# HELP: Mains transformers

Discussion in 'General Electronics Chat' started by hamopp, Sep 27, 2009.

1. ### hamopp Thread Starter Active Member

May 13, 2009
68
1
Hi All

I have got some transformers that are 230v input and 4.5v - 0 - 4.5v at 6va
can anyone tell me what 6va means in ma ?

I have been looking on the net for some maths to work this out, but not found
any that i could understand.

Regards
Howard

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
With the secondaries wired in series (4.5V-0-4.5V = 9V total) you can draw a continuous current from the connection of Irms = 6/9 A = 667 mA. This is based on my assumption that the total transformer rating is 6VA.

If the secondary comprises two separate windings of 4.5 V each (there would be four wires coming out on the secondary side), you can also connect these in parallel (check the winding 'polarities' first). In this case you could draw a total current of 1.33A (at 4.5V total) from the parallel combination.

3. ### hamopp Thread Starter Active Member

May 13, 2009
68
1
Thanks for that info
but how would one go about
So am i right in saying that a 12VA transformer of the same voltage = about 1Amp ?

Thanks again
Howard

4. ### rjenkins AAC Fanatic!

Nov 6, 2005
1,015
69
The basics are:

Watts = Volts * Amps

VA = watts / power factor

A resistive load has a power factor of 1, so in that case, watts = VA

(Power factor is in principle a measure of how well the peak current aligns with the peak voltage of the AC waveform. 1 = perfectly aligned).

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Good question - hopefully the manufacturer included a data sheet with enough information to enable you identify which wires are the correct ones to wire in parallel - if they have made provision for that in the construction. Otherwise you'd need an oscilloscope to check the relative phases of the secondaries. The only other way is to try one connection with a fuse (say 1 Amp) link included in the parallel loop - if the fuse doesn't blow you've picked the correct connection. If it does blow, reverse one of the secondary connections and try again.

12VA at 4.5V-0-4.5V would give you 12/9 = 1.33 A or 2.66 A if you wire the secondaries in parallel for 4.5V total.