Most efficient means for measuring current drawn by laptops?

[tawnytim]

Hi!  I own a computer repair and I like to be able to see the current being drawn by laptops in the shop if there is a no power complaint or other power issue.  I was wiring https://www.amazon.com/diymore-Digital-Voltmeter-Ammeter-Red-Blue/dp/B08HQM1RMF/ref=sr_1_20?crid=3JS3HA0OAZ819&keywords=amp%2Bmeter&qid=1673735459&sprefix=amp%2Bmeter%2Caps%2C83&sr=8-20&th=1 directly onto different power adapters and it works well, but I'm sick of wiring these for every type of power adapter(we work on PC, mac, you name it) and I like each tech to have their own set of diagnostic equipment.  So, I was looking for something more universal, and so I bought https://www.amazon.com/dp/B09M6D5BDD?psc=1&ref=ppx_yo2ov_dt_b_product_details.  It is not accurate at all.  So I'm wondering, is it inaccurate because it's a cheapy?  Would I have better luck with Fluke or equivalent?

That's a thought, I do own Dave's multimeters, never thought to see if he makes amp meters too.

Anyhow, can I get some feedback on this please?  Thanks!

[EHT]

The clamp meter you showed a link to is for measuring AC current, and for this, it needs to be looped through the clamp (probably explains in manual). At best you can measure the AC line current.

You haven't really explained *why* you want to measure the current drain in the case where there is no fault. What are you trying to do?

FWIW I powered my a laptop from a bench PSU, when the adaptor cable broke (magsafe one). Bench PSUs give a very accurate reading of DC current drain.

[fzabkar]

I have something similar to this:

http://www.p3international.com/products/p4400.html

It cost me around AU$20.

My laptop is using 7.5W as I'm typing this (battery fully charged).

[tawnytim]

I don't have a need to measure current draw when there is no fault.  Only when I get a machine in with a no power complaint or some other power related issue.  I can tell if it is charging the battery, if there is a parasitic draw when the lid is closed, etc.

[tawnytim]

I accidentally posted a reply to you without replying directly to you.  However, I have a question about your bench supply methods.  So what, you just cut off the end of your magsafe connector and made a cable using your bench supply?  Does it all seem to work correctly without the machine complaining about an improper charger, etc, due to there being no communication with the charging IC?

[bdunham7]

If you are measuring the AC input current, your clamp meter will work better with a 10X line splitter for small currents.  Even better would be a P3 Kill-A-Watt.  You'd start by plugging in just the power brick to see what its quiescent draw is, then plugging in the laptop to see how much the draw goes up, if any. 

If you want a closer look at the DC power that the laptop is using, I think a multi-connector breakout box of some type is the way to go, not using a separate PSU.  With a BOB, you can look at both the performance of the power brick as well as the current draw of the laptop itself.  The EEVBlog 121GW with its unique power measuring would be a handy way to check voltage, current and power all at once, although you can also buy gadgets on Aliexpress that sort of do the same thing.  Another worry with a bench PSU is that a mistake could fry something pretty easily.

[tawnytim]

Yeah, I'm really after DC current draw.  That gives me a good picture as to what's happening on the board.  I actually have the little brother to that device, the BM235 I believe.

[james_s]

You can plug the power brick into a Kill A Watt type device, power drawn from the wall is going to be more or less proportional to power drawn by the laptop.

[bdunham7]

Yeah, I'm really after DC current draw.  That gives me a good picture as to what's happening on the board.  I actually have the little brother to that device, the BM235 I believe.

OK, that's going to be really difficult with a current clamp like that, no matter the brand.  If you loop the wire 10X or even 100X it might work a bit better, but if you are willing to cut some cables up there's no need.

The 121GW is unique in that it measures voltage and current at the same time, thus the power feature.  That may or may not be helpful.

[tawnytim]

Yeah, I'm really after DC current draw.  That gives me a good picture as to what's happening on the board.  I actually have the little brother to that device, the BM235 I believe.

The 121GW is unique in that it measures voltage and current at the same time, thus the power feature.  That may or may not be helpful.

So if I were to remove the cover of the laptop and connect this voltmeter to the pins on the motherboard corresponding to + and - from the DC jack, it will read current too?  That's... amazing.

[bdunham7]

So if I were to remove the cover of the laptop and connect this voltmeter to the pins on the motherboard corresponding to + and - from the DC jack, it will read current too?  That's... amazing.

Um, no?  Where did you get that idea?

[abdulbadii]

No idea at shop, at home, yours on your own displace battery, extend the poles with copper wires & alligator clips at every end, one of which is interposed by a MM probe set on 1-10 Ampere meter from which the other probe continue to the other battery pole

run laptop while read the MM

[inse]

@EHT: Nope, it says AC/DC current.
But what is crucial: you cannot simply pass the supply cable through the clamp as forward and return lines will cancel out.
You need to split the wires…

[Swake]

If you really are in repair then you have a lab power supply. That thing likely has U & I meters on board already (If not replace it with a decent one). What stops you from using that supply to feed the laptop?

On the lab PS dial 19V (or whatever the laptop expects, but it is going to be 19V nine times out of ten). Plug two test leads in the power supply and point at the input barrel of the laptop. Takes 10 seconds.

- This method excludes the laptop PS from the equation. In any case you have to use your own known good PS or first confirm the customer's PS is good.
- Using the lab PS you can play with the current limiter to avoid burning things if you suspect shorts in the laptop.
- With the Lab PS many other diagnostics procedures are possible such as injecting appropriate voltages directly behind the regulators or feeding battery's directly to confirm these are taking a charge.

[tawnytim]

Um, no?  Where did you get that idea?

It didn't make sense, but you mentioned the 121gw measured current and voltage at the same time.  I wasn't sure what you meant.  https://www.amazon.com/dp/B07NXRPNPR?psc=1&ref=ppx_yo2ov_dt_b_product_details Now that little device, that works wonderfully.  I wish I had something like that with an input and output jack for the most popular laptop adapter DC jack styles.  The 4.5mm Dell, HP bluetip(also 4.5mm), Lenovo yellow Lego style, etc.

[tawnytim]

If you really are in repair then you have a lab power supply. That thing likely has U & I meters on board already (If not replace it with a decent one). What stops you from using that supply to feed the laptop?

On the lab PS dial 19V (or whatever the laptop expects, but it is going to be 19V nine times out of ten). Plug two test leads in the power supply and point at the input barrel of the laptop. Takes 10 seconds.

- This method excludes the laptop PS from the equation. In any case you have to use your own known good PS or first confirm the customer's PS is good.
- Using the lab PS you can play with the current limiter to avoid burning things if you suspect shorts in the laptop.
- With the Lab PS many other diagnostics procedures are possible such as injecting appropriate voltages directly behind the regulators or feeding battery's directly to confirm these are taking a charge.

Yes of course I have a bench power supply, and I do what you're speaking about all the time when I've got a board short and need to inject voltage.  That works wonderfully and of course I get a real time and very accurate reading of voltage drop and current draw.  The only thing it's missing is on many models of laptop, there is a data line in the adapter as well which ID's itself to the laptop.  You sound like you know PC repair so I'm sure you know some power management chips in certain model laptops expect to see specific power supplies/adapters.  This all comes from my disinterest in wiring current meters into all of our shop adapters as we have quite a great many.