Continuity test on resistor

Question

I have a newbie question.

I'm learning about electrical engineering and I bought my first multi-meter. It has a feature called "Continuity Test". I believe this test is to make sure conductors such as wires and cables are continuous and not severed or perhaps you want to find the right circuit.

When I inspect a resistor with the multi-meter probes I can find voltage and current, but when I test for continuity the meter does not beep.

It's as if the two ends of the resistor were not connected somehow. I'm sure this is a laughable question for veteran electrical engineering people but why is it that a multi-meter would not register continuity through a resistor? What goes on in the continuity test so that it registers positive for continuity? Does a certain amount of voltage, current or perhaps the frequency needs to be equal through the circuit in order for the meter to consider continuous?

Answer 1

The continuity function is designed to give an audible indication of resistance that is less than some threshold value.

To be useful, it will be designed to respond much more quickly than the display so that a tech can quickly 'buzz out' wiring and such like without waiting for the reading to settle or even taking his or her eyes off the test probes. It's specifically designed so that the voltage does not turn diodes on, it won't respond to resistors above a certain value and so on, so that it (usually) responds to just a fairly solid electrical connection.

The values will vary somewhat by manufacturer, but here is an excerpt from the Fluke 177 DMM manual:

As you can see it has hysteresis and a pulse stretcher that allows brief breaks to be detected. This is done by circuitry that is mostly operating in parallel to the main ADC function. Some crummy cheap meters have a continuity beep that is dependent on waiting for an ADC result, but they are not very useful. Avoid!

Answer 2

You're right about what a continuity test is for: determining that two wires are connected. That implies a low resistance between them. So try it: test smaller and smaller resistor values until your meter shows continuity. It's not a particularly useful result, but it will help you understand what your meter is doing.

Answer 3

The continuity tester is meant as a pass/fail test. If the resistance is too high, then an open is assumed and no beep. If the resistance is below the preset threshold then it is assumed that a connection exists and a beep is heard.

Answer 4

The continuity range usually has a certain resistance threshold in which it considers a wire to be closed. A long copper wire might be about 1 ohm, and you could probably expect that threshold to be about 100 ohms or so; I don't know what the typical value is. If your resistor is of greater resistance than the threshold on your meter, then even though current can flow through it, the meter will not register continuity.

Answer 5

The continuity test applies a very small voltage to the circuit and check what current flows through it. If the current is larger than the lower limit (for continuity test of your multimeter), it beeps. For example, you cannot have a beep testing a diode, because it has a voltage drop (about 0.7V). If you measure a resistor with less than 10 ohms, probably you will hear the beep. If you check your multimeter DS it will not be hard to specify the maximum resistance or voltage drop to make it beep.