Equivalent circuits of DMM measurements on certain DMM settings - Page 1
Forum section: electronics / testgear

author: Cicada
author_rank: Regular Contributor
timestamp: January 18, 2023, 02:32:34 pm
content: HiI have a fluke 177. But I guess my question will extend to most/all digital multimeters (DMM).I try to repair equipment PCBs by measuring components in circuit. I know you need an ESR meter to measure capacitors in-circuit. This link to the correct place in the video and requires on 20 seconds of listening.I would like to know how one can use the other measurement settings on a DMM to do in circuit measurement on components. I think if you know what the meter applies to the circuit and what it measures you will be able to deduce something about the circuit that you are trying to understand and repair.For example. I have my DMM on the ohms measurent setting i.e. position (red)1 in attached picture. When I measure over a Varistor (over voltage protection) that sits between live and neutral (input section of SMPS) I get a ohms reading that starts in the 100kohm range then jump to Mohm range and then to OL. It then loops around and repeat the cycle.https://www.youtube.com/shorts/D6tjjgemU3IMy interpretation of what happens here follows below.Interpretation:The attached picture indicate how a DMM implement ohms measurement. [Or see link 1 below]The DMM apply a current source to the circuit and measure voltage. My guess is that my circuit have some sort of capacitance that gets charged by the current source until a certain voltage is reached and then the DMM switch the current source off. The cap then discharge and the measurement repeat. Or the current source stay on but something in the circuit discharge the capacitor when the voltage reach a certain value.Its maybe a bit difficult in this case to figure out what really goes on here, buit this is sort of the idea.BUT. I would like to know (like in the ohms measurement case) how the DMM apply signals and measure on the others settings i.e. 2 Capacitance, 3 Continuity and 4 Diode.Maybe there is a good source document that discuss:1. The equivalent DMM circuits as asked for above and the voltages/currents operated at,and/or2. tips and trick on how one can use the DMM on these settings to learn more about your circuit with full or limited prior knowledge of how your circuit actually lookThanks a lot for any help.CicadaLinks1.https://www.pickeringtest.com/en-za/kb/hardware-topics/sensor-simulation-technical-articles/resistance-measurements-with-a-dmm

author: Kleinstein
author_rank: Super Contributor
timestamp: January 18, 2023, 05:17:47 pm
content: How the capacitance, diode and resistance modes are made can vary between meters.For the newer DMMs there usually are no schematics available. With some luck the chip set is known and the data-sheet (if available and readable and not in Chinese only) may give a few hints.The diode mode could have a constant current source for something like 1 mA or it could be just a resistor (e.g. ~10 K but likely including a PTC for protection) to a fixed voltage of some 3 or 5 V.So the first step to measure the meters characteristics.Continuity is often like diode (often even combined), just with a beep when close to short.The resistance mode can also be a fixed current or a series connection with variable current depending on the resistor (slightly more current with lower resistance).