Newbie Question regarding measuring volts on a laptop AC adapter - Page 1
Forum section: electronics / beginners

author: Jim4Wheels
author_rank: Contributor
timestamp: March 06, 2023, 02:35:49 pm
content: Hello everyone.I'm completely new to multimeter usage (and the intricacies of electricity in general) so please forgive this very basic and dumb question!Did a bunch of searching beforehand on this forum, google, bing, semaphore, etc. to no avail, so either I'm the first person to ever ask this question, or I wasn't phrasing it correctly.The QUESTION!I have an AC adapter for a Dell Alienware laptop (large & heavy) with an output of 19.5V and 12.3AI want to test the voltage with my multimeter (an ANENG 621a), but the meter is only rated for 10A maximumIs it okay to test only the voltage, on the voltage input of the meter without worrying about the limits of the meter on the amp input?... or would I be risking life and limb with this endeavor?Bonus question, are there any good resources to learn more about the relationship between volts and amps, without using the common waterfall analogy, that doesn't work for me.😜Thanks.

author: tooki
author_rank: Super Contributor
timestamp: March 06, 2023, 02:49:15 pm
content: 1. Relationship of volts and amps (voltage and current): see Ohm's law2. A power supply's current rating is themaximumcurrent it candeliver. It is up to theloadto actually draw current.3. A multimeter's current rating is the maximum current it canmeasure. When measuring voltage, the current rating is irrelevant. So yes, you can safely measure the voltage of your power supply.4. Don't try to measure the current of a power supply by connecting it to the current jacks of the multimeter. You'll just blow the meter's fuse.5. There are videos about how to use a multimeter.

author: TimFox
author_rank: Super Contributor
timestamp: March 06, 2023, 02:52:17 pm
content: On such an adapter, it should be safe to probe the output with a DMM.However, you cannot load the device to its full current capability with the meter, you need an external load, such as an external 1.6 ohm, > 250 W (!!) resistor, to see what happens when the full rated current is drawn from the device.Without a load, you may find a strange voltage, if the supply was designed for a minimum load current (probably not).Such a power supply is designed to supply a fixed voltage, and has a maximum current capability: it is not designed to supply a fixed current.

author: Jim4Wheels
author_rank: Contributor
timestamp: March 06, 2023, 03:19:10 pm
content: Thanks for the answers.So, if I understand correctly, measuring voltage in my situation is generally safe, but I might not get an accurate reading?YouTube was part of my searching adventure, that's how I got the idea to measure voltage with my multimeter to begin with 🤣 -- when you're just starting out, youtube can be a double-edged sword.This is one of several videos I saw:

author: TimFox
author_rank: Super Contributor
timestamp: March 06, 2023, 04:42:30 pm
content: Quote from: Jim4Wheels on March 06, 2023, 03:19:10 pmYou should get an accurate measurement of the voltage at the output of the device, but that voltage might well change when you draw a large current from the supply.

author: MrAl
author_rank: Super Contributor
timestamp: March 08, 2023, 02:38:13 am
content: Quote from: Jim4Wheels on March 06, 2023, 02:35:49 pmVoltage is an "across" variable. Voltage is measured across an element like a resister. Voltage 'exists' across an element.Current is a "through" variable. Current is measured through an element like a resister. Current is a flow and it is thought of as moving through an element.These properties mean the two are measured differently.

author: Jim4Wheels
author_rank: Contributor
timestamp: March 08, 2023, 01:52:34 pm
content: Thank you everyone for all of the replies and advice.Still having some difficulties understanding, voltage, amperage, and how they are measured differently, but for now it’s enough to know that I’m not going to blow up everything trying to measure the voltage on my laptop charger.(my confusion comes from the thought that, even though I am only measuring voltage, the amps would still need to go somewhere, and my first thought was they would go into my meter.)Been looking up different tutorials on YouTube to understand electricity better. So far this is the best one I found.Going to see if I can get the rest of his material.https://youtu.be/OGa_b26eK2c

author: Brianf
author_rank: Regular Contributor
timestamp: March 08, 2023, 02:04:03 pm
content: Imagine a tank of water on top of a tower with a hose coming down to the ground.As the tower gets higher the pressure will increase, that pressure is like the 'voltage'. Voltage = Pressure.Now turn on the tap. Water will flow from the end of the hose, the higher the flow rate the greater the 'current'. Current = Flow Rate.But the flow rate will be limited be the size of the hose, a small diameter hose will resist the flow and reduce its rate. That's resistance at work.

