multimeter AC voltage and current measurement

hi all,
ow does multimeters measure AC voltages and currents ????? can some one point me to a circuit for doing that.
thanks in advance!!!!!!!!!!!

Analogue or digital multimeter?

thanks for the reply.
i would like to know about both ways.

Are you asking for a circuit to convert AC to a DC voltage that represents what? Peak AC voltage? Average AC Voltage? Root Mean Square of AC voltage? etc?

Tell us what you are trying to do?

i am designing a power meter. so i need to measure the AC voltage and current going into an appliance. the calculations are being done by a pic. i just need to give the analog inputs to the pic as accurately as possible. thats why i am asking how the multimeter does it.

Depends on what you need. My $3 Harbor Freight Chinese DMM does it differently than my $150 Fluke DMM. If you are measuring power, you need the RMS value like in the Fluke, which is quite complicated (opamps, integrators, multipliers). It correctly reads the RMS value regardless of waveshape (non-sinosoidal).

The $3 meter uses a $0.05 cent diode, but that can only be calibrated for one waveshape, namely sinosoidal without much harmonic distortion.

Spend some quality time with Google searching for Root Mean Square converter circuits.

Strictly speaking, multimeters don't measure AC voltage or current, because the magnitude changes from one instance to the next. They measure the result of some math function of the AC voltage/current, and in some less expensive meters the AC waveform being measured is assumed to be sinusoidal (whether it actually is sinusoidal, or not) to get an accurate measurement result directly. The result of the math function implemented in a particular multimeter is a DC voltage that can be measured. To get a detailed measurement of an unknown waveshape from instant to instant, an oscilloscope is required. There are severe limitations on the frequency and crest factor of the input AC for a measurement to have a certain accuracy using a multimeter.

The methods used for measuring the result of the math function and the way the math function is implemented are several. Sometimes the math function (average, peak, or RMS) is performed digitally after the AC input is sampled with an ADC. Most often the AC input is rectified and applied to a LP filter to extract the average value of the AC input. There are specalized ICs that perform an RMS function of the AC input. There are numerous circuit examples on the Internet, easily found. AC current is usually converted to AC voltage by passing the current though a low value resistor and measuring the voltage across the resistor.

A couple of novel/clever circuit ideas for a wattmeter are:

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