Measuring voltage with digital multimeter - Page 1
Forum section: electronics / beginners

author: mike_mike
author_rank: Super Contributor
timestamp: November 23, 2018, 08:20:50 pm
content: Hello,I have some questions regarding the measurement of voltage using a digital multimeter.1. What will happen if I want to measure a DC voltage but my multimeter is set to Resistance ?2. What will happen if I want to measure a DC voltage but my multimeter is set to AC Voltage ?What will be displayed on the multimeter screen in both cases ?I am using a UNI-T UT52 multimeter.

author: pelule
author_rank: Frequent Contributor
timestamp: November 23, 2018, 08:34:47 pm
content: 1. What will happen if I want to measure a DC voltage but my multimeter is set to Resistance ?DMM reisistance is a voltage measurement method, it mesures the voltage over the resistor by an internal generated constant current. Most DMM use their lowest voltage range (i.e. 200mV).If the DC voltage is inside the limit of the max. input voltage:= the DMM shows the DC voltageIf the DC voltage exceeds that limits:= a cheap DMM may be damaged= a good quality DMM should just show "overload" as it is input protected.2. What will happen if I want to measure a DC voltage but my multimeter is set to AC Voltage ?If the DC voltage is inside the limit of the max. input voltage:= The DMM will measure/show the AC part (noise) of the DC voltage.If the DC voltage exceeds the limit of the input voltage:= a cheap DMM may be damaged= a good quality DMM should just show "overload" as it is input protected.Have had a look to the UT52 specification. There is noted:"Overload protection: 200mV is 250VDC or AC RMS. All other ranges is 750Vrms or 1000Vp-p."/PeLuLe

author: IanB
author_rank: Super Contributor
timestamp: November 23, 2018, 08:42:50 pm
content: Quote from: mike_mike on November 23, 2018, 08:20:50 pmAlthough good quality multimeters have protection to guard against damage if you do this, you should try really hard to avoid doing it. Try to cultivate good habits, for example always check the range setting before connecting probes to a device under test, always detach the probes when you have finished a measurement, and never turn the meter selection dial while the probes are attached to a device.

author: Vtile
author_rank: Super Contributor
timestamp: November 24, 2018, 10:35:23 pm
content: Quote from: IanB on November 23, 2018, 08:42:50 pmI'll say the same as two gents above. While especially new / modern multimeters are tried to be made safe for all ranges, you still should try to learn the "proper measuring procedures".That is (at top of my head and not in strict order of importance):- clean working environment, no clutter.- Read the fucking manual.- Identify the DUT and go it through with though to determinate what kind of energy levels you are facing (high currents, high voltages, high frequencies etc.)- Rule of thumb is that every 1000 volts can jump 1 mm through air, so 10'000 V will jump 10 millimeters.- All unnecessary cables and wires attached and non insulated ends ended ie. with some terminal strip.- All unnecessary circuit separated from power (especially with mains or high voltages )- All capacitors should be threaten as potentially lethal and should be shorted with small resistor before measuring, bleeders might be malfunctioning.- In slightest doubt use only off-line measurement methods (turn power off and attach the measuring device and then turn power on).- Active control circuitry should be stopped and locked in the safe state (both hardware and software based)- Double check your meter is in safest mode available (typically highest voltage range) and your test circuit is connected as should (test leads in voltage ports not in current etc.)- Verify your measuring device is working correctly from known source.- Start with verifying the voltage in the circuit both in AC and in DC starting with the highest range in the measuring device- In case of current measurement, be absolutely sure that you have right side of load in series with your meter. The load is only thing that will prevent a short circuit at it should be connected in any circumstance series with your meter (meter in current measuring mode is same as bare wire between measuring points).- The old mantra of electricians "keep one of your hand at back pocket" is still relevant while working with (*unknown) live circuitry.*unknown - a circuit which voltage, current or general peak energy levels are unknown or a known circuit that is behaving abnormal way or a prototype of some form without a known separation from high energy circuitry.

author: Brumby
author_rank: Unknown
timestamp: November 25, 2018, 03:07:38 am
content: I can only underline what others have said - and I will stress the safety concern your questions raise:If you are asking these questions,then you are not thinking safely.Forget the fact that you may damage your meter. Forget the fact that whatever is displayed will be meaningless. REMEMBER THIS: If you are concerned you might do this sort of thing, then one day you are going to pick the wrong combination of meter setting and circuit and you could injure or kill yourself.This is not scaremongering. This is a real concern and cannot be emphasised enough.One of THE most dangerous things you can do is connect ANY external device (a meter or a scope for example) to a circuit. Double and triple check setting of the meter and the sockets used BEFORE connecting ... and please, PLEASE,PLEASEread the manual before you start using a meter for the first time. While this may seem to be a waste of time for common measurements, you can come across specific warnings as well as non-obvious features. It is time well spent.The bottom line is simple: BE SAFE.