author: Jim4Wheels
author_rank: Contributor
timestamp: March 08, 2023, 04:28:59 pm
content: Hey Brianf, thank you for that explanation, it helps.Can perhaps you, or someone, explain why my thought above (about the amps flowing recklessly into my meter when measuring voltage) isn't true?Again, this is something I've been searching for, but nothing I've found has given any explanation as to why this isn't a concern.EDIT-- I believe everyone who said that it's safe, I just want to understand the answer better for my own learning.

author: MrAl
author_rank: Super Contributor
timestamp: March 08, 2023, 04:36:32 pm
content: Quote from: Jim4Wheels on March 08, 2023, 04:28:59 pmHi,You are right that current flows into your meter. In theory though voltage can exist without any flow of current. To measure that though, we usually end up causing a flow of current as a by product.Say we have a voltage of 10 volts we want to measure. That voltage is there, whether we measure it or not.When we go to measure it, our equipment can only measure it by drawing some current so that the instrument can sense the voltage.This current is almost always very very very tiny. Most new meters have an input resistance of 10 megohms, so if we measure a 10 volt voltage it only draws 1 microamp. That's very small.An actual product like a tape recorder might draw 100 milliamps or more, so you can see the difference.This also shows the difference between theory and practice. Theory is about what is happening in the ideal situation under ideal conditions with ideal components. In practice we almost never have ideal conditions or ideal components so we see some small variations from theory. The voltmeter is a practical instrument, so it draws some current in order to make that measurement. If it was ideal, it would draw no current at all. As far as i know right now there are no ideal voltmeters, so every voltmeter draws at least a tiny current in order to make the measurement.The same is true about a current meter, although that drops a small amount of voltage in order to make the measurement.The voltmeter draws a tiny amount of current to make the measurement, the ammeter drops a small amount of voltage to make the measurement. They are both not ideal but they are pretty close.

author: Jim4Wheels
author_rank: Contributor
timestamp: March 08, 2023, 05:56:58 pm
content: Thank you for that very thorough and easy to understand explanation.That makes a lot of sense to me, and I think I understand now.You got my curiosity going, so I looked at the user's guide for my meter, and didn't see anything listed for Input resistance so that's a bit concerning...I watched someone do a teardown of the ANENG 621a on youtube, and he didn't jump out of his chair in horror, but admittedly I don't have enough knowledge to know what to look for myself.

author: james_s
author_rank: Super Contributor
timestamp: March 08, 2023, 06:15:13 pm
content: Typically the input resistance of a modern DMM will be about 10M, it doesn't matter for measuring voltage from a low impedance source like a power supply. You only have to think about the input resistance when dealing with sensitive circuits that are affected by the load the meter presents.

author: Zero999
author_rank: Super Contributor
timestamp: March 08, 2023, 06:40:41 pm
content: Look up Ohm's law. The actual current drawn is proportional to the voltage and inversely proportional to the resistance.The basic formula can be depicted as a triangle:VI | RA more complicated formula wheel.In your case the meter has an input resistance of 10M to 20M Ohms and the power supply voltage is 19.5V.Suppose the meter has a 10M input impedance.I = V/RV = 19.5R = 10×106I = 19.5/(10×106) = 1.95×10-6A or 1.95µA.Hint:Obviously dividing Volts by Ohms gives the answer in Amps, but using decimal prefixes and basic algebra simplifies thingsM = 106k = 103m = 10-3µ = 10-6So1/106= 10-61/103= 10-3So dividing volts by M Ohms always gives the answer in microamps and dividing volts by k Ohms always gives the answer in milliamps.

