# Why does the input current waveform change as per load in an electric circuit?

#### Devika B S

Joined Mar 8, 2017
144
Consider a rectifier circuit feeding an inductive load. As the load becomes highly inductive, the output current becomes smooth and ripple free. But the input current waveform takes a square shape. However the input voltage remains sinusoidal in shape. Why does this happen? Isn't both the voltage and current at source supposed to be constant for a given circuit?

#### MrAl

Joined Jun 17, 2014
10,075
Hello,

You might want to post a pic of what you are describing.

However, an inductor in a rectifier circuit can keep the current flowing through the diode(s) because it acts like a short term constant current source. This means the waveshapes could change compared to those we see without an inductor because without the inductor the current drops to zero for much of the time.

The other effect of the inductor is to lower the peak current into the circuit from the line. That keeps the input current peak lower as well as the diode peak current lower which reduces voltage drop in the diodes.
This is not as effective at higher and higher input currents because the inductor also limits average current though so we often would see a swinging choke in the circuit rather than a fixed inductor so we could get higher efficiency at full load.

The older style wall warts use leakage inductance as a means to limit output current and thus help keep the circuit safe. The newer regulated style have built in electronics to limit current.

Last edited:

#### Alec_t

Joined Sep 17, 2013
13,435
Does the 'rectifier circuit' have a reservoir capacitor?

#### Devika B S

Joined Mar 8, 2017
144
Hello,

You might want to post a pic of what you are describing.

However, an inductor in a rectifier circuit can keep the current flowing through the diode(s) because it acts like a short term constant current source. This means the waveshapes could change compared to those we see without an inductor because without the inductor the current drops to zero for much of the time.

The other effect of the inductor is to lower the peak current into the circuit from the line. That keeps the input current peak lower as well as the diode peak current lower which reduces voltage drop in the diodes.
This is not as effective at higher and higher input currents because the inductor also limits average current though so we often would see a swinging choke in the circuit rather than a fixed inductor so we could get higher efficiency at full load.

The older style wall warts use leakage inductance as a means to limit output current and thus help keep the circuit safe. The newer regulated style have built in electronics to limit current.
Above is the circuit

#### Devika B S

Joined Mar 8, 2017
144
Does the 'rectifier circuit' have a reservoir capacitor?
Not there

#### MrAl

Joined Jun 17, 2014
10,075

Hello again,

In that circuit once current flows down through the load the inductor will try to keep that current flowing for a while even if the AC line voltage goes to zero. The time period this happens in depends on the value of the inductor and the line frequency. So it's just a matter of the inductor storing energy during the diode normal conduction period, then releasing some of the energy back into the circuit through the diodes later when the line voltage goes lower. This will happen on both half cycles of the line.
To see this effect another way you can replace the inductor with a constant current source for a short time period with the current in the same direction as when the inductor charges.

In the attachment i labeled the top component L and the bottom R, but it does not matter unless we start to look at voltages across each element.

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