# What will happen if a DC voltage is applied to the transformer coil?

This is a very common question asked by those who are starting up with transformer science. You may come across the same question during interviews too.

Before explaining what will happen if a DC voltage is applied to the transformer primary, please keep in mind a few points.
• The resistance of transformer winding is very less.
• When we apply DC voltage to the transformer, the reactance of its winding will be zero since reactance is frequency dependent and the frequency of a DC supply is zero.
• Flux produced by the flow of direct current will be constant.
Now let us enter into the topic.

## What will really happen when a DC voltage is applied to the transformer primary?

When a DC voltage is applied to the primary winding of a transformer, due to low resistance, the winding acts as a short circuit across the terminals of the DC source that lead to the flow of heavy current through the winding resulting in overheating of the winding. Eventually, the source or the winding will be damaged. The effect of current on winding depends on the applied DC voltage, rating of the transformer, winding resistance and how powerful the DC source is. “Nothing will happen if 5Vdc is applied to a 50KVA transformer!”.

## Can we get voltage at the secondary side of a transformer when DC voltage is applied?

No, there will be no output at the secondary side. But a sudden spike can be recorded due to initial current inrush and nothing else later.

### Caution:

Never connect DC supply to a transformer. For testing, current controlled source can be connected.

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### This Post Has 5 Comments

You will also get a voltage spike on the secondary when power is disconnected from inductive kick.

That is a very good point to be noted. You will also get a voltage spike on the secondary when power is disconnected from the inductive kick. Thank you for your contribution.

2. Rahul

What if we short the output of transformer and give it the d.c supply at input. Having a load connected in the input terminal.

It does not matter whether you connect a load at the secondary of the transformer or not, due to low resistance, the winding acts as a short circuit and lead to the flow of heavy current and damage the winding.

3. Jar Jar

How come it is possible to use DC to power an electromagnet though, as in, for example, an EI shape microwave transformer core with the original primary coil, and the work piece as a (single turn) secondary coil?

I know it is not effective or smart, but this is the arrangement most people seem to go for.