how does connectivity on a multimeter work - Page 1
Forum section: electronics / beginners

author: SirRage
author_rank: Contributor
timestamp: July 29, 2016, 06:22:51 am
content: Not how to use it. But what is the multimeter actually doing in order to do that test? Is one of the leads sending out some kind of charge?

author: Signal32
author_rank: Frequent Contributor
timestamp: July 29, 2016, 06:27:08 am
content: The continuity function on multimeters works basically the same as measuring resistance.The multimeter outputs a voltage and sees what current flows through the leads.By doing this it can calculate the resistance of the circuit you are measuring.Ex: On continuity, my UT61E outputs 3.0v with a max of 1.5mA.

author: timb
author_rank: Super Contributor
timestamp: July 29, 2016, 02:13:53 pm
content: Quote from: Signal32 on July 29, 2016, 06:27:08 amExactly. You may also notice that some multimeters will display a rough resistance in continuity mode (up to the cutoff of what's considered "continuity", which is generally 10 to 20 ohms).Other meters might show a voltage in continuity mode, allowing it to show the Vf of a diode. (Though I've only seen this a couple of times. Most meters have a dedicated diode function separate from the continuity.)On meters that lack a continuity function, you can simply place them into resistance mode on the lowest range, to achieve the same thing, albeit without the beep indication.There also exists dedicated continuity meters, which can be as simple as two wires in series with a lamp/buzzer and 9V battery. When the circuit is completed, the buzzer/bulb activates.

author: KL27x
author_rank: Super Contributor
timestamp: July 29, 2016, 06:07:43 pm
content: They way I have built one:3V battery. Red probe = high impedance voltage. Black probe = a comparator set to just south of Vcc /2. And a high impedance shunt to ground (which forms a resistor ladder with the red probe high impedance output.... say 10K on each probe). Tune comparator setpoint.. typically to where continuity is somewhere between 10-500 ohms. Voltage drop of, say, a P-N junction is too great to show continuity, of course. It will only beep on a low impedance connection and send somewhere around 0.15mA through the connection.... not enough to harm anything.Across a 1M ohm connection, voltage drop across the probes could be approaching 3V, but at current approaching zero. But if there's any conduction, voltage drop is very minimal and max current 0.15mA. Completely safe, IMO. Max power dissipation by the circuit-in-test would be when circuit is ~20K, which would equate to 0.1125mW. Across a PN junction, power dissipation would be sub 0.1mW. Perfectly safe for any electronics, I think.I don't suppose a multimeter does it much different, although there are many ways to skin a cat. Comparator. Op amp plus ADC plus software. W/e. Since the multimeter also has to measure resistnace, it has the opamp and ADC, already. But a comparator has lower latency. The good DMM's also have a latching circuit, so there's a minimum duration of beep when continuity threshold is reached. Done in software, 99% of the time, most likely. But could be done with a one-shot monostable multivibrator circuit.