About The Impedance Of Multimeters
Ever wondered why when you measure the voltage of a particular circuit with your digital and analogue meter, the result that you got were different? The reason for it usually is due to the impedance of the meter you are using. It is also depends on what kind of circuit you are measuring.
So what is impedance?
The definition of impedance is the measure of the opposition to electrical flow. It is measured in ohms and represented by the symbol Z.
Generally digital multimeter has a high impedance input of 1 Mega Ohm and greater and for analogue multimeter would be 20 Kilo Ohm for DC and 9 Kilo Ohm for AC.
So what does this Ohm value mean? If a multimeter that has high impedance input, it means when you are measuring the voltage of a point (or nodes) in an electronic circuit, very little current would goes through it (the multimeter) thus it would not disturb the circuit under test and you will get the voltage what you are expecting. If you are using a low impedance input multimeter such as the Analogue Multimeter, the volt meter will draw too much current from the circuit and will give you a false voltage reading. This is called as circuit loading (loading effect). In other words, your meter is an external load to the circuit you are measuring and will alter circuit performance.
For example, if you are measuring a point that has 3 volt using a high impedance input meter and you got approximately 3 volt, then you know that the point you are checking is good. For that same point if you use a low impedance input meter to check, you may get 2.90 or even 2.8 volt (depending on the type of circuit you are checking)! This means the low impedance input meter is disturbing the circuit you are measuring (loading effect) and gave you an inaccurate reading. If you do not know about the impact of multimeter impedance to a circuit, you may have thought that the circuit you are measuring have problem but the fact is that particular circuit you are checking is actually in working condition!
If this is the case then why do we still need to use a low impedance meter?
Now, don’t get me wrong that Analogue meter is of no use in checking the DC voltage due to its lower impedance (20 K ohm) as compare to digital multimeter (1 Mega ohm and above). If you just want to know if a DC voltage is present in an electronic circuit and the value is not that important to you, you can always use an Analogue meter to test it because it can give you a fast result by just looking at the pointer. For example, if you are checking a 5volt DC line of a power supply and your Analogue meter shows you 4.85 to 4.9 something volt, you know that this voltage would be acceptable because there would be tolerance in the output DC volt.
However, if you really want to know the precise voltage of the DC output then need to use a Digital multimeter to test it. The result could be between 4.98 to 5.01 volt depending on the accuracy of the digital multimeter. As for me when checking DC voltage of a circuit I would usually use a Digital Multimeter because it has a higher impedance input. Actually the 20K ohm impedance of Analogue multimeter would not severely impact the result you got as compare to the Multimeter that has even lower impedance value that I will briefly explain in the next paragraph.
Dual impedance meter
Nowadays you can find Digital multimeter that comes with dual impedance features. The first feature would be the high impedance input (usually 10 Megaohms) that built into the voltmeter range and the second feature (low impedance input usually between 2k to 5kohm depending on the brand of meter). Fluke named it as Auto-V/Lo Z and you need to deliberately set it to use this feature. Take a look at the Fluke 117 meter video below explaining about the Ghost voltage and how the low impedance range can help to solve it:
Another example would be testing a button cell or coin battery (or watch battery). If you test these kinds of batteries using a high impedance input meter, a weak battery may still show good 3 volt! However, once you have selected your meter to Lo Z test, the result would be different, you may get between 2 to 2.5 volt only because this Lo Z feature is sort of like putting a heavier load (2k to 5k ohm depending on the brand of meter) in which the weak battery can’t sustain the load draw thus the voltage will drop. Although UNI-T 191 Model has LoZ feature but that feature is only for the AC voltage and not DC voltage.
This is why before you buy any Digital multimeters you need to know the specification of the meter. In other words, not all Digital multimeter are designed/created to be the same in term of features and specification. Expensive and branded Digital multimeters has its own advantages over cheap Digital multimeters.
Conclusion- If you already have a digital multimeter with high impedance of 10 Mega Ohm, it is not necessarily you must get another meter that has the low impedance feature. It depends on your job application. Having a Digital and Analogue multimeter already can solve lots of electronics circuit problems. Of course with more specialized test equipment your troubleshooting job would be much easier because these test equipment can remove the guesswork and shortening your troubleshooting and repairing time. Hope you have learned something new today and if you have any questions, you can post a comment below that is related to this topic.
