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Is it safe to test for continuity with multimeter

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c64:

--- Quote from: bdunham7 on July 16, 2020, 05:25:04 am ---Surely you mean the 200R range, not 200M!

--- End quote ---
Nope. I really mean 200M. In 200R range it's 0.25V / 0.5V

bdunham7:

--- Quote from: c64 on July 16, 2020, 05:36:40 am ---Nope. I really mean 200M. In 200R range it's 0.25V / 0.5V

--- End quote ---

Wow!  I just looked--that's fairly impressive for a $5 meter.  I might just have to upgrade!

Ian.M:
Ohms and continuity measurement has certainly come along way since the days of analog meters with a 22.5V dry cell battery for their high ohms ranges and a D cell for the low ohms range.  IIRC the high voltage was needed to get enough current through high value resistors under test to deflect the fixed sensitivity movement adequately, as with no active electronics, the ohms scales had to be calculated based on the reciprocal function, (with an offset for a short-circuit current limiting resistor), as current sources were impractical, which meant they were always very cramped at the left hand end.  IIRC ohms zeroing was handled by an adjustable shunt across the movement, which 'threw away' sensitivity to match the full scale (zero) deflection to the current battery voltage.  The analog meter polarity reversal on ohms ranges was because it was usual to basically use the current ranges shunts and switching, and to maintain the same direction of current flow in the external circuit driven by an internal voltage source, the polarity had to be reversed. 

If anyone's got an Avometer or similar old pro grade analog multimeter handy with good batteries in, perhaps you could post the open circuit voltage and short circuit (zeroed) current on its highest and lowest Ohms ranges, together with the nominal range and the meter make and model.

EEVblog:
FYI, beware measuring ohms output voltage in the high resistance ranges, as then the typical 10M input impedance of the test meter comes into play.

mikerj:

--- Quote from: Peabody on July 14, 2020, 06:22:07 pm ---Would this not also apply to any test for resistance between two points?  Isn't continuity just a special case of an ohms measurement?

Also, am I the only one that worries about lead polarity?  My digital meter puts out positive voltage on the red (+) lead when measuring resistance, as you would expect, but my analog meter is the reverse.

--- End quote ---

The vast majority of analog meters have polarity reversed w.r.t test lead colours in ohms mode, something you quickly learned if you grew up using them.  This makes sense when you think about the design, ohms mode is effectively a current mode with a voltage source connected in series.  Correcting the polarity would require more complex switching.

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