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Unread 14-02-2016, 19:46
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Re: spark controller dropping voltage in reverse

Quote:
Originally Posted by dlsmith View Post
Cheap DVMs on AC scale use a diode to rectify the signal, average the rectified half wave, and then multiply that by some fudge factor (based on an assumed sine waveform). It works OK for sine waves but gives nonsense readings for square waves or PWM.
If you have one of those $3 Harbor Freight DVMs and a battery, you can see this in action:

1) using the DC mode, measure the battery voltage and write it down.

2) using the AC mode, measure the battery voltage (with red lead to +) and write it down.

3) using the AC mode, measure the battery voltage (with black lead to +) and write it down.

If the voltage in (3) is zero, you very likely have a "average of half-wave rectification multiplied by fudge factor" meter, and you probably got a reading in (2) of about 28 volts !!

Now let's do some math to see why.

The true RMS of a sine wave is peak/sqrt(2)

The half-wave-rectified average of a sine wave is peak/pi

The ratio is the fudge factor: f = (peak/sqrt(2)) / (peak/pi) = pi/sqrt(2) = ~2.2

So a AC voltage in (2) will be approx DC*2.2


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