# Purpose of a rectifier in a positive oscillating circuit

#### 8dm7bz

Joined Jul 21, 2020
199
Hello,
I found a circuit (transmitter) design that would create an oscillating magnetic field. Another circuit (receiver) would pick the induced voltage up and amplify it.
Now as I understand it, the oscillating part has only positive voltage. But in the receiver part they use a recitfier. Why would you need this if the induced voltage is only ever positive ?

thanks,
8dm7bz

#### AlbertHall

Joined Jun 4, 2014
11,317
When you first apply a voltage to a coil the current rises exponentially. The rising current generates an increasing magnetic field. When you remove the voltage the magnetic field decreases.

At the receiver, the increasing field will generate a voltage in the receiver coil. When the magnetic field reduces the receiver coil will generate a voltage in the opposite direction. So the output from the receiver coil will be AC even if the voltage at the transmitter coil is pulsed DC.

#### 8dm7bz

Joined Jul 21, 2020
199
Thanks a bunch for the clear explaination

#### AnalogKid

Joined Aug 1, 2013
9,260
The receiver uses a rectifier to form an "envelope detector", a form of AM detection. The output of the receiver is not the transmitted waveform. It is the shape of the waveform - how it changes in amplitude as it is turned on and off.

But the real answer to your question lies in the physics and math of Faraday's Law of Induction.

The receiving coil cannot tell the polarity of the transmitting wave because that is not what a coil responds to. It sees only the instantaneous *changes* in the wave's amplitude. It cannot tell the difference between something going from -3 to -1 and back and something going from +2 to +4 and back. Both are a change of 2 in the positive direction. Also, the output of the coil is a bipolar version of the transmitter waveform, no matter what the transmitting waveform offset is. For both cases mentioned, the coil sees something going from -1 to +1 and back, a net change of 2 centered about 0.

NOTE to more experienced forum members: YES, I KNOW that that explanation is so over-simplified that it will offend. Waves do not "go from -3 to -1". I think that differential calculus is not going to convey useful information. This is *not* a dig at the TS, just my read of his experience level.

ak