How does a multimeter's continuity function work?

[Braeden Hamson]

This sounds like it should be really simple, but I'd like to know. I wonder because I can't test the continuity through a component, only the continuity on the same node/wire. I tested my meter with another meter and when I'm in the continuity function there's a 2.5 v potential on the leads (but there isn't any load) it can actually turn on and LED with the leads. My other meter which is a cheap but really good $10 homedepot meter Has a 0.5 v potential on the leads but wont turn on the LED. Mind you these LEDs are amazing they will turn on with any voltage, that includes me holding the positive lead in my hand and touching the negative to the faucet in my house. So measuring continuity seems a lot more complex than I thought it was. If I had a scope I'd hook it up to that (Come on scopemonth)

Anyway, if anyone has any more info on how continuity measurements are done I'd love to hear. Also if there are any drawbacks to how it's done.

 

[Minder]

Most high end meters, Fluke etc, have very low current capability on the resistance range and cannot test a diode unless they are directly shorted, as they do not have the current capability to forward bias the diode or semi.
So the diode range actually passes a current through a diode or semiconductor and the meter indication is actually voltage drop across the diode.
M.

 

[Braeden Hamson]

Ahh so its a question of current not voltage. I need to watch out for current more.

 

[davenn]

This sounds like it should be really simple, but I'd like to know. I wonder because I can't test the continuity through a component, only the continuity on the same node/wire

what Minder said is part of it

this is the other part
you will find that continuity mode on most meters is designed for any resistance of around 100 Ohms or less ( and you will get the beep and the resistance reading)
the continuity mode isn't designed to work through 100's or 1000's of Ohms

 

[hevans1944]

Most continuity functions on multimeters apply a fairly large "compliance voltage" in a constant-current "measuring mode" so they are really measuring resistance as a voltage drop between the two probes. As @davenn said, they usually produce a "beep" if the measured resistance is low enough to satisfy whatever the meter manufacturer defines as "continuity," and this is typically anything less resistive than about a hundred ohms or so. For real continuity tests, where you are interested in the integrity of the connecting wiring... completeness, good solder joints, tight connections, etc. ...one hundred ohms might actually be too high. You might want to see continuity on the order of ten ohms or much, much, less than that. All depends on why you are testing the continuity of a circuit.

The meter manufacturer will set the threshold level for the beeper at somewhere between zero and about one hundred ohms. Any resistance higher than the threshold will be defined as "no continuity" (beeper is silent) and any resistance lower than the threshold will be defined as "continuity" (beeper sounds).

Lighting up LEDs with some multimeters is just an added cherry feature to the continuity test function. As Dave mentioned, some meters also have a "diode" test function that provides sufficient compliance voltage to turn on a single junction diode and display the voltage drop across the diode. Very useful for sorting germanium from silicon diodes, and useful of course to test sensitive LEDs.

 

[davenn]

For real continuity tests, where you are interested in the integrity of the connecting wiring... completeness, good solder joints, tight connections, etc. ...one hundred ohms might actually be too high. You might want to see continuity on the order of ten ohms or much, much, less than that.

all the meters I have display the resistance ( as well as sounding the beep) when the when the resistance is at or lower than the manufacturer set value ...
as I said around 100 Ohms or less

 

[Audioguru]

My Fluke meter uses 0.2V for resistance testing and about 2.5V for diode testing which is not enough voltage to light modern white, blue and green LEDs.