# AC voltmeters

The instrument, which is used to measure the alternating voltage between two electrical points the circuit is called ** AC voltmeter **. If the AC voltmeter consists of a rectifier, it is said to be an AC voltmeter based on a rectifier.

The DC voltmeter only measures DC voltages. If we want to use it to measure AC voltages, we have to follow these two steps.

We get a ** rectifier based AC voltmeter **, simply by including the rectifier circuit to the base DC voltmeter. This chapter deals with rectifier-based AC voltmeters.

## Types of AC voltmeters based on rectifier

Here are the ** twox types ** of rectifier based AC voltmeters.

- AC voltmeter using Half Wave Rectifier
- AC voltmeter using Full Wave Rectifier

Now let's talk of these two AC voltmeters one by one.

### AC voltmeter using a half wave rectifier

If a half wave rectifier is connected before the DC voltmeter, then this entire combination is called an AC voltmeter using a half wave rectifier. The ** block diagram ** of the AC voltmeter using a half-wave rectifier is shown in the figure below.

The diagram above consists of two blocks: half-wave rectifier and DC voltmeter. We will get the corresponding circuit diagram, simply by replacing each block with the respective component (s) in the block diagram above. So the ** wiring diagram ** of the AC voltmeter using a half wave rectifier will look like the one shown below.

The ** rms ** value of the sinusoidal input voltage (AC) signal is

$$ V_ {rms} = frac {V_ {m}} {sqrt {2}} $$

$$ Rightarrow V_ {m} = sqrt {2} V_{rms} $$

$$ Rightarrow V_ {m} = 1,414 V_ {rms} $$

Where,

$ V_ {m} $ is the maximum value of the sinusoidal input voltage signal (AC).

The ** CC ** or average value of the half wave rectifier output signal is

$$ V_ {dc} = frac {V_ {m}} {pi} $$

** Replace **, the value of $ V_ {m} $ in the equation above.

$$ V_ {dc} = frac {1,414 V_ {rms}} {pi} $$

$$ V_ {dc} = 0.45 V_ {rms} $$

Therefore, the AC voltmeter produces an output voltage, which is equal to ** 0.45 ** times the rms value of the sinusoidal input voltage (AC) signal

### AC voltmeter using a full wave rectifier

If a full wave rectifier is connected before the DC voltmeter, then this entire combination is called an AC voltmeter using a full wave rectifier. The ** block diagram ** of the AC voltmeter using the full wave rectifier is shown in the figure below

The above functional diagram consists of two blocks: a full wave rectifiere and a DC voltmeter. We will get the corresponding circuit diagram simply by replacing each block with the respective component (s) in the block diagram above.

So the ** circuit diagram ** of the AC voltmeter using a full wave rectifier will look like the illustration below.

The ** rms value ** of the sinusoidal input voltage (AC) signal is

$$ V_ {rms} = frac {V_ {m} } {sqrt {2}} $$

$$ Rightarrow V_ {m} = sqrt {2}: V_ {rms} $$

$$ Right arrow V_ {m} = 1.414 V_ {rms} $$

Where,

$ V_ {m} $ is the maximum value of sinusoidal (AC) input voltage signal.

The ** DC ** or The output signal value of the full wave rectifier is

$$ V_ {dc} = frac {2V_ {m}} { pi} $$

** Replace **, the value of $ V_ {m} $ in the above equation

$$ V_ {dc} = frac {2 times 1.414: V_ {rms}} {pi} $$

$$ V_ {dc} = 0.9: V_ {rms} $$

Therefore, the AC voltmeter produces an output voltage, which is equal to ** 0.9 ** times the rms value of the sinusoidal input voltage (AC) signal.