How Does a Multimeter Meaure Voltage and. . . . - Page 1
Forum section: electronics / beginners

author: DW1961
author_rank: Frequent Contributor
timestamp: June 25, 2023, 01:42:05 am
content: When you use a 100 ohm resister across the probes the voltage flows between the positive and negative probe around the resistor and the battery (see image). But if you don't use the resistor and you just use the probes, I assume it goes back to the meter? If so, why does it go through the resistor when connected as in the picture, and not just right back to the multi-meter and bypass the resistor?Edit:I should have made it clear that using a resistor with the meter is because the meter has no battery test function, thus no internal test resistance.

author: ledtester
author_rank: Super Contributor
timestamp: June 25, 2023, 01:54:30 am
content: In voltage mode your multimeter has a very high impedance between the probes - typically greater than 1 megaohm. So some current will pass through the meter but compared to the 100R resistor it will be insignificant.

author: IanB
author_rank: Super Contributor
timestamp: June 25, 2023, 01:57:39 am
content: Quote from: DW1961 on June 25, 2023, 01:42:05 amVoltage does not flow. So the whole premise of your question is flawed.

author: Brumby
author_rank: Unknown
timestamp: June 25, 2023, 02:09:25 am
content: Quote from: IanB on June 25, 2023, 01:57:39 amThat's a somewhat pedantic response.Considering the nature of the question and the place it was presented, I would be inclined to give the OP some slack and/or provide some education. Just saying they are wrong is not helpful.

author: xrunner
author_rank: Super Contributor
timestamp: June 25, 2023, 02:09:44 am
content: Quote from: DW1961 on June 25, 2023, 01:42:05 amWell ... Your wording of the problem is a little strange. Are you suggesting the meter is in the DC voltage mode? You need to model the situation to understand it. There are two more "resistors" in the circuit as shown. The battery has internal resistance, and the meter is also placing a high resistance in parallel to the battery and resistor shown.

author: IanB
author_rank: Super Contributor
timestamp: June 25, 2023, 02:29:31 am
content: Quote from: Brumby on June 25, 2023, 02:09:25 amI suppose you are right. But I find it hard to think of someone who has been on the forum for three years with over 600 posts as being a beginner.

author: Brumby
author_rank: Unknown
timestamp: June 25, 2023, 02:31:26 am
content: Quote from: DW1961 on June 25, 2023, 01:42:05 amTo be more accurate, it is current that flows. In simple terms, voltage is just the pressure that makes current want to flow.QuoteRemember this: Electricity will flow ineachandeverypart of a circuit that it is able.In your example, when you have the resistor and meter connected, some current will flow through the resistorandsome will flow through the meter. How much flows through each will depend on the nature of each part.If we just use simple resistance**in our calculations, the resistor is 100 ohms and a typical digital multimeter is at least 10,000,000 ohms. Use ohms law to work out how much current flows through each.**Actual circuit parameters in the real world can get really, really tricky, especially as the frequency of things increase - but it is not necessary to worry about that with the example you are trying to understand.

author: DW1961
author_rank: Frequent Contributor
timestamp: June 25, 2023, 02:31:41 am
content: Quote from: IanB on June 25, 2023, 01:57:39 amElectricity doesn't flow either, but we gotta talk about it somehow. Would it help if I had said "electrons bumping into each other?"

author: IanB
author_rank: Super Contributor
timestamp: June 25, 2023, 02:33:19 am
content: Quote from: DW1961 on June 25, 2023, 01:42:05 amTo elaborate on my previous response, voltage is not something that flows, it is a potential. Measuring voltage is a bit like measuring the weight of an object on a scale.An ideal voltmeter does not allow any current to flow through it when measuring voltage, in much the same way that an ideal scale does not move when you place a weight upon it.

author: DW1961
author_rank: Frequent Contributor
timestamp: June 25, 2023, 02:33:48 am
content: Quote from: ledtester on June 25, 2023, 01:54:30 amThen the meter measures voltage between the probes and resistor?

author: Brumby
author_rank: Unknown
timestamp: June 25, 2023, 02:35:03 am
content: Quote from: IanB on June 25, 2023, 02:29:31 amHmmm.... Fair point.

author: IanB
author_rank: Super Contributor
timestamp: June 25, 2023, 02:36:01 am
content: Quote from: DW1961 on June 25, 2023, 02:31:41 amElectric current flows. Electricity has two aspects, voltage and current. Current is the thing that flows, and voltage is the driving force that makes current happen.