author: Nominal Animal
author_rank: Super Contributor
timestamp: March 08, 2023, 10:33:30 pm
content: Apologies for the following wall of text in advance.Let's start from the basics, measuring the voltage of a battery.If you set your DMM to DC voltage, and probes to the battery contacts, you'll measure theno-load voltage.This is because the DMM voltage probe resistance is very high, and does notloadthe battery; does not draw much energy from the battery at all.Unfortunately, battery chemistry is such that without a load, the voltage is much higher than when you try to draw energy from it, especially when the battery is depleted. So, we need to draw a current from the battery while we measure the voltage across the battery.Let's say we have a 1.5V battery we want to check. Let's say we use a 47 Ohm burden resistor, which should draw a current of 1.5V / 47 Ohm = 0.032 A = 32 mA, or what two 5mm LEDs might draw. We connect one end of the voltage probe to one end of the battery and one end of the resistor, and the other end of the voltage probe to the other ends of the battery and resistor. Essentially, we have the resistor and meterparallelwith the battery. Then, the DMM tells us the voltage of the battery when trying to draw about 32mA from it. There are well-known approximate curves for different battery types and chemistries, but they all look more or less like ╰─╮, with the middle part sloping only slightly downwards.Now, let's switch to the "switchmode power supply", your laptop AC adapter.If you stick the voltage probes directly to the adapter output (the end you connect to the laptop, being DC at about 19 V), you measure the no-load voltage of the adapter. It can be slightly higher than the nominal voltage, but switchmode supplies are typically well regulated to some voltage. On old heavy transformer-followed-by-rectifier supplies, it can be significantly higher than the nominal voltage.Measuring the voltage on the adapter output while the laptop is charging from it –– typically requiring you to sacrifice a suitable DC jack extension cable ––, you can find out what the voltage output is when the laptop draws some current from it.To measure both the voltage and the current, we need two DMMs. One measures the voltage across the adapter output, in parallel with the laptop. The other is put in DC amps mode, and is connectedin series: one of the two DC lines between the adapter and the laptop is cut, and the ammeter probes connected to the cut ends. (Again, this is usually done by sacrificing a suitable adapter extension cable.)Of course, that still only tells us the voltage and current at whatever current the laptop decides to draw from the adapter.If we want to see what the voltage is at different loads, we need a programmable or adjustable load. The ones with both voltmeter and an ammeter built in are most useful. The USB ones you can get for < 10€ or USD $10 off eBay and elsewhere are pretty good, but not necessarily very precise, but sufficient for hobbyist work. For a 19.5V 12.3A direct current (DC), you need a programmable load that can dissipate up to 19.5V × 12.3A = 240 W.You connect the adapter to the load, starting with zero load, and then increase the load up to the maximum, and record the voltage and current, pairwise, whenever either changes. If you put voltage on the vertical axis, and current on the horizontal axis, you'll get one of ─┐, ─╮, or ╰─╮. The more the center part slopes down right, the worse the voltage regulation. Seeing the last, i.e. no-load voltage high, means the adapter needs to be always loaded, or the circuits have to tolerate the higher voltages.One way people have built adjustable loads is to use old-fashioned incandescent light bulbs that can handle the specified DC voltage (typically rated much higher than the voltage to be measured). They are basically just resistors, that turn electrical energy into light and heat. You put them in series, with positive end of one connected to the negative end of the next one. The more you put in series, the higher the load. You connect the voltmeter probe to the adapter output, the ammeter between one end of the adapter and the light string, and the other end of the light string to the other end of the adapter. For each number of lights, you record the voltage and the current. This tells you what kind of voltage the adapter outputs at different current draws, just like with a programmable load, but with fewer load current sample points.

author: MrAl
author_rank: Super Contributor
timestamp: March 09, 2023, 06:44:28 am
content: Quote from: Jim4Wheels on March 08, 2023, 05:56:58 pmHi,If you put a resistor in series with your meter after measuring a voltage without it, you can get an estimate of the input resistance.For example, if you measure 10 volts with no resister then with a 10 megohm resistor in series and you measure close to 5 volts, then the input resistance is 10 megohms. When you measure half the voltage the input resistance is about the same as the series resistor.They often call it input impedance but its mostly resistive.

author: Jim4Wheels
author_rank: Contributor
timestamp: March 09, 2023, 08:17:23 pm
content: So, apparently someone did another tear down of my meter, and it only has a 0.8 M ohm input resistance … I don’t know if that’s safe… thoughts?

