+ 24 I2 I4 I6
- 24 I1 I3 I5
In addition to a PLC, relay, and coil, a typical hydraulic or pneumatic control circuit has
connectors (167 and 172), a fuse, and a terminal block (121 and 122).
OV NC Q2 Q4
24V Q3 Q1 PE
If the machine was designed with
LEDs on the relay and the connector on
the coil, you’re in luck—these are really helpful when troubleshooting.
• Verify that the voltage on the output of the PLC is at the proper level. If
it is a DC circuit, the voltage level may
be active high or active low depending
on how the circuit is designed. Also, the
PLC may have an LED dedicated for
that output, which tells you when the
PLC’s output should be active.
• Check the relay. If the LED illuminates when the PLC is active, the relay
is receiving a signal. If the relay doesn’t
have an LED, use a voltmeter to verify
that the primary (input) side of the
relay is receiving enough voltage. Also
measure the voltage level on the secondary (output) side of the relay.
• Check the connector. Is its LED
illuminated? If it has no LED, measure
the voltage at its output. If the LED on
the connector is illuminated but the
cylinder is unresponsive, you probably
have a faulty coil on the valve itself.
Another approach is to take a look
at all the LEDs to see which ones are
active and which are not, then troubleshoot accordingly.
Some control circuits do not use a
relay to control a cylinder. Some valves
draw so little current that a control
relay isn’t necessary. However, it is
common to find a fuse in the circuit
protecting the coil on the valve. Making
sure the fuse is not blown is always a
George Winton, P.E., is the founder and
president of Winton Machine Co., 470
Satellite Blvd., Suite F, Suwanee, GA
30024, 888-321-1499, gwinton@
METAL MARKET Advertising Section
A TPA PUBLICATION
APRIL/MAY 2012 • TPJ 27