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What Happens When an AC Line Touches a DC Line?

What Happens When DC Equipment Operated on AC Voltage and Vice Versa ?

AC and DC Mixed Operated Devices

Well, It is a complex question and the answer depends on multiple factors. We will discus the possible factors affecting the situation as follow.

First of all, there are some devices which operates on both AC and DC voltage and currents such as universal motors (series-wound motors), vacuum cleaner motor, AC and DC operated fans etc. These equipment may run faster on DC than AC at the same level of voltage. Also, the AC and DC signals are mixed and used for multiple purposes when needed. For example, in clamping (by using clamper circuit) for amplification. In this process, If  positive DC voltage are added to the sine wave, it gets shifted upwards (with reference zero crossing axis). If Negative DC is added, the opposite will happen.

In the above scenarios, (i.e. when AC mixed with DC)

Equation of Pure AC Current:

i = Im Sin ωt + θ

Where:

  • i = Instantaneous value of current
  • Im =  Max or Peak value of current
  • ω = 2πf = Angular frequency
  • θ = Phase angle difference

Now, If DC is mixed with AC, it becomes

i = Im Sin ωt + θ ± I’

Where I’ = is DC Current and we use it for multiple purposes. But keep in mind that it is not always the case i.e. you can’t apply the same mixture of AC and DC to all electrical and electronic devices and equipment. In next section, we will discuses what happens if we apply DC to the AC equipment and vice versa and what are the results.

Generally, the residential and home supply voltage is 120V AC (230V in EU) and 3-18V DC in case of electronic circuits. If accidentally AC line touches the DC line connected to the electronic circuit and components, the result may be bad smell smoke, flames or even worse. This is because the DC devices are designed to be operated at specific and pure DC voltage and current. In case of AC components when DC connected to it, it may not work properly (that’s why a transformer can’t be operated on DC voltage) or even start to smoke and burn.

What Happens When an AC Line Touches a DC Line

What Happens when AC Devices Operated on DC?

We know that most of the AC machines uses inductors and coils such as motor, transformer etc. The Impedance (overall resistance) of AC machines depends on the frequency of the supply voltage.

Mathematically,

Resistance in AC Circuit : Z = V / I in Ω

Where:

  • Z = the overall resistance of AC = Z = √ (R2 + (XL + XC)2
  • X= Inductive reactance in Ω
  • X= Capacitive reactance in Ω
  • V = Voltage in Volts
  • I = Current in Amperes

But Xand XC depends on freqeuncy i.e.

  • X= 2πfL
  • XL = 1/ 2πfC

Now, we know that there is no frequency in the DC. i.e. f = 0 in DC circuits. Now the overall resistance for DC would too much low as compared to the AC. this way, excessive current will flow in the field coils or inductor which may burn or blast the overall circuit.

Lets see the simple example below where the applied voltage is 50V and 10Ω in both AC and DC:

Current in AC Circuit:

  • I = V/Z
  • I = 50V / 10Ω
  • I = 5A

Current in DC Circuit:

  • Z = zero because of 0 frequency (putting XL = 2πfL = 0 because f = 0)
  • I = V/Z
  • I = 50V/0Ω
  • I = 50A.

This shows that excessive current will flow in the circuit if we connect an AC machine or device with DC supply which leads to burn the rotor or stator coils. If they survive, they wont be able to operate at normal condition.

Another case is that AC circuits using capacitor where we know that a capacitor blocks DC and let pass AC through it. This is another reason that an AC device won’t work perfectly on DC supply.

In short, if we connect an AC device to the DC supply:

  • Some machine like motors may not work properly or even damage (except the universal motors operated on both AC and DC).
  • A Transformer may start to smoke and burn if DC supply is connected to the primary of a transformer. Same is the case of alternators.
  • Coils and solenoid may rapidly burn if connected to the DC supply.
  • In some cases, filters rectify the AC and provide DC. Some devices needs only peak value of AC which is 40% higher than the RMS value of AC. If operated with high DC voltage, the operation may be acceptable.
  • Overall, if DC applied to the AC rated devices, some may burns, start to smoke, work partially or not at all according to the designs and operations.

Related Post: What is the Role of Capacitor in AC and DC Circuit?

What Happens when DC Devices Operated on AC?

If we connect an AC supply to the DC devices and equipment:

  • The positive and negative voltage will destroy (this is not always the case) some of electronic components such as transistors and electrolytic capacitors. In case of higher AC voltage, they may burn with blast and catch fire.
  • Batteries which charges on DC only, If you connect an AC source to the battery. It will not charge and expect an explosion and fire. 
  • In relays and audio amplification devices, the AC signal variation generate humming noise which is unacceptable.
  • Some components may not reset or operate properly even they survive the negative half cycle.
  • Microcontrollers and microprocessors in digital computer works on digital logic “1” and “0” as ON/OFF. AC will generate lots of ON/OFF signals, where microprocessor wont be able to decide a direction as ON or OFF which leads to annoying or useless operation of the system.

Related Post: Why Do Electronic Circuits Use DC Current instead of AC?

What Will Happen when AC Line Coronets to the DC Line ?

All of the above discussed scenarios will happens at once. If accidentally an AC line touches a DC line connected to the DC rated devices and components, they may destroyed such as  thyristor , transistor, resistors, capacitors, inductors, microchips, ICs. If the level of voltage is low and the duration is small, the fuse or circuit breaker may blow and disconnect the circuit from the power supply. You may notice a spark flash light and bad smell smoke as well.

Conclusion:

If you ever wonder that what happens when AC is connected to the DC and vice versa, keep in mind the Murphy’s law (anything that can go wrong will go wrong). i.e. AC equipment wont work properly on DC and DC devices may not function well on AC. So please be safe and never try this at home (i.e. touching AC line to the DC line) as serious injuries may occur due to electric shock and fire. Stay safe. If there is no proper protective devices installed on the systems, this may damage both the AC and DC rated equipment and components.

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