How do you use transformers on DC supply?

Answer 1

In order to pass DC current through a transformer you would need some basic knowledge of what a transformer actually is.

A transformer is simply put a coil of copper wire around metal plates of iron.

When you put current onto the coil, the metal plates will build up a magnetic field.

During the build up of this magnetic field, the current will be 'low'

When the magnetic field is maximized, the coil will work as a shortcut on a DC power source.

In order to use a transformer, ie transform voltage up or down, we need a fluctuating magnetic field. This can not be done without electronics of some variety, and the resulting voltage will be AC, not DC.

The easiest way to use a transformer with a DC power source is to make a circuit that electronically change polarization of voltage back and forth. Depending on transformer, this can be as low as 50 times per second (EU standard 50 Hz AC 220 Volt, US standard 60 Hz AC 115 Volt) or higher like 100.000 times per second (100 Khz) as is in many high speed switch power supplies. It all depends on the transformer you use and what you want to use it for.

If say you want to make a power source for an electric drill and you want to use a car battery as the source, then I would forget it. An electric drill need a lot of juice and in order to do so via a transformer, this transformer would need to be very very large.

Every ampere of 12 Volt would only give you approx 0.07 Ampere of 115 Volt.

A drill easily drain 800 to 1000 Watts. Approx 8 Ampere at 115 Volt.

Following this, an ideal transformer with hardly any loss would eat away 100 Ampere every 8 ampere on output 115 Vac.

With the loss factor (some of the transformed voltage will be given away as heat) and you would probably need approx 110 Ampere 12 Volt in order to generate 8 Ampere 115 Vac. A normal 60-65 Ampere car battery would then last a mere half hour fully charged. Another problem is that most batteries are not designed for such a load over any prolonged time (more than 30-60 seconds).

Systems have been and are buildt that can withstand such a drain but they consist of many batteries connected in paralell in order to even the drain between all batteries.

Answer 2

You don't. Transformers are used in AC, not DC.

Certainly, you can use a transformer and a rectifier, followed by a filter, to produce DC from AC, but I don't think the question meant that usage. Transformers depend on varying magnetic flux, and that only comes from use in AC.