author: Zero999
author_rank: Super Contributor
timestamp: March 09, 2023, 08:38:20 pm
content: Quote from: Jim4Wheels on March 09, 2023, 08:17:23 pmThat's very low. I know some cheap meters have a low input impedance. Some older, analogue meters switch in a different resistor, for each voltage range, because the meter just measures the current through the resistor.0.8M is no safety risk. Your power supply is more than capable of providing adequate current. The voltage is 19.5V, so a 1M resistor would give 19.5µA and 0.8M, or 800k, would be a little more, but way below the 12.3A maximum.Cheap meters can pose a safety hazard, but it's for other reasons i.e. insufficient over voltage protection, causing it to violently explode, when subject to high voltage spikes, or insufficient fusing, so it overheats if too much current flows. In your situation, lack of over-voltage isn't an issue, as it's not mains. Does it have a fuse? If not, don't use it to measure current on any circuit which might exceed 10A.

author: Jim4Wheels
author_rank: Contributor
timestamp: March 09, 2023, 09:01:05 pm
content: It does have a fuse, which looks like an HRC (I learned about that from EEV blog videos.😎)Mainly got it to test my laptop charger, measure some household batteries, and check to ensure there is no voltage on outlets when I flip a switch.I read on another forum that there’s a danger that some outlets provide voltage even when turned off, and the pluggable outlet testers don’t detect it— I would need a multimeter for it.I don’t plan to ever measure amps at all, just voltage.I quite enjoy doing things myself and learning how things work, I just want to stay safe in the process and not burn anything down either 🤪

author: IanB
author_rank: Super Contributor
timestamp: March 09, 2023, 09:15:42 pm
content: In case you are curious, I just measured the voltage on a Dell 130 W power adapter (output 19.5 V, 6.67 A). With no load I saw 19.501 V, so the voltage regulation on the power adapter is apparently quite accurate.I also checked a Dell 240 W adapter (19.5 V, 12.3 A) and got 19.726 V. So I have to take back what I said about voltage regulation. All adapters are not the same.

author: TimFox
author_rank: Super Contributor
timestamp: March 09, 2023, 09:26:08 pm
content: One possible confusion when measuring AC voltage on an outlet (or similar connection) with a DMM.Some "upstream" switches before the outlet may have a capacitor across the contacts to protect the switch.In the old days, a Simpson 260 analog meter in AC mode might have an input impedance of only 1000 ohms/volt, or 250 kohm on the 250 VAC full-scale.(The current 260-8 is 5000 ohms/volt.)With a measly 250 kohm load, the small current through 1 nF, would only give a voltage indication of only 11.3 V in a 120 V, 60 Hz circuit.A Fluke 117 modern DVM, has approximately 5 megohm input impedance for ACV, so it is hard to tell the difference between no connection and a 1 nF capacitor from 120 V.That DVM, among others, has an optional "Low-Z" mode that switches in 3000 ohms across the input for that reason.

author: MrAl
author_rank: Super Contributor
timestamp: March 10, 2023, 05:14:07 pm
content: Quote from: Jim4Wheels on March 09, 2023, 08:17:23 pmThat's close to 1 megohm. That is not too bad dont worry too much.120v will draw 120 microamps and may help eliminate ghost voltage readings.

author: Jim4Wheels
author_rank: Contributor
timestamp: March 10, 2023, 07:39:38 pm
content: OK. Thank you again I’m just being overly cautious (and a little nervous) because I hear a lot of remarks about electrical safety and I’m just starting out Don’t want to let my curiosity get in the way of safety.I was about to go out and buy another multimeter, such as the fluke 117, just to get the extra safety.

