Continuity testing - Why does swapping DMM probes give differing results?

Question

Continuity query - Can anyone explain this?

Circuit is a simple 12v DC, wired as follows -battery pos to bulb to spst switch back to battery earth. Testing with DMM in the dedicated audible continuity mode.

With switch open, all points from battery pos up to the switch read continuity as one would expect.

If one closes the switch (live circuit) repeating the above test gives a negative, no continuity result - which i understand to be correct as continuity testing should be done on non-live circuit.

BUT... if I then swap the DMM probes over (neg DMM to Battery pos) (still on live circuit) all points on the circuit now read continuity (beep)

Also this situation occurs just testing across battery terminals - pos to pos and neg to neg no continuity, but swapping so DMM neg to battery pos gives continuity.

Answer 1

It sounds as though you are connecting your ohm-meter to a live circuit. This could destroy your meter.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Equivelant circuit of ohm-meter.

Most budget digital multimeters use a 0 - 199.9 mV (0.2 V) voltmeter circuit in them. All of the voltage and current ranges on the meter switch in and out resistors so that at full scale for that range (200 mV, 2 V, 20 V, 200 V) 200 mV will be presented to the internal voltmeter.

For resistance measurement a constant current is sent out on the red probe and when attached to the device to be tested this results in a voltage reading across the multimeter leads. In Figure 1 the 200 mV voltmeter is represented by VM1 and the resistance range selected is giving out 100 μA. From Ohm's Law we can calculate that this will give us a reading of 200.0 mV at \$ R = \frac {V}{I} = \frac {0.2}{100\mu} = 2 \ \text k\Omega \$ and that any resistance greater than 2k (including open circuit) will give the over-range indication, usually 1---. You can work out the currents for the other ranges on your meter.

Now it should be clear that by connecting your probes to a live circuit you have (1) messed up the test current and (2) risked applying too high a voltage to the internal voltmeter.

If you have been using the diode test / continuity range the internal circuitry will be somewhat similar and you risk similar damage.

Resistance and continuity checks can only be made on dead circuits.