Electronics Forum / Basics / June 2006

My project requires operation of a single relay to control mainly
resistive loads up to 5amps ac @ 230v. The main project control
circuitry (5v DC) requires very little power (few micro amps idle to
10mA active) - however, the relay coil does require quite a bit of
power when the relay coil is energised, details as follows:

- From the datasheet: Relay is 5v coil, non-latching, SPCO, nominal
power consumption is 400mW.

I need to select a pcb mount transformer to safely and conservatively
provide power for the above control and relay circuitry, but am going
round in circles with my understanding and basic calcs as to the size
of the transformer I need.... can someone help me with my understanding
as follows:

Project Power Requirements:
Control power: 10mA
Relay coil: 400mW / 5v = 80mA
Therefore total project power = 90mA

If my calcs above are correct, then what size transformer do I need?

- Is is good practice to apply a nominal 'margin' of lets say 1.5 *
project design power (to allow for safe working of the relay, and some
inductive loading)? this would mean I need a transformer with output of
135mA.

Here's the bit I'm stuck on.... calcs above relate to DC power @ 5v,
how do I relate that to the Va rating of the transformer, which primary
is at 230v?

Thanks.