KMC Controls XEE-6311 - Manual (Page 5)

Digital Designer’s Guide
AN0604D Revision B
Tips for connecting 24-volt power
Connecting to controllers
1–19
If grounded transformers are used on a network containing both full-wave
devices and half-wave devices, problems may occur because of the circuit
differences in the power supplies. The common connection (direct or indirect) on
the network and the grounds on the transformers may set up an inadvertent short
circuit. In the example shown in Illustration 6 below, the transformer on the
full-wave device is shorted across one of the rectifier diodes during half the AC
cycle. Likely symptoms of such a situation may include:
◆ A circuit breaker or fuse opens.
◆ The power supply and/or transformer becomes overheated or damaged
.
120 VAC
24 VAC
Circuit
Breaker
Common
24 VAC
Circuit
Breaker
Common
Full-Wave Device Half-Wave Device
+ +
– – – –
120 VAC
Filter/
Regulator
Filter/
Regulator
Even with grounded transformers, this should not be a problem:
◆ If all the devices are full-wave or all the devices are half-wave. (Nearly all
currently available KMC devices are half-wave.)
◆ If the full-wave and half-wave device connections remain truly isolated.
If circumstances require the use of a grounded transformer with a full-wave
and a half-wave device on a network,
one possible solution may be the use of an
isolation transformer. The example shown in Illustration 7 below, shows a single
XEE-6000 series transformer and a 24 VAC isolation transformer. (Be sure the
transformer is rated to handle the extra load—the total nominal load on a
transformer should be no more than 80% of the transformer’s rated value.) An
isolation transformer could also be used between a transformer and a single
device.
Illustration 6 Shorting of circuit from connected (grounded) full-wave and half-wave devices