This article is brought to you by Jestine Yong. He is from Kuala Lumpur Malaysia and he loved electronics repair and blogging about electronics repair information. He is the author of the famous Basic Electronics Repair and SMPS Repair ebook . He is also a trainer and conduct electronics repair courses at Noahtech Electronics Training Center.
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Mark
October 23, 2021 at 7:50 am
Great topic Jestine,
I personally have 23 multimeters and had a love of these meters for many years.
I'm actually teaching a night course right now dealing on multimeters.
I have found that most digital meters are within the 10 MegaOhm range.
The easiest way to find out what the impedance on a meter is, is to use 2 meters, one set on volts, the other on ohms, connect the leads together, red to red, black to black and you will be able to read the impedance on the Ohms reading. I am developing a series of videos on my channel that is purely discussing multimenters.
Keep up with these very interesting articles Jestine - I find them fascinating!
Jestine Yong
October 23, 2021 at 7:58 am
Hi Mark,
Wow! 23 multimeters! Thanks for your input sharing and the videos.
Jestine
Prasanna mendis
January 27, 2022 at 12:52 am
Useful article thanks for you
Yogesh Panchal
October 23, 2021 at 2:17 pm
Mark, Thanks! for sharing your Video Link.
Kushal Saini
April 9, 2024 at 1:26 pm
Hi sir iwant a Multimeter for low current AC 100ma dc 200ma
Anwar Y Shiekh
October 23, 2021 at 9:09 am
I've started to use rechargeable lithium 9V batteries in my multi-meters.
Jestine Yong
October 23, 2021 at 2:44 pm
Hi Anwar,
Can save money by recharging the lithium battery.
Jestine
Debasis Ray
October 23, 2021 at 9:21 am
When I was age 20. I doing and experiment to remove Ghost voltage reading. I used analog and digital multimeter same time parallel.
Jestine Yong
October 23, 2021 at 2:45 pm
Hi Debasis,
I have not tried that method but it is a good idea. Thanks for sharing.
Jestine
Parasuraman S
October 23, 2021 at 1:21 pm
Excellent informative article. However, looks like you have skipped mentioning about the use of analogue multimeters (low impedance) to measure diodes and transistors, which the digital multimeters (with high impedance) show as ok.
Jestine Yong
October 23, 2021 at 2:43 pm
Hi Parasuraman,
Thanks. About the diodes and the transistors test I have included it in my ebook here.
Jestine
A. Fitzgerald Downes
October 23, 2021 at 2:03 pm
Thanks for your indepth study of quality meters. When I was at technical school about 50 years knowing about meter performance was something we had to be familiar with. For example, the difference in "measuring" with a VTVM (Vacuum Tube Volt Meter)and the NEW FET (Field Effect Transistor) multimeter. But what do you think about using the oscilloscope performing the same function?
Jestine Yong
October 23, 2021 at 3:42 pm
Hi Fitzgerald,
Sorry I don't own any VTVM Meter thus would not be able to comment on this but from article found in the internet it has very high input impedance. Same goes to FET Tester (from Sencore) that has high input impedance. Most oscilloscope have 1 Meg ohm and above thus would not really load down the circuit under test.
Jestine
Yogesh Panchal
October 23, 2021 at 2:08 pm
Sir,
Thanks!for sharing tips.
Jestine Yong
October 23, 2021 at 2:36 pm
Hi Yogesh,
You are welcome.
Jestine
A. Fitzgerald Downes
October 23, 2021 at 2:09 pm
And by the way where can I get the UT 191T Professional Multimeter? I need one for my workbench.
Jestine Yong
October 23, 2021 at 2:47 pm
Hi Fitzgerald,
You can buy it from aliexpress or from ebay.com.
Jestine
Albert van Bemmelen
October 23, 2021 at 3:46 pm
A digital multimeter with Lo Z measuring option was new to me. Thanks for the tip Jestine.
