View Full Version : Transformer information question

Black Forest
06-22-2011, 12:54 PM
I am now working on my sheep shearing machine control system.

The PLC reguires 24volts as do the hydraulic valves. So I need a transformer.

What information do I need to supply for you all to tell me what kind of transformer, how big, etc. etc.?

06-22-2011, 12:58 PM
You need to know the current requirements for each device to be connected to the transformer. Add up all the currents, then multiply by 24 (the voltage) this will give you the VA requirement for the transformer, which is all you need to size it (well the primary voltage too).

06-22-2011, 01:38 PM
Is that DC? if so for the solenoids and the inputs you can usually get away with a common 24vac secondary with just a bridge, no capacitor, BUT, the PLC may need the ripple removed with a Cap, in that case the secondary of the transformer is DC x .7, so, 24DC x .7 would be a ~16-17vac.
For a ball park on sol current @ 1amp apiece the VA size will be ACV secondary X total current.

Jaakko Fagerlund
06-22-2011, 04:25 PM
And add 20 % to the VA rating so the transformer won't be on full load all the time.

06-22-2011, 04:48 PM
For extra esthetic bonus points you can get DIN rail power supplies from Siemens that matches your Logo. A quick look shows at least 1.3A and 4A versions like this:

It's a bit cheaper though to find another switch mode 24V power supply.


J Tiers
06-22-2011, 09:22 PM
If your relays or valves require AC, which they might, then you need to know the "pull-in" VA, and must be able to supply the corresponding AC current for as many as must pull-in at the same time. Pull-in VA may be many times the "sealed" or "closed" VA.

For a DC supply, a small SMPS with 24DC out is a very good solution. All approved and so forth, one-piece, no design work past deciding the needed current.