78 VAV Controller—Variable Air Volume Controller
Before connecting the transformer to the VAV, connect the input
power to the primary leads of the 24 VAC transformer.
Note: If you ground the secondary of the power transformer, you
must use a separate 24 VAC to 24 VAC isolation transformer
(such as the Y65GS.)
Measure the voltage of each secondary transformer lead to earth
ground with the DVM in parallel with a 100k ohm 1/4 watt resistor
(Figure 39; if using double banana plug, insert plug into DVM).
- If you read 5 VAC or greater, the transformer is earth grounded.
You need an isolation transformer in order to isolate the
connections from earth ground and protect system components.
Wire a separate 24 VAC to 24 VAC isolation transformer (such as
the Y65GS) to the VAV.
- If you read less than 5 VAC, the circuit is properly isolated.
Usually a value less than 0.10 VAC indicates a completely isolated
circuit.
Line
Voltage
24 VAC
TRANSFM1
100K ohm
1/4 watt
DVM
100K ohm
1/4 watt
DVM
Figure 39: Testing the Transformer
Determine the polarity of the transformer’s leads by using a DVM
referenced to earth ground without the 100k ohm resistor. Connect the
transformer’s secondary lead with the higher voltage potential to the
24 VAC terminal on the VAV. Connect the transformer’s secondary
lead with the lower potential to the 24 VAC Common terminal on the
VAV.
102FPower
Transformer
Isolation –
VAV100/101
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