# Why Can’t a Transformer Be Operated on DC Supply?

## What Happens When the Primary of a Transformer is Connected to the DC Supply?

A transformer is a device which step-up or step-down the level of AC current or voltage without changing the primary (i.e. input source) frequency.

Transformer only works on AC and can’t be operated on DC i.e. it has been designed to be operated only and only on alternating current and voltage. To know that what will happen if we connected a DC source to the primary of a transformer, see the following examples where we connect a transformer to the AC first and DC then.

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### Transformer Connected to the AC Supply

Suppose we connect a transformer to the AC supply with the following data.

• Primary Voltage =  V1 = 230V
• Resistance = R1 = 10 Ω
• Inductance = L = 0.4 H
• Source Frequency = 50Hz

Lets see how much current will flow through the primary of a transformer in case of AC.

We know that resistance in AC = Impedance

Impedance = Z = V / I in Ω

Where Z = √ (R2 + XL)2 in case of inductive circuit.

X= 2πfL

X= 2 x 3.1415 x 50Hz x 0.4H

X= 125.67Ω

Now for impedance

Z = √ (R2 + XL)2

Putting the values

Z = √ (102 Ω + 125.672 Ω)

Z = 126.1 Ω

Now current in the primary

I = V / Z

I = 230V / 126.1Ω = 1.82A

The Primary current in case of AC = 1.82A

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### Transformer Connected to the DC Supply

Now connect the same transformer to the DC voltage and lets see what happens.

We know that there is no frequency in DC i.e. f = 0. Therefore, the inductive reactance Xwould be zero if we put f = 0 in the X= 2πfL.

Thus, current in the primary of a transformer in case of DC source.

I = V / R

I = 230V / 10Ω

I = 23A.

The Primary current in case of DC = 23A

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The above calculation shows that excessive current will flow in the primary of a transformer in case of DC supply which will burn the primary coils of the transformer. This is not the only reason as the current will be DC, now lets see what happens in case of stady state current in the transformer.

If the primary of a transformer is connected to the DC supply, the primary will draw a steady current and hence produce a constant flux. Consequently, no back EMF will be produced. They primary winding will draw excessive current due to low resistance of the primary because we know that inductive reactance (XL) is zero due to the inductive reactance formula (XL = 2πfL) where frequency of the DC source is zero. Thus result is that the primary winding will overheat and burn out or the fuse and circuit breaker will blow. Care must be taken not to the connect the primary of a transformer across the DC Supply.

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### Why a Transformer Can’t be Operated on DC instead of AC?

If we apply the DC voltage or current to the primary of a transformer, The following are the results

We know that

v = L (di/dt)

Where:

• v = Instantaneous voltage across the primary coils
• L = Inductance of the inductor
• di/dt = Instantaneous rate of current change in A/s

Now in this case, the voltage are constant i.e. DC, Now the current (i) will rapidly increase until the the iron core of the transformer saturated.

At this stage, current (i) will increase to the dangerous level and stop changing. When there is no change in current (i), the induced voltage in the primary will be zero as di/dt = 0 which leads to short circuit the transformer winding with the appalled DC source.

When current exceeded the safe level, high power loss will occur as P = I2R. which will rise the temperature to the dangerous level and there may a chance of explosion of the transformer and transformer oil may catch fire as well.

Or let’s see by Faraday’s Second Law

e = N dΦ / dt

Where

• e = Induced EMF
• N = the number of turns
• dΦ = Change in flux
• dt = Change in time

In case of DC voltage to the transformer, there would be constant flux (Φ) induced in the primary due to constant current.

Now the induced EMF in the primary will be zero as (dΦ/dt = 0) i.e. e = N dϕ/dt = 0 due to constant flux induced by constant current.

We also know that there is no frequency in DC supply and flux is inversely propositional to the frequency (Φ = V / f) which saturates the transformer core.

Its mean, the primary of the transformer will act a short circuit path to the additional DC current which may blow the overall transformer. That’s the exact reason we should not connect a transformer to the DC supply instead of AC.

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### Under What Condition is DC Supply Applied Safely to the Primary of a Transformer?

In most cases, this is an electrical and electronics engineering interview type question, so lets see how to connect a transformer to the DC supply.