author: DW1961
author_rank: Frequent Contributor
timestamp: June 25, 2023, 02:36:29 am
content: Quote from: xrunner on June 25, 2023, 02:09:44 amIt's a meter without a battery tester, so no resistance other than what is necessary to get the meter to work. If I use a meter with a dedicated battery test setting, I get a different voltage reading than the other meter w/o that setting.I should have made it clear that using a resistor with the meter is because the meter has no battery test function.

author: Brumby
author_rank: Unknown
timestamp: June 25, 2023, 02:39:00 am
content: Ah .... so you want to make a meter into a battery tester by adding a load. Well, that IS the simple way to do it - but for a valid measurement, you will need the right load for the particular battery.

author: DW1961
author_rank: Frequent Contributor
timestamp: June 25, 2023, 02:49:37 am
content: Quote from: IanB on June 25, 2023, 02:29:31 amYour personal opinions do not give you a license to disrespect me (or others) for any reason. It's unacceptable, sophomoric, and abusive.I apologize to the community for this response, but it's a matter of self respect, and I don't need it or want these types of fruitless disrespectful comments. These types of down-talking and disrespectful comments never help anyone -- ever.

author: TimFox
author_rank: Super Contributor
timestamp: June 25, 2023, 03:14:11 am
content: The cell has an internal resistance in series with its chemical electromotive force, which resistance should be much smaller than the very high input resistance of the voltmeter.The current through the 1 or 10 M\$\Omega\$ voltmeter input from a 1.5 V cell is only 150 nA to 1.5 uA.To see the effect of the cell's internal resistance, which increases dramatically as the cell discharges, a suitable and relatively small resistor will draw enough current to drop the voltage at the voltmeter input.100\$\Omega\$ will draw 15 mA from a 1.5 V cell.Notice that I wrote "cell": a "battery" is a series connection of cells (despite the labeling on commercial cells).

author: DW1961
author_rank: Frequent Contributor
timestamp: June 25, 2023, 03:40:36 am
content: Quote from: Brumby on June 25, 2023, 02:31:26 amI do get that. But how do the meter know what is passing between the resistor and probes?

author: DW1961
author_rank: Frequent Contributor
timestamp: June 25, 2023, 03:48:26 am
content: Quote from: Brumby on June 25, 2023, 02:35:03 amIs it? Talking down to someone is fair because they have been on a forum for x years?The error in his logic is that I have been here for 3 years so I should know this stuff. In reality, I visit when I have a question and then go away sometimes for weeks or months even, w/o even thinking about electronics. My total time on this forum typing and replying is minuscule, in actuality. Add up how long it takes to type and reply 600 times average, and you get total time on the board. Perhaps, maybe, several hours or so.I mean, what he is saying is that since I have been her for so long my question was to low for him to reply nicely to.It makes the hair on the back of neck stand up when I think people think that sort of reply to another human being is okay.

author: DW1961
author_rank: Frequent Contributor
timestamp: June 25, 2023, 03:49:49 am
content: Quote from: Brumby on June 25, 2023, 02:39:00 amIsn't 100 ohms correct for 1.5 to 9v?

author: Jwillis
author_rank: Super Contributor
timestamp: June 25, 2023, 03:52:51 am
content: Quote from: DW1961 on June 25, 2023, 01:42:05 amYour multimeter measures aPotential Differencebetween the positive probe and negative probe. It then compares and calculates that difference from a reference value set in the chip. This gives a readout on the display.In old analog volt meters the voltage and current would create a magnetic field in the meter coil that has a specific impedance this would cause a deflection . And as you know, with a set impedance (resistance), when voltage rises so must current. So as voltage rises the meter deflects more.In your picture, Your measuring the potential voltage across both the resistor and the battery. Any battery will drop in voltage under enough load. Since the meter "draws' very little current because of a very high impedance. (ussually 1 to 10 mega ohms) the multimeter can't simulate a load high enough to drop the voltage of the battery. The resistor draws more current across it because of it's much lower impedance dropping the voltage. So the meter can measure an accurate voltage because it effects the battery very little compared to the 100 ohm resistor.This is a load test of the battery at approximately 15mA. If there is no significant drop in voltage then the battery is good. If it drops to less than ( I believe ) 1V the battery is considered depleted.Suppose that's the best way I can describe it.