author: TimFox
author_rank: Super Contributor
timestamp: March 10, 2023, 08:01:25 pm
content: Electrical safety, especially around mains power, is very important.I think the main problem, addressed by the "Cat III", etc. labels is that accidental connections will occur from full mains voltage (120 or 240 V) to the ACcurrentinput on a DMM.In a proper DMM, the very low input resistance of the AC amps range will draw such a huge current that the very expensive fuse will blow quickly, before the meter blows up in the face of the operator.On current ranges, the input resistance can be estimated by assuming that 10 A will make 100 mV for the voltmeter, i.e. only 0.01 ohms.Obviously, the current through that resistance from 120 V will be limited by the external circuitry and can be life-threatening.(Many writers here have complained about the very high price of these safety fuses, and inquire about cheaper substitutes.)The official Fluke fusehttps://www.fluke.com/en-us/product/accessories/fuses/fluke-11a-1000v-fusecosts $21 USD.A related tirade from Fluke:https://www.fluke.com/en-us/learn/blog/safety/10-dumb-things-smart-people-do

author: Jim4Wheels
author_rank: Contributor
timestamp: March 10, 2023, 08:11:08 pm
content: So, I guess I should just drop the money on the better fluke multimeter and ditch the one I have?I certainly don’t want anything blowing up in my face. And human life is worth more than a few hundred dollars.Appreciate all the advice from the experts, that’s exactly why I registered before I do something stupid😅

author: TimFox
author_rank: Super Contributor
timestamp: March 10, 2023, 08:26:15 pm
content: That depends.When dealing with line voltage, you need a good safe meter with an appropriate "Cat" rating: see the Fluke references for discussion, especially with respect to the current available from the source to blow up your meter.If you are only measuring voltages inside typical low-voltage circuits, not directly connected to your electrical utility, the available current is much lower and the safety requirement is less onerous.

author: james_s
author_rank: Super Contributor
timestamp: March 10, 2023, 08:28:29 pm
content: When dealing with 120V from a residential branch circuit I wouldn't worry about it too much. A cheap meter is less than ideal for that, but there isn't enough energy for something to really blow up in a nasty way. I would most certainly not poke around in a 480V industrial panel or even the bus bars or high power circuit for a major residential appliance without a proper CAT rated meter but for typical household stuff it should be fine.

author: Jim4Wheels
author_rank: Contributor
timestamp: March 10, 2023, 08:58:14 pm
content: That makes sense. I did go ahead and buy the fluke 117. It was the safest one in my budget that still seems to be useful for basic measurements, while having that extra bit of protection.Especially for a newbie like myself, I’d rather have extra protection and never need it, than need it and not have it.Speaking only for my own opinion, I like, and actually prefer the larger screen, and the touch buttons on the ANENG 621A, but because I am inexperienced I just don’t have the confidence that it will protect me if something bad were to happen, either from my own stupidity, or a random surge, etc.. That’s probably just my ignorance talking, I don’t know.

author: james_s
author_rank: Super Contributor
timestamp: March 10, 2023, 09:01:00 pm
content: Well you can't go wrong with a Fluke.

author: IanB
author_rank: Super Contributor
timestamp: March 11, 2023, 02:02:59 am
content: Quote from: Jim4Wheels on March 10, 2023, 08:58:14 pmIf you like the Aneng meter, don't be afraid of using it. Everyone who starts out uses inexpensive meters, and even very experienced people like "Big Clive" will happily use inexpensive meters on camera.For safety, just keep one thing in mind: don't measure the mains. However tempting it is, just don't.When you have more experience, that might change, but for now, just stay away and stay safe.As a matter of interest, trained electricians do not use a DMM to check whether a mains circuit is energized or not. They use instead a dedicated voltage tester designed specifically for that purpose.

author: Jim4Wheels
author_rank: Contributor
timestamp: March 15, 2023, 02:05:29 pm
content: Just wanted to update this again, to say thanks for all of the advice and help.My Fluke 117 arrived the other day, and the difference between that and my other meter is quite noticeable to me. The speed is quite a bit faster. The leads snap in with a reassuring click, the tilt stand makes working with it easier, and the best part of all, the manual states quite clearlyInput impedance >10MΩIt also lists all the certifications on the back of the meter, which gives me some extra confidence -- not that I'm going to purposefully test those claims, but it's nice to know.And the NCV (VoltAlert) actually works reliably for AC outlets, so I don't need to stick my probes on the socket to test voltage -- that's a huge plus, and much safer.One other recommendation I decided to take, was to get a pair of TL175 leads, which are a LOT nicer than the TL75's that the meter came with, although I am already noticing some white dots on the wires, which as I understand, are meant to indicate insulation wear... I don't think I should be seeing that after only a few days... I haven't been doing much at all with the leads yet. Is this normal, or should I try to get a replacement?EDIT: Contacted Fluke, and they seemed to indicate that the white spots were nothing to be concerned about as long as the cladding seemed intact. So that's good, I suppose. Another benefit of Fluke is reachable support with quick answers.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 15, 2023, 05:35:13 pm
content: Just don't make the error of trying to measure the current output by sticking a meter across the terminals.Bang and bang goes your meter as well. If you're lucky only the internal meter fuse will blow. Else....