There are two Conditions where we can connect a transformer to the DC.

• Pulsating DC as Input
• High Resistance in Series with primary winding

Pulsating DC in Transformer

In this method, a pulsating DC (which contains ripples and not a pure form of steady state current) to the primary side of a transformer. In this case, the negative cycle reset the flux and time integral of voltage is zero in one complete cycle which again helps to reset the flux in the winding. This concept is used in SMPS (Switched-Mode Power Supply.

High Resistor in Series with Transformer

As we know that a transformer works only on AC. in case of DC supply, the primary of a transformer may start to smoke and burn. But there is a way where we can operate a Transformer on D.C (although the circuit is useless with no output) by adding a high value resistor in series with the primary of the transformer.

When the primary winding of a transformer is to be connected to a DC supply. a high resistance is connected in series with the primary. This series resistance limits the primary current to a safe DC value and thus prevent the primary from burning out.

Please be advised that don’t connect a transformer to the DC supply without a high resistance in series with primary. Because there is no frequency in DC and the impedance (Z) of inductor is zero. If you put Z = 0 in the I = V/Z, Current will be too much high i.e. inductor acts as a short circuit to the DC voltage and currents.

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### Electrical Technology

1. Sonia says:

If we give dc supply to transformer with high series resistance. Then the transformer output will be ac or dc?

1. Rahul Singh says:

Another case is also possible if we supply very low DC voltage I.e almost one tenth of rated value then transformer will not going to burn because than it draw very low current and that current will going to bear by the transformer resistor.

And as per the answer of question what is the output if we supply DC since due to DC inductor act as short circuit there is no magnetization of primary and therefore no flux will going to produce by primary so there is linkage of flux with secondary winding so there is no output

1. Sourav Sawhney says:

Yes because inductor coils works with change in flux linked with coil
In DC there is a constant supply and thus no change in flux hence no induced emf or voltage.
In ac as we know sinusodial wave is there due to which current and voltage changes continuously and which in turn is responsible for change in magnetic flux linked with coil ang thus induced emf.

Thanks

2. Randy Charles Abramovitz says:

Alternating field is required to produce voltage and flux linkage to windings on a common core. Ldi/dt =V represents excitation to set up the magnetic field.

The transformer will not give any output. Transformer works on mutual induction. Concept of connecting high resistance is totally wrong.

1. It is already mentioned that there would not be an output and the circuit will be useless.

4. Jagapathi Babu says:

Transformers is not like rectifier

It’s only Change voltage either step up or step down with out change in frequency

2. Kiran Shankar Akhnoor (SEECO) says:

A.C. current generates Frequency 50-60Hz, and due to magnetic effect on the pri flux is produced and transformed into secondary output.
Since the D.C. current doesn’t produce any frequency therefore magnetization does not occur so no output is measured.

3. Suvankar chakraborty says:

We know ,, infective reactance =2πfl
So, dc has no frequency . So, the inductive reactance = 2×π×0×l = 0 … that’s why the transformer will be short circuited ..

4. Aman choudhary says:

Because dc current
has Zero frequency

5. Arunmurotia says:

What about automotiv ignition coil ?
This is step up transformer which works on 12 volts DC supply.

1. Greg Vincent says:

An automobile ignition coil is not operating off of 12 volt dc. The ignition points (transistors in modern ignition systems) open and close once per revolution for each cylinder. In a single cylinder engine running at 1500 rpm, the motion of the points will produce a time varing (AC) signal of 25 Hz on the primary of the ignition coil.

6. SUDHARSHAN NAIDU BOMMASANI says:

Transformers are frequency machines where sinusoidal voyages are pass.

7. Jagapathi Babu says:

I like this type of questions

8. Sumit Kumar Mishra says:

Sir your efforts is very nice every explanation is very good sir love your content sir❤️✌️????

9. brian says:

transformer just change alternating voltage or current to the required level depending its step up or step down .
But if we apply dc voltage to see the reaction ,remember dc has zero frequence and given by its formular. SO it produces zero inductive reactance to the applied voltage or current. so primary winding receives much current result in burning of the device plus induction need alternating current

10. Indra says:

Now I know why a transformer can be operated at DC supply. Thanks

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