# Too High Voltage Drop across Transformer's Output

I want to get very high current flow thought secondary winding of transformer on my 3 Ohm load. As I apply high voltage of secondary to the load the voltage across it drops dramatically. Only about 2% of initial voltage (of unloaded secondary) is presented at the load. What can I do to achieve larger voltage drop across the load?

If the secondary winding would have same ohmic resistance as load R1, that is 3 Ohms, would then half of secondary's unloaded voltage be then dropped across R1 (and half across secondary winding)?

Here, I used two transformers to power up my load. I need higher voltage (+100V), so 220V was only accessible, but I couldn't plug my load directly into wall socket because the fuse would break at the same moment I would plugged in my load - so I used transformer which would allow me to use same voltage but higher currents. In addition, I was counting for load to sink few tens of amps, but transformers weren't made for such currents. But I meant to use them just for a few milliseconds - in pulses so transformers would have time to cool down and not melt. Here are actual specs for both transformers: 220V to 15V - secondary's current is 4A and primary's 1A, 15V to 220V - secondary's current is 0.5A and primary's 1A.

I know that none of two transformers meet my requirements but I was sure that second transformer could generate few tens of amps only for a fraction of a second. And that is really basically what I want; a transformer that could supply around 220V or so while generating current of few tens of amps for a few milliseconds then a second or so of pause until next current spike. But would I need a transformer that can supply 100A of continuous current through secondary, even if wanted to use same amount of current for only a fraction of a second?

simulate this circuit – Schematic created using CircuitLab

You seem to be thinking that transformers can store significant amounts of energy. This is simply false.

You can't get any more power out of a transformer than you put into it. If you can't get the current you need from the mains connection, no number of transformers will "amplify" the current for you, unless you're willing to take that current at a lower voltage.

It seems you are seeking the current demands of a small spot-welder. The secondary winding produces the voltage, it does not drop the voltage across itself. If you want 30 amps across the 3 ohm load, the secondary needs to be 90 volts. To produce that current (even for milliseconds) requires an output of 2700 VA. That is a very big transformer.
A smaller transformer can give you the current you need (but at a lower voltage) or give you the voltage you need (but at a lower current.) You will always get AC power from any transformer. If DC will work for you, look at a capacitor circuit.

Whereas it's true that you can over-current a transformer briefly and stay within its thermal limits, that doesn't improve the resistance of the transformer windings, which will still limit the maximum you can draw.

In your case of dropping to a low voltage and back up again, the killer is probably the excess resistance in the low voltage windings, and the wire you use to connect them. Impedance across a transformer goes as turns^2. The turns ratio between 220 and 15 is about 15 (in round numbers), so every ohm on the 15 v windings looks like 225 ohms at the 240v windings. If you had only 10mohm winding resistance in both 15v windings in series that would add over 2 ohms to your output resistance, and you've not even considered the resistance of the 220v windings yet, which also appears in series.

For short pulses of high power you need a high power rated transformer. The only thing that may be reduced is cooling. The material of the transformer will store and slowly dissipate short pulses of thermal losses.

If the transformer's core is too small for the needed power it will be driven into saturation and this limits the transfered power. The peaks of the sinus waves are cut.

If the wires of the windings are too thin, the resistance of the windings is too high and limits the maximum current. The resulting voltage drop reduces the output voltage. So you get neither the necessary current nor the necessary voltage.

• You don’t saturate the transformer core with current, you saturate it with voltage-time area, which is fixed on your input. Also, if you would drive it into saturation, it’s not the peak of the voltage waveform which is cut, it’s the very tail close to zero crossing. – winny Dec 24 '18 at 14:31
• I know I need higher rated power transformer, but how higher? If I want to generate a pulse for a fraction of a second that goes as high as 220V and outputs around 100A, would I still need 220V/100A rated secondary of transformer? Or is 100A of current (for a fraction of a second) achievable with lower current rating of transformer's secondary? Because I doubt I could afford a transformer with 100A secondary current rating. – Keno Dec 24 '18 at 18:01
• @Keno You need a transformer rated for 22 kW. Of course you need a connection to the power grid capable for 22 kW. You don't need a transformer rated for continous operation at full power for hours. A low duty cycle of 5 % or less may do. So a blower cooled transformer should not be necessary. – Uwe Dec 24 '18 at 21:19