VA's on a transformer?

I'm busy with a project which I need to power on 5 relays at the same time.
The relay's are 5V each and consume 150mA when powered.

So in total they should draw 750mA

I bought a transformer but now am not sure its 'powerful' enough?


Its a 230V/12V - 3VA 50Hz transformer.

Does that mean it can put out 3V at 1A (ie 5V at 3/5A?)

Would that mean is too small to drive all my relays on at the same time? I would need a 750mA * 5V (3.75VA) Transformer?

I would have to put the output of the transformer through a rectifier and then a 5V regulator - would that make things even worse?

What would happen if the transformer is too small to power all the relay - would they just not all power on, or would the relay eventually burn out?

gregmcc said:

I'm busy with a project which I need to power on 5 relays at the same time.
The relay's are 5V each and consume 150mA when powered.

So in total they should draw 750mA

I bought a transformer but now am not sure its 'powerful' enough?


Its a 230V/12V - 3VA 50Hz transformer.

Does that mean it can put out 3V at 1A (ie 5V at 3/5A?)

Click to expand...

Yes, but as it's a 12V transformer it's maximum current will only be 3/12A

Would that mean is too small to drive all my relays on at the same time? I would need a 750mA * 5V (3.75VA) Transformer?

I would have to put the output of the transformer through a rectifier and then a 5V regulator - would that make things even worse?

Click to expand...

Yes - if you're using a higher voltage transformer, then you should use higher voltage relays and feed them from before the regulator - but 3VA 1s still too small a transformer.

What would happen if the transformer is too small to power all the relay - would they just not all power on, or would the relay eventually burn out?

Click to expand...

It would burn out the transformer, and almost certainly the device wouldn't work properly before that happened.

Nigel - thanks for the explanation. Guess its back to the electronics shop tomorrow.

So a 0-12V 1A transformer should do the trick?

Your transformer won't do. You need a 10VA transformer. But why not get a 5/6 volt wall-wart that puts out the right current and is allready DC out! E

Thats a good idea - didnt think of that

I have one right next to me I"m not using - a Nokia 5V 2A charger.

Does that mean it can put out 3V at 1A (ie 5V at 3/5A?)

Click to expand...

No. It means it can put out 12V at up to ~250mA (1/4A). So the tranny is unsuitable and would burn out. You need a lower voltage higher current transformer, e.g. 9V with at least 9VA rating. Bear in mind that the rectified peak voltage is 1.4 times the rated RMS voltage of a transformer, the rectifier drops ~1.4V if it's a bridge type, the rectifier output needs a high value smoothing capacitor, the regulator input voltage needs to be a few volts (depending on regulator type) above its 5V output, and at 750mA the regulator will create a lot of heat so will need a heatsink.

LOL

Sounds like a trap.

1. According to the stuff I know, your little Transformer will not cut it. It will run very hot while it lasts
2. As per the last question.....the Relays will not burn out eventually as they have never really worked anyway....

Strange questions deserve strange answers.

tvtech

Never mind. You guys covered it above.

Regards,
tvtech

Thx all - you learn something new every day

As another possibility you can easily get 5v relays where the coil runs from 5v 30mA or 5v 50mA, they are quite common.

Mr RB said:

As another possibility you can easily get 5v relays where the coil runs from 5v 30mA or 5v 50mA, they are quite common.

Click to expand...

It rather depends on his loads though - small relays only have small capacities as well.