Unit installation

64 OMRCPMICRO
Testing Solid-State Relays
The input resistance of most digital voltmeters is on the order of 10,000,000 to 12,000,000 ohms (10 to 12
megohms). Analog meters with 20,000 ohms/volt movements would have an input resistance of 5 megohms on
the 250 volt scale. Connecting a voltmeter into the circuit adds very little load to the relay. When the solid-
state relay switches to "off", the resistance increases to approximately 100K ohms. This is a 100:1 ratio of
resistance between the digital meter and the load, and a 50:1 ratio between the analog meter and the load. This
will cause 100 times more voltage to be dropped across the digital meter as is dropped across the relay, and 50
times more voltage to be dropped across the analog meter as is dropped across the relay. A low wattage
lightbulb (of the appropriate voltage) will load the circuit enough to eliminate the false voltage signal. (Note:
The cold resistance of a 25 watt/120 volt bulb is approximately 48 ohms.) In this instance, an incandescent test
lamp is a better tool than a voltmeter.
The output contacts of each solid-state output relay on the Output Board is equipped with a 5 amp fuse.
These fuses resemble tiny resistors and are located on the Output Board adjacent to the relay they protect.
These are pressed into place and can be removed by lifting them out with a needle nose pliers.
The LED located on the solid-state relay base responds to the input of the solid-state circuit. If the LED does
not glow, the relay is probably not energized. These relays require the existence of 5 VDC to initiate closure.
When the relay is energized, the LED should glow.