author: Jim4Wheels
author_rank: Contributor
timestamp: March 16, 2023, 12:48:31 am
content: Quote from: Terry Bites on March 15, 2023, 05:35:13 pmWhat do you meansticking a meter across the terminals?I don't want my precious new meter, or various body parts to go bang bang... Please explain

author: james_s
author_rank: Super Contributor
timestamp: March 16, 2023, 01:14:53 am
content: Quote from: Jim4Wheels on March 16, 2023, 12:48:31 amHe means poking the probes in the hot and neutral slots of a receptacle for example, or across the terminals of a car battery or other high current source while trying to measure current, that will result in a direct short and you'll get a flash & bang. With a Fluke the meter will probably survive, but you'll likely burn the tip off the probe and blow a $10 fuse.To measure current without a clamp probe you have to interrupt the circuit and place the meter in series with the load, not across a voltage source. The 'A' jack on your meter is for all practical purposes connected directly with the 'Com' jack. You should never connect a voltage source across these two terminals.Voltage - ParallelCurrent - SeriesPlay around with something like a pair of AA cells and a flashlight bulb until this is second nature before you mess with higher power stuff. If unsure, please ask, there are no stupid questions when it comes to safety and longevity of your gear.

author: Jim4Wheels
author_rank: Contributor
timestamp: March 16, 2023, 03:20:44 am
content: Oh, right...I never heard it stated that way, but makes perfect sense.As I said before, I have zero intentions of measuring amps at all... only voltage My meter will never be turned to the A positions, nor will any lead be inserted into that port on the meter...I appreciate the invitation to ask more dumb (but not stupid) questions -- I have no doubt I'll make use of that invitation

author: james_s
author_rank: Super Contributor
timestamp: March 16, 2023, 04:10:04 am
content: Being able to measure amps is very useful, there's no reason to avoid doing it, just don't be dumb about it.

author: Jim4Wheels
author_rank: Contributor
timestamp: March 16, 2023, 05:20:30 pm
content: I'm taking a beginner's course for diagnosing electronic circuits, and the things I'm doing right now is measuring voltage, resistance, and continuity.Measuring amps isn't even in view yet, except to calculate values using Kirchhoff's voltage and current laws.My initial question in this thread was me trying to put my limited knowledge to work to find out why my laptop wasn't powering up... (P.S. the charger is fine, it was the power button itself, BTW)So I'll probably have a lot more questions as I proceed with this coursework. De Morgan's Theorem is making my head hurt.

author: Eraldo
author_rank: Regular Contributor
timestamp: March 20, 2023, 04:28:33 pm
content: Quote from: Jim4Wheels on March 16, 2023, 03:20:44 amAs someone else said, measuring current is quite necessary depending on your project. Maybe you want to know the power consumption of one of your circuits or check the capability of current sinking of your circuit. You're definetly gonna use it eventually so learn about it.But be careful when doing measurements with it and not go over its specifications. In general the most common accidents that happened to me with multimeters were trying to measure the voltage while having the leads connected to the current measurement. Only once though did i have a big incident with it and that was while trying to measure the mains voltage (230 ac). Gotta say it was like a flashbang before my eyes. The strange part tho was that after that i immediately opened my multimeter to check it out and to my surprise it was completely fine (not even the fuse was blown).The breaker acted fast enough as to not blow anything exept the probe tip.My advice on measuring current is to always probe where you're confident it works and always start with the amps range and then step down to milliamp range if it's safe. As you know the multimeter needs to be in series while doing the measurement and never short circuit the load (unless it's a bench power supply with current limiting and you need to measure its output current capabilities or accuracy). And always buy multimeters with a fuse in the high current measuring mode or else your shunt resistor will become your fuse (not pleasant).