exists on the ballast side but not on the lamp
side, change the capacitor and re-measure the open circuit voltage on the lamp side. If there is still no voltage, disconnect
the lamp socket from the ballast and measure open circuit voltage again. Once a voltage is measured test the lamp socket
for shorts with an Ohmmeter or replace the lamp socket. An ohmmeter test is not conclusive as the test is at low voltage
and the failure may be due to the open-circuit voltage.

Are there any precautions that should be taken when measuring Open Circuit Voltage (OCV)?

Any fixture that includes a ballast with an ignitor will have high voltage pulses at the lamp socket, when power is applied
to the ballast and the lamp is not operating, that can damage a voltmeter used to measure OCV. The ignitor should be
disabled or the meter protected by a high voltage capacitor (approximately .0027 uF, 2,000 V) across the input. Metering
could also be connected after the lamps starts (at which time the ignitor shuts off).

What instruments and test equipment are required for troubleshooting HID Ballasts?

Only the input to HID lighting systems is a sine wave. Once the voltage and current is processed through the ballast and
lamp, it is changed and is no longer a sine wave. As a result of this transformation, only TRUE RMS volt and amp meters
will give proper readings. TRUE RMS clamp-on current meters are also available and are most convenient when reading
lamp current.

There are many brands of test meters available. Some indicate RMS and some indicate TRUE RMS on the meter. They are
not the same. Only those that have TRUE RMS will read non-sinusoidal waveforms accurately. The RMS meters will give
readings 10 to 20% low depending on the shape of the voltage or current waveform. Some of these instruments will also
read capacitance directly when connected to a disconnected, discharged capacitor.

There is no field usable meter to test ignitors.

Troubleshooting: What do I need to know about conducting a Short Circuit Lamp Current Test?

Do not be concerned about shorting an HID ballast output. They will not instantly burn up. An HID ballast is designed
to limit current at the specified value range.

To assure that the ballast is delivering the proper current under lamp starting conditions, a measurement may be taken by
connecting an ammeter between the lamp socket center pin and the socket shell with rated voltage applied to the ballast. If
available, a lamp socket adapter may be used as described in the open circuit voltage test.

The proper test procedure is as follows:

  • Energize ballast with proper rated input voltage
  • Measure current with ammeter at A1 and A2 as shown in the diagram
  • Readings must be within test limits shown in the table
  • A clamp-on TRUE RMS ammeter may also be used to perform this test by placing an 18 gauge wire between the
    lamp and common leads of the ballast. When using a clamp-on ammeter for this measurement, be certain the meter
    is not near the ballast magnetic field or any steel object that may affect the reading.

The short circuit current test will also determine a defective capacitor in constant wattage circuits. A shorted capacitor will
result in high short circuit current, while an open capacitor or low value capacitor will result in no or low short circuit
current.