author: DW1961
author_rank: Frequent Contributor
timestamp: June 25, 2023, 03:54:45 am
content: Quote from: TimFox on June 25, 2023, 03:14:11 amSo the resistor loads the battery and the meter reads and calculates the volts from the pos and neg battery connection from the probs to the meter?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 25, 2023, 04:25:50 am
content: Quote from: DW1961 on June 25, 2023, 03:49:49 amThe suitable resistor depends on the size of the battery. 100 Ohm is quite low for a coin cell, but quite high for a car battery.It depends on the battery type if one needs a load resistor to test a battery.For the alkaline cells one can get away without a load resistor, as the open circuit voltage of the cells goes down. Usually 1.2 V per cell are considered empty, as not much charge is left than. However it depends on the circuit and some work down to less than 1 V while others already complain (stop working) at 1.4 V.Old style dry cells and Li coin cells like CR2032 need a load to get a meaningful result. When empty the open circuit still stays nearly the same, but the internal resistance goes up and the voltage thus drops under load. A those cells are usually used with light load one can use a larger resistor, like 1000 or even 10000 ohm for testing. Such a battery may be empty for circuits that need relatively high current like a flash light, but may still work for a low load like a clock.I noticed the difference between the battery cells with an alarm clock: with a dead alkaline cell the clock stoped / slowed down but the alarm still worked till the end. With a dry cell the clock still worked, but the alarm failed when the battery was empty.

author: DW1961
author_rank: Frequent Contributor
timestamp: June 25, 2023, 07:14:16 am
content: Regardless of how is measures voltage using the resistor, I just tested my MM using it's built in test mode using an AAA A battery:100 Ohm resister and MM set to 20VDC = 1.57vNo Resister and MM set to 20VDC= 1.58vBuilt in 1.5v test mode = 1.516vThis indicates to me the battery is not only good, but fully charged. That's why the resistor test and the non resistor test look almost identical. As far as the built in test goes, I dunno why the discrepancy in voltage, except that the built in circuit uses a larger resistor?What I need now is a low battery to see the difference between the resistor and no resistor.

author: Jwillis
author_rank: Super Contributor
timestamp: June 25, 2023, 10:36:05 am
content: I know we're no really supposed to advertise or promote products here. But i picked up one of these and it really helped to understand ESR and how it changes as the battery depletes.https://www.aliexpress.com/item/1005004269893522.html?spm=a2g0o.order_detail.order_detail_item.11.7cb2f19cLsM6OuWith the assistance of the battery data sheet of course you need to select a load resistor that will pull an average normal use current . In the case of a AA 1.5V cell it is somewhere around 15mA. So this is probably why a 100 ohm resistor was used.

author: BeBuLamar
author_rank: Super Contributor
timestamp: June 25, 2023, 11:01:09 am
content: Quote from: IanB on June 25, 2023, 02:29:31 amI am fine with someone who doesn't know that's what we here for. But as you said I suspect that the OP is trying to pull our legs.

author: BrokenYugo
author_rank: Super Contributor
timestamp: June 25, 2023, 01:48:21 pm
content: Do you have two multimeters? Put the one in battery test mode, and measure resistance across the two relevant jacks of the meter, whatever you measure is the built in load resistor.

author: tooki
author_rank: Super Contributor
timestamp: June 25, 2023, 02:38:30 pm
content: Quote from: DW1961 on June 25, 2023, 03:40:36 amIt doesn’t know and it doesn’t care. The meter simply measures the voltage differential between any two points in a circuit.When you apply a load to a battery, the battery’s internal resistance is also in series with the battery’s voltage source. So in essence, your load and the internal resistance create a resistive voltage divider, and your meter is measuring the center tap, so to speak. That’s why the voltage drops under load. The different internal resistance values of different battery types is why you need a different load resistor.See the attached sketch. (The section in blue is for the pedants who will otherwise remark that a thorough equivalent circuit also shows parasitic inductance and capacitance. You can completely ignore that for your purposes.)

author: DW1961
author_rank: Frequent Contributor
timestamp: June 25, 2023, 07:07:46 pm
content: Quote from: BeBuLamar on June 25, 2023, 11:01:09 am"pull our legs. . . ." Like a conspiracy theory or something? I'm wondering why you would think that?I've never really thought about how a multi-meter measures voltage. The image I posted made me curious. I didn't really mean how a MM theoretically measures voltage, but, rather, how does it measure voltage when the circuit seems to be independent of the meter, seems to just bypass it. In the image, the green line actually doesn't go through the battery. I think maybe that is what threw me off.

