How to get different current from traansformer of same volt connected to main

[polashd]

as the equation of transformer (Vs/Vp= Ns/Np = Ip/Is), I need to know if the ratio of Amp and Voltage (output) is same or not, when input (of mains) is constant.

to clarify: From a fixed input (I and V like main line) how can i get 12v 1Amp and 12v 2amp.

According to the equation (Ns/Np= Ip/Is) if I want to change Amp I've to change the ratio of Ns & Np. but this will also change Vs (Vs/Vp = Ns/Np).

I know it's possible and I've lake of knowledge. I only need how it is possible (different output current without changing output voltage (from main line)

 

[ericgibbs]

hi,
The (Vs/Vp= Ns/Np = Ip/Is) are ratio's.

If you have say a 240Vac step down to 12Vac transformer, you should get 12Vac on the output winding.
The important point is the VA , volt/ampere rating of the transformer.
For an ideal say, 12VA transformer you could draw 1amp from the 12V secondary, the primary current would be 1*[ 12/240] = 0.05A

For a 24VA transformer you could draw 2amps from the 12V secondary, the primary current would be 2 * [12/240] = 0.1A

OK.?

 

[polashd]

hi,
The (Vs/Vp= Ns/Np = Ip/Is) are ratio's.

If you have say a 240Vac step down to 12Vac transformer, you should get 12Vac on the output winding.
The important point is the VA , volt/ampere rating of the transformer.
For an ideal say, 12VA transformer you could draw 1amp from the 12V secondary, the primary current would be 1*[ 12/240] = 0.05A

For a 24VA transformer you could draw 2amps from the 12V secondary, the primary current would be 2 * [12/240] = 0.1A

OK.?

Thanks for your reply

as you wrote VA rating is important. What determines VA rating, how to calculate it?

Is the primary current variable (on demand of the transformer). If so can't we use the equation (Ns/Np = Ip/Is) to get desired output current (by changing/calculating number of turns of the winding)?

 

[ericgibbs]

Thanks for your reply

as you wrote VA rating is important. What determines VA rating, how to calculate it?

Is the primary current variable (on demand of the transformer). If so can't we use the equation (Ns/Np = Ip/Is) to get desired output current (by changing/calculating number of turns of the winding)?

hi,
Transformers are sold in various VA ratings, you choose the one that is a little over the VA rating thats needed to power your project,
Say you were making a bench power supply for 12V at 5Amps, you would choose a 60VA transformer.
You could use that same transformer/power supply for a simpler project that required only say 1amp.... so its the maximum loading that you would expect from your projects.

To get the higher VA ratings of the transformer, the manufacturer uses the same ratio's but different gauges of wire for the windings. [ also the core is calculated to suit] , heavier gauge wire for higher currents..

The turns ratio determines the step up or step down ratio of the input voltage to output voltage, not the current.....the core size/material and wire gauge is chosen to suit the VA..

OK.?

 

[polashd]

I got it
thanks

 

[KeepItSimpleStupid]

I'll add the following. A transformer has some losses which might me core losses, (eddy current and flux) and I^2R losses and temperature variations. These will contribute to "regulation". Regulation may be the differences in voltage with and without a load.

You can, for instance, wire transformers with wire that has a square cross section to reduce the magnetic losses.