120v - 12v transformer connected to 12v - 120v transformer

[dinesh401]

Normally if we give a 120v to primary of a 120v/12v transformer the output will be 12v .
If we reverse it i.e., if we give 12v with 1A to the secondary of the same transformer we will give get 120v AC with 0.1A

If we give 120v to the 120v/12v transformer we will get 12v output. If this output is connected to the 12v secondary of another transformer can i get 120v of AC with 0.1 A ?

 

[BradtheRad]

I once tried to make my own homebrew inverter, with the idea that I could take a 120VAC to 12V step-down transformer, and reverse it.

It didn't work. I could not get sufficient voltage across a 25 W light bulb. Even a 7w bulb only glowed dimly.

Years later I continued wondering what was the problem.

I finally realized the high V side is constructed to present a high impedance to the power company. The power company might be 2 ohms. The transformer primary has to present a substantial impedance to the power company. This comes from a combination of resistance and reactance.

Then we turn it around. If the primary was 100 ohms impedance for 120 V incoming, it will still be 100 ohms outgoing (assuming frequency is unchanged).

And that impedance will be added to any load you attach.

Your first transformer admits 120 VAC at 0.1 A. Therefore it has an overall impedance of 1200 ohms. As for the low voltage windings, we'll just say their impedance is low in comparison.

The output from your second transformer might conceivably have .1 A maximum going through it, if you short the output wires. Because it also has an impedance of 1200 ohms. So any load you attach will be in series with that 1200 ohms. You won't get .1 A through the load.

The most power you can get is by matching the impedance, namely with a load of 1200 ohms.
Net will be 60 VAC at .05 A.

 

[dinesh401]

thank you so much for your explanation.

So i forgot the impedance offered by the transformer in my calculation.

 

[The Electrician]

Normally if we give a 120v to primary of a 120v/12v transformer the output will be 12v .
If we reverse it i.e., if we give 12v with 1A to the secondary of the same transformer we will give get 120v AC with 0.1A

If we give 120v to the 120v/12v transformer we will get 12v output. If this output is connected to the 12v secondary of another transformer can i get 120v of AC with 0.1 A ?

Why do you want to do this? Are you trying to make an isolation transformer out of two transformers?

 

[rfredel]

Hello dinesh401,

you can do it, and it will function. The problem is only, that your output voltage is a little bit lower then 120V about the losses inside the transformers (DC-resistance of the windings and iron-losses). The smaller the power of the transformers, the greater the losses.

I do it like this, to make power supplies for little valve amplifiers. So I have a cheap isolation to the main input. Also I can take the filament voltage from the first transformer.

For your example you need 120V and 12VA. In this case I would take for the first transformer 120V / 12V 15VA or 20VA (12V 1,25A or 12V 1,6A) and for the second transformer a secondary voltage of 9V as input and power of 12VA. So you can minimize the losses.

Regards

Rainer