author: TimFox
author_rank: Super Contributor
timestamp: June 25, 2023, 07:33:30 pm
content: An old-fashioned meter (e.g., Simpson 260 VOM) measures the voltage between the two input terminals by connecting a largish resistor in series with a microammeter."20,000 ohms/volt" means a 50 uA meter with 200 k\$\Omega\$ total resistance to make a 10 V full-scale.New-fangled meters (the usual 3.5-digit DMM) connect the two input terminals to a switchable voltage divider to obtain, say, 200 mV at the full-scale setting of the voltmeter.The internal ADC is scaled to obtain 1999 counts at 199.9 mV at the divider output: it's an electronic circuit.Nothing in the usual hand-held meter (of either type) is connected to ground, and "voltage" is always the difference between two points.At higher frequencies, consider a coaxial transmission line. Cut the cylindrical line with an imaginary plane perpendicular to the wire. At a given position along the line, the voltage is the difference in potential between the center conductor and the outer shield. The current is the flow along the wire through that plane. At high frequencies, this is a local measurement.

author: DW1961
author_rank: Frequent Contributor
timestamp: June 25, 2023, 07:35:15 pm
content: Quote from: BrokenYugo on June 25, 2023, 01:48:21 pmIt reads out 13.5 ohms. That can't be right?I tested a dead AA battery that read .8v in normal voltage mode. Then I tested in using the meter's internal battery test function, which replied with .0v. I then connected the resistor to the leads and it also read .0v.So even though the readings were a little different between a fully charged AA when measured with the MM's internal battery test function and the resistor using normal voltage function, both the resistor and the internal battery function measured 0v (actually blipped on both .01v). So that tells me the 100 ohm resistor method and the internal battery test function are working.I'd still like to test them again with a battery that isn't so dead, such as one that has around 1.0v -1.1v measured with the internal battery tester and then the resistor.

author: BrokenYugo
author_rank: Super Contributor
timestamp: June 25, 2023, 09:19:23 pm
content: That sounds about right, probably a 10 ohm resistor (measuring low resistances to high precision is hard). You need to load the battery down to show the voltage dip if it's nearing end of life, 10 ohms is only 150mA@1.5V. I have one of those generic testers with the little analog meter, with the specs molded into the case, the stated load is 4 ohms on the 1.5V position. A good AAA (or larger) will accelerate the needle to the green zone hard enough it overshoots and makes an audible click against the stop, a handy feature/bug.

author: DW1961
author_rank: Frequent Contributor
timestamp: June 25, 2023, 09:37:28 pm
content: Quote from: BrokenYugo on June 25, 2023, 09:19:23 pmI'm just wondering what the different readings would be between a 100 ohm resistor and the built in 15 ohm resistor. Everyone says use a 100 ohm resistor for AAA AA 9V and C and D batteries.Checking the 9V side I get a reading of 1500 ohms?

author: BrokenYugo
author_rank: Super Contributor
timestamp: June 25, 2023, 09:50:48 pm
content: A higher resistor will load the battery less, who is "everyone"? What's their logic behind 100 ohms?Then 1500 ohms is what they picked, my analog tester uses 220 on the 9V contacts, my cheap harbor freight meter with similar function probably uses yet another set of values.There's no standard for this sort of thing, at the end of the day it's a go/no go sort of test. That's why I have the little analog battery tester. I think it's silly to measure such a thing to such high precision and I don't care to drag a DMM out every time I change a battery.

author: BeBuLamar
author_rank: Super Contributor
timestamp: June 25, 2023, 10:31:21 pm
content: Quote from: DW1961 on June 25, 2023, 07:07:46 pmThe current flow into all things connected to the battery in parallel that including the resistor and the DMM. But let assume the DMM is an ideal voltmeter that no current goes thru it still there is a voltage accross the 2 battery terminal whether you have current goes thru something or not (like a resistor).

author: DW1961
author_rank: Frequent Contributor
timestamp: June 26, 2023, 01:13:53 am
content: Quote from: BrokenYugo on June 25, 2023, 09:50:48 pmOh right I forgot about that. Higher resister lets less power through so less of a load?And that explains the discrepancy between using the 100 ohm resistor and using the battery test function, as the battery test function resulted in less voltage.I wonder why such a high resistor on the 9V side?The 100 ohm resistor seems to be a thing on the internet. Search string: test AA battery multimeter