Measuring those 3V CR2032 and similar lithium cells indeed often gave high 3V DMM readings on still not working batteries when used in IR remotes or other higher amp devices. In lower current consuming devices like watches these cells probably still would work. Also a good reason never to keep these 3V battery cells on pink antistatic carbon sheets because these sheets definitely slowly discharges them. In the end leaving us with only empty lithium cells.
Jestine Yong
October 23, 2021 at 3:50 pm
Hi Albert,
You are welcome. Thanks for sharing about not keeping the 3 volt battery cells on pink antistatic carbon sheets.
Jestine
Babu M S
October 23, 2021 at 10:23 pm
very good information sir.Thank you very much for sharing.
Jestine Yong
October 23, 2021 at 10:52 pm
Hi Babu,
You are welcome.
Jestine
Paris Azis
October 24, 2021 at 12:00 am
Hi Jestine
For high accuracy, in the past, in the vacuum tube era, it was not possible to measure (as an example) the AGC voltage of a TV receiver with a conventional analog multimeter having usually a sensitivity of 20kOhms in its DC ranges...This was due to the fact that this voltage was developed across a high value resistor of some MOhms. It is obvious that such a measurement would be erroneous, because of paralleling a very high resistance with a relatively very low one, that is, the low impedance of the instrument itself. The resultant resistor when paralleling was always smaller than the instrument ‘s impedance... Therefore, in order to cover such needs, those VTVM were developed... Their input impedance was typically 10MOhm, making the AGC and relevant other measurements quite easy and reliable..
Now, about ghost voltages, it is always easy to “degrade” a good modern multimeter paralleling it with a relatively small resistor (or with an analog multimeter as described above) in order to identify if a voltage is a ghost one...
In other words, I don’t think that this feature is so important to have...
On the contrary, the high impedance of an “ancient” VTVM is of utmost importance for example if one needs to test the leakage of a capacitor...
Simply you charge it, then connect the VTVM across it, and watch the discharging time...
If the cap happens to be a healthy electrolytic one, the instrument’s pointer stays at the indicated voltage for incredibly long time...If it drops quickly, the cap is leaky...This is perhaps one of the best applications to “see” how time works on a RC circuit (where C the cap under test, and R the instrument ‘s impedance)...
Jestine Yong
October 24, 2021 at 9:07 am
Hi Paris,
Wow! That was a superb explanation and a good tip of checking the cap!
Thanks so much
Jestine
Henrique J. G. Ulbrich
October 25, 2021 at 11:04 pm
Thanks for sharing again another useful and didactic article. The impedance of an analog multimeter is not really expressed as 20 kΩ. The correct specification is Ohm/V. In an analog multimeter specified as 20kΩ/V, this value is only found if there is a 1V scale, not common in these instruments. If a voltage of 3V is measured using such device in the 10V scale (a very common situation), the impedance of the multimeter in this case is 20kΩ * 10 = 200 kΩ. In the same sense, the same multimeter in the scale of 500 V has an output impedance of 20kΩ * 500 = 10 MΩ. So, in this case the impedance of the analog multimeter will not really affect the measurement, as compared to the digital ones. Eventual differences will be due to inherent precision (and quality/price) of the instruments, as well as their calibration status. There will be problems only when measuring low voltages in a high-impedance circuit (as the AGC voltage in the old BW TVs, as pointed out by Paris Azis). In the remaining trivial situations, the impedance of an analog multimeter will not in practice affect the precision of the measurements taken. This is true, for example, in the measurement of the 5V line from a power supply. Such a circuit have inherently a very low impedance. By the way, the load imposed by a device is also known as burden.
Jestine Yong
October 26, 2021 at 10:11 am
Hi Henrique,
Thanks for the clear explanation and example. This is also why using digital meter (with 10 Megaohm input impedance) to check the DC voltage of electronic circuit be it a low or high impedance circuit would be the best choice.
Jestine
michaelgp3
November 19, 2022 at 4:47 pm
I would add that digital multimeters are, by themselves, not very useful in troubleshooting ground faults where water and the resultant corrosion is involved as continuity readings go all over the place due to the high impedance of that type. Analog multimeters will provide steady and reliable readings in these circumstances due to their lower impedance.
Jestine Yong
November 20, 2022 at 7:28 am
Hi Michael,
Thanks for the comment!
Jestine