TESTING SWITCHES and RELAYS

Switches and relays have contacts that open and close mechanically and you can test
them for CONTINUITY. However these components can become intermittent due to dirt
or pitting of the surface of the contacts due to arcing as the switch is opened.
It is best to test these items when the operating voltage and current is present as they
quite often fail due to the arcing. A switch can work 49 times then fail on each 50th
operation. The same with a relay. It can fail one time in 50 due to CONTACT WEAR.
If the contacts do not touch each other with a large amount of force and with a large
amount of the metal touching, the current flowing through the contacts will create
HEAT and this will damage the metal and sometimes reduce the pressure holding the
contact together.
This causes more arcing and eventually the switch heats up and starts to burn.
Switches are the biggest causes of fire in electrical equipment and households.
A relay also has a set of contacts that can cause problems.
There are many different types of relays and basically they can be put into two groups.
1. An electromagnetic relay is a switch operated by magnetic force. This force is
generated by current through a coil. The relay opens and closes a set of contacts.
The contacts allow a current to flow and this current can damage the contacts. Connect
5v or 12v to the coil (or 24v) and listen for the "click" of the points closing. Measure
the resistance across the points to see if they are closing.
You really need to put a load on the points to see if they are clean and can carry a
current.
The coil will work in either direction.
If not, the relay is possibly a CMOS relay or Solid State relay.
2. An electronic relay (Solid State Relay) does not have a winding. It works on the
principle of an opto-coupler and uses a LED and Light Activated SCR or Opto-TRIAC to
produce a low resistance on the output. The two pins that energise the relay (the two
input pins) must be connected to 5v (or 12v) around the correct way as the voltage is
driving a LED (with series resistor). The LED illuminates and activates a light-sensitive
device.