Digial Voltmeter DC and AC mode

Discussion in 'Electronic Basics' started by aman, Apr 26, 2005.

1. amanGuest

I dont understand the real meaning of DC and AC readings on the Digital
voltmeter. Is it correct to say that if the signal = DC component + AC
component then DC reading gives the DC component and AC reading gives
the AC component ?

2. John LarkinGuest

Usually, yes. On most DVMs, the DC range is heavily averaged/lowpass
filtered, so the AC component is mostly ignored. And on the AC range,
it's AC-coupled, and response drops off below 10 Hz or something like
that, so the AC range ignores any DC component entirely.

It needn't be so, though: some meters read the *true* RMS voltage on
their AC range, which includes any DC component; but that's rare.

I think the old analog Simpson-type VOMs may have responded to DC on
their AC range. Anybody know for sure? Did they have a series cap, or
just a rectifier and the meter movement?

John

3. Peter BennettGuest

Probably - although there may be some meters where this is not so.

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Peter Bennett, VE7CEI
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4. John LarkinGuest

Cool: the AC range is DC coupled, but the OUTPUT range adds a series
cap!

http://www.simpsonelectric.com/pdf/manuals/test/260-8Ximan.pdf

John

5. JamieGuest

no.
the DMM (digital Multimeter) will give you DC (direct current)
from a steady source of energy , like a Battery for example.
AC voltages are alternating types of energy meaning, take the battery
and keep reversing the leads on the meter at 60 times per second or
50 times depending from what country your coming from. this Voltage
is changing polarity..
lets look at it from a more simpler view.
a common cell(1 part of a battery) (carbon) puts out aprox 1.56 volts
new. imagine putting this cell on a rotating plate on it's side.
now imagine the probes are on the edge of the plate equally spaced
apart.
as the plate turns with the cell lying on it, the - + terminals are
getting closer to the probe tips, lets say the meter is reading the
approaching terminals of the cell giving you a stronger reading until
they are perfectly lined up with the 2 probes! at this point the reading
is at its strongest. note that the DMM may have the - or + sign on the
left of the reading indicating the polarity. now the cell is still
turning and as it does the voltage is now going down. when the 2 cell
tips are at the far points of each probe tip, the voltage reading will
become 0. the cell keeps turning and you then will see voltage start
increasing again ! but this time the polarity is now the opposite as it
was before.
this continues until we get back to the starting point. this is
call alternating current because the voltage is raising and falling
back and forth alternating its polarity as it does so.
Hence the need for a meter to have a different mode to display to you
the Average or RMS. (root mean Square) reading which is really not the
highest voltage of the sine wave (PEAK).
for example.
10 Volts RMS. would be something like 10.4 if this AC wave was
converted to a clean DC to represent the peak..
so when measuring that 120 Volts of house current , the actual peak
voltage is around 160 volts.
any ways..
enough of this.