My digital multimeter has a continuity test, but my new analog VOM does not. Can I check continuity with my VOM, and if so, how?
The simple answer is yes. It’s easy, and it can be more meaningful than the continuity function on a DMM (I’ll explain that in a minute). But first, here’s how easy it is to check continuity with a VOM:
Set your VOM for its lowest Ohms range. Touch the test leads together and set the meter for full scale, exactly as if you were going to measure resistance. Apply the probes to the circuit and watch for the meter to swing full-scale (or very close to it). If it does, you have continuity. If the needle doesn’t move, there is no continuity.
OK, how is that more useful than the continuity function on a DMM? The continuity function provides an audible signal or beep when “continuity” is “detected,” allowing you to perform a number of tests without having to look at the meter. But what, exactly, is continuity? Continuity is not something that can be measured– it is a condition that exists (or does not) between two points in a circuit. The condition is simply that current can flow between the two points. Strictly speaking, there is continuity between the two leads of a resistor. There might be a whole lot of resistance (which can be measured), but there is still continuity between the two points.
In practice, however, we use the “continuity function” mostly to determine that we have good connections on our circuit board, to check for shorts, etc. In other words, we are looking for a relatively low resistance, and that is exactly what the continuity function on a DMM does. The meter turns on the beep if resistance is below a certain figure,
typically a single ohm or so, which is entirely arbitrary and often misleading. Let’s say you are using the continuity function to look for a point in the circuit that is shorted to ground. It could very well be that the path to ground is through a low-value resistor, perhaps less than an Ohm. You hear the beep and think “short,” but in fact everything is as it should be. You can also have the opposite problem– you think you have an open circuit because you don’t get the beep, but in fact your circuit includes a resistor that is higher than the continuity function’s threshold.
Some DMM’s display the actual resistance, to a fraction of an Ohm, when in “continuity mode.” Others do not.
One last advantage of using an analog VOM is that it is instantaneous. Digital multimeters, especially the less expensive ones, require a noticeable amount of time to calculate and update the display, or trigger the continuity beep. Sometimes it can be tricky to get the test leads onto the circuit for long enough to register on the DMM.