# How to tell how much current can pass through a transformer

Transformers have a ratio value. For instance, 10:1 will drop voltage 10 times less but increase amperage 10 times more.

Usually when shopping for transformers I see this voltage values for primary and other voltage values for secondary/auxiliary/etc. so 120V primary to 12 V would be an example of a 10:1 transformer.

My issue is, I can’t see any information on the max power/current for the transformer.

The same transformer 10:1 doing 120 V to 12 V can draw 0.2 A -> 2 A or 2 A -> 20 A. These are very different scenarios... Since it it made up of coils, the transformer's size and wire gauge will determine the max current and too much current can burn the coil's thin wires, causing a short and ruining the transformer.

How do I get this information? How do I know how much current I can pass through a transformer before buying said transformer? Also, I believe the frequency that the transformer is being used at will also affect the max current, right?

• for those of you talking about power transformers... I am wanting to build SMPS so I do need pulse transformer! Nov 14, 2023 at 13:30

When you discover that you are looking at the wrong sort of transformer and, move-on to find the right sort for power AC applications, you would use the V.A rating of the device to calculate the maximum RMS output current. So start here at digikey and look for the column called "power". The column has units of V.A. You will also find that pretty close to that column is maximum output current column should you prefer to sort devices based on maximum current: -

Your first sentence has an error. A transformer does not change voltage down and necessarily boost up current.

And the currents you seek for the transformers are already in the first column.

And finally, those are not even regular mains transformers you are looking at, but switching mode power supply transformers, which cannot be used on 50/60 Hz mains but with high frequency driver circuit.

Here's a basic transformer specification (assuming there are no losses).

1. Primary voltage: 240 V 60 Hz

2. Secondary voltage: 24 V

3. Volt Amperes: 480 VA

Volt Amperes is the product of voltage and current either on the primary or the secondary side.

The full load primary and secondary currents may be determined as follows:

Primary current = Volt Amperes / primary voltage = 480 / 240 = 2 A

Secondary current = Volt Amperes / secondary voltage = 480 / 24 = 20 A