Transformer Amps

[The Master]

Hi, I need to buy a transformer that can output just over 800mA. I found one with the following spec:-
Input Voltage: 230V
Output Voltage: 12+12V
Output Current: 416mA

I'm a bit confused with the output current. If i join both secondaries together in parallel does that give me 832mA?

[ronsimpson]

I do not know how your transformer is wound. (center tapped or two windings)
Paralleling the windings will add the current.

[The Master]

I dont know how the transformer is wound either. I know that putting the secondaries in parallel with double the current and that putting them in series will double the voltage. 2 voltages are specified so i know that each one is 12V but only 1 current value is specified. Im not sure if that is the current per secondary or the total current of both in parallel.

All transformers seem to be marked this way so i assume its a standard but i cant find any information on it

[rsocor01]

Master,

How many cables (total) do you have on the secondary side? What is the input voltage, 120 VAC?

Robert

edit: Oops forget about the input voltage. How many cables do you have on the secondary side?

[The Master]

There are 4 pins for the secondaries, 2 for each one and they are not internally connected.

Heres the datasheet (http://www.rapidonline.com/netalogue/specs/88-5036.pdf) if that helps. The part number is VTX-121-4810-212.

I read somewhere on the internet that the amps given are for both secondaries total. Im just doing the maths though and 10VA / 12V = 0.833A. The value given for that coil is 416mA which is half 0.833A. I think this means that each secondary is capable of 416mA and using both together will give 833mA.

[Byte_Butcher]

Yes, 833mA @ 12V, or 416mA @ 24V

Your clue is in the 10VA rating in the first column.

12 * 0.833 = 9.996 VA.

Are you really going to ask it to output over 800mA? Is that continuously, or for short duty cycle? I don't think I'd feel comfortable running a little transformer continuously at 96% of it's maximum rating...





Steve

[rsocor01]

Yes, 833mA @ 12V, or 416mA @ 24V

Your clue is in the 10VA rating in the first column.

12 * 0.833 = 9.996 VA.

Are you really going to ask it to output over 800mA? Is that continuously, or for short duty cycle? I don't think I'd feel comfortable running a little transformer continuously at 96% of it's maximum rating...





Steve

I was going to say the same, but took too long to respond :(. I completely agree with Steve about running this transformer continuously at 96% of it's capacity. If you do so, you might be able to fry eggs on top of this transformer :D.

Robert

[The Master]

I could be using anything from 50mA to 800mA. It is possible that it will be on full load for a few hours.

What is a good % to run at constantly? There are some bigger transformers i could use (upto 30VA). Would it be a better idea to use a chassis or toroidal transformer instead of a PCB mount one?

[rsocor01]

What is a good % to run at constantly?

For power design (buildings), 80% is a safe maximum number. Now, for small transformers I don't know what the percentage is.

Robert

[ronsimpson]

For power design (buildings), 80% is a safe maximum number. Now, for small transformers I don't know what the percentage is.

Robert

The transformer power rating is with a resistive load. If you use diodes and a capacitor to make DC then the transformer should be de-rated to 50%. Because of the diode/capacitor, current only flows near the peak of the sign wave. The current only flows for about 20% of the time and is 5x larger than with a resistive load.

[The Master]

I think ill go with a chassis transformer then. Ill get a much bigger one so i can run more lights from it too.

Thanks for your help. I understand transformers a little better now

[rsocor01]

The transformer power rating is with a resistive load. If you use diodes and a capacitor to make DC then the transformer should be de-rated to 50%. Because of the diode/capacitor, current only flows near the peak of the sign wave. The current only flows for **Databases
about 20% of the time and is 5x larger than with a resistive load.

There are ways of using 100% of the current cycle with diodes and capacitors, so there is no need to "de-rate to 50%". The VA rating is given for RMS values and yes it is for resistive loads. By the way, the power rating of the transformer depends on the maximum Vrms and Irms that the windings can handle at any given time. How much of the sine wave is utilized in the load is irrelevant.

Robert