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October 12, 2011 / Dario Torres

Measuring Pick Up & Drop Out Voltage of Relays

One common electrical characteristic that is specified of relays is the pick up and drop out voltage.  These values are defined as follows:

Pick up voltage (also defined as pull in voltage, or must operate voltage): As the voltage of a de-energized relay is increased, the value at or below which all contacts must function.

Drop out voltage (also defined as release, or must release voltage): As the voltage on an energized relay is decreased, the value at or above which all contacts must return to their de-energized position.

These values are important to engineers when certain designs are required to function within a certain voltage range.  For example, let’s say a 12V relay is being used to turn on a motor.  The motor will function as long as the contacts are closed.  However, if for some reason the voltage input to the relay is decreased to a certain level, the contacts will open and the motor will turn off.  Therefore, it is important that pick and and drop out voltage values are considered when designing any type of project that may cause the input voltage of the relay to fluctuate (i.e. battery input).

Pick up voltage and drop out voltage values are simple to measure.  A power supply and a digital multi-meter are the only two components needed.  The following schematic can be used for measurement:

Follow these steps to measure the pick up and drop out voltage:

  1. Set the DMM (Digital Multimeter) in Continuity mode
  2. Connect t he leads to the common and the normally open terminal pins.
  3. Slowly begin to increase the “coil power supply.”
  4. When the DMM beeps, it means the contacts have engaged and current is detected.
  5. The voltage reading on the “coil power supply” is called the Pick Up Voltage.
  6. Continue to increase the “coil power supply” to the nomical coil voltage of the relay.
  7. Now slowly decrease the “coil power supply” output.
  8. When the DMM turns off (or stops beeping), it means the contacts have disengaged.
  9. The reading on the “coil power supply” is the Drop Out Voltage.

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