A simple Class 2 Transformer provides the Control Power for standard Douglas Lighting Control Panels

Douglas Transformer power supplies are current limiting transformers that provide power for safe, Class 2 type lighting control circuits. In most cases, only one transformer is required by a lighting control panel.

Douglas WR-4075 Series Transformers

The WR-4075 series Transformers are current limited to provide a safe Class 2, 24VAC power source. In the event of an overload, an internal thermal fuse will trip, shutting the transformer off. The internal fuse will automatically reset after the transformer has cooled down.

There are 2 models of WR-4075 series Transformer:

• WR-4075-120/277
  Dual tap primary 120V and 277V
  Power: 40VA steady, 75VA momentary
  Certifications: UL and CSA
• WR-4075-120/347
  Dual tap primary 120V and 347V
  Power: 40VA steady, 75VA momentary
  Certifications: CSA (CANADA ONLY)

Transformer Loading

In the vast majority of cases, only 1 transformer per relay panel will be required. Theoretically, there is no limit to the number of relays that can be powered by a transformer, since a Douglas relay only uses power when it switches. It uses no power when it is in the ON or the OFF state.

There are other lighting control devices that do use low amounts of steady power, such as time clocks, scanners, sensors and switches.

If a large number of these lighting control devices are powered by one transformer, you should do a Loading Calculation to ensure that there is no overload. In the rare case of an overload, split the circuits and add extra transformers.

Transformer Loading Calculation

Douglas WR-4075 series Transformers are able to provide a steady current up to 1600mA and can supply a momentary current up to 3000mA. Calculate the steady draw requirements to ensure that the 1600mA limit is not exceeded. The excess momentary power that is available can be used to switch relays.

The following is an example loading calculation:

1. Calculate the steady current draw.
   • 100 Douglas LED switches are connected, each using 4mA. The total steady draw from these devices is 400mA.
   • 2 Douglas relay scanners and 1 time clock are installed, each using 100mA. The total steady draw from these devices is 300mA.
   • The sum of the above steady draws is 700mA, which is only a portion of the 1600mA steady state current available.
2. Calculate the mA available for relay switching.
   • Subtract the steady state draw from the momentary current limit of 3000mA.
   • (3000mA) less (700mA) yields 2300mA. Each relay uses 350mA to switch, so up to 6 relays can be switched at the same instant.
   • (2300mA available) / (350mA per relay) = 6.6. Do not connect more than six relays to the same switch in this case.
   • NOTE: More than six relays can be triggered by a switch when a relay scanner is used.