[REQUEST] Should I test a laptop's PSU's amperage?

[Appletax]

Got a Dell Vostro 7620 laptop with an original Dell OEM AC adapter (130W, 19.5V, 6.7A).

My multimeter is an Innova 3310.

I just wanted to make sure that the adapter is working properly, so I tested the voltage and it output 19.7V. I got to thinking that perhaps I should also test the amperage. I looked up how to do this. I placed the positive red probe into the DC 10A jack and set the multimeter to DC 10A. I put the negative black probe on the outside of the tip and the positive red probe and the inside of the tip and it shut down the AC adapter. I unplugged the adapter and plugged it back in and it appears to be working fine. I ran the built-in UEFI diagnostics on the power supply and it passed.

Apparently, this caused a short-circuit or an overload (not sure which) and the power supply's built-in current limiter / foldback device/fuse prevented this from killing the power supply. It sounds like had the multimeter settings been at a higher amperage, it could have killed the PSU. "The PSU reduces its voltage to limit the short circuit current, while not cutting it off completely so that it can detect if the short is removed."

Should I even bother with testing DCA? How would I go about doing this properly? What did I do wrong?

Also, perhaps it would be nice to have alligator clips for the black probe so I can go hands-free with that probe. There's one at Harbor Freight for only $3. Not sure if it would work with larger PSU tips.

I see that Fluke makes alligator clip probes that the probe can go into. I do not see this type of product as being available from other manufacturers.

Edit:

 

[River Valley Computer]

To measure current you have to be 'in series' with either the positive lead or the negative lead. The way you tested it was a direct short thru the multimeter.

 

[River Valley Computer]

If you have the DC voltage e.g. 19.5 VDC it would be very rare that there not be current available.

 

[Larry Sabo]

If you have the DC voltage e.g. 19.5 VDC it would be very rare that there not be current available.

Agreed, but it does happen. I had such a case. 19.5v unloaded but couldn't produce enough current under load. Substituting a known good charger is best if there's any doubt about the original charger, IMO.

 

[HCHTech]

To measure current you have to be 'in series' with either the positive lead or the negative lead WHILE IT'S DOING WORK/POWERING THE LAPTOP. The way you tested it was a direct short thru the multimeter.

Just a clarification in bold, there. While this is possible, it's not simple. Ideally, there would be some kit available (with a bunch of different adapters to account for the various tip designs) that would give you simple places to get your multimeter probes on for an amperage test.

As far as I know, this kit doesn't exist (well, it probably does as a $5K or $10K piece of lab equipment, oscilloscope included!), so doing an amperage test just isn't going to be simple. PLUS, it's likely not good enough to do a single test to rule out problems. You would need to measure the amperage throughout a range of power draws to get a true picture of the power supply's performance -- or to be more precise, measure the amperage over time at each point on a range of power draws.

As a result, it's way easier to test with a known-good supply and just call the old one borked if the new one fixes the problem. My analytical brain really wants the full dataset before pronouncing that I've found the problem, too. That simple little question (can I test the amperage) is a really a huge ask.

 

[NviGate Systems]

Got a Dell Vostro 7620 laptop with an original Dell OEM AC adapter (130W, 19.5V, 6.7A).

My multimeter is an Innova 3310.

I just wanted to make sure that the adapter is working properly, so I tested the voltage and it output 19.7V. I got to thinking that perhaps I should also test the amperage. I looked up how to do this. I placed the positive red probe into the DC 10A jack and set the multimeter to DC 10A. I put the negative black probe on the outside of the tip and the positive red probe and the inside of the tip and it shut down the AC adapter. I unplugged the adapter and plugged it back in and it appears to be working fine. I ran the built-in UEFI diagnostics on the power supply and it passed.

Apparently, this caused a short-circuit or an overload (not sure which) and the power supply's built-in current limiter / foldback device/fuse prevented this from killing the power supply. It sounds like had the multimeter settings been at a higher amperage, it could have killed the PSU. "The PSU reduces its voltage to limit the short circuit current, while not cutting it off completely so that it can detect if the short is removed."

Should I even bother with testing DCA? How would I go about doing this properly? What did I do wrong?

Also, perhaps it would be nice to have alligator clips for the black probe so I can go hands-free with that probe. There's one at Harbor Freight for only $3. Not sure if it would work with larger PSU tips.

I see that Fluke makes alligator clip probes that the probe can go into. I do not see this type of product as being available from other manufacturers.

Edit:

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If I may, I would like to suggest keeping things simple. I too was once upon a time very interested in technology and always tried to do various projects, some of which worked, others ended up as disaster.

Your attempts at trying to diagnose something could have ended much differently. I say this out of concern and the hope that you can stay safe.

When technology isn't properly understood, it can be dangerous. Anytime you have a direct short like that it can cause issues, damage and more. Imagine trying that with a battery or mains ATX PSU!!

This is why we used to keep a decent quality generic AC Adapter and a few older but working used adapters for specific brands handy for just these scenarios. That way we could test with a known good one.

I gotta be honest in my days I nearly wore myself out at times until I found a simple routine to test things and not over complicate things.

You are doing break and fix, not engineering sample QA.

It's not bad to improve your skills but you need to be balanced so you aren't caught off balance and have the right tools for the right job.

 

[wavey]

The simple way to measure amperage is to use a clamp meter, I say this having never tried it, so...

Well I just looked this up and it doesn't work because you have both live and negative wires in the lead.

You can only measure 1 wire at a time so while the laptop may be taking 2.5a from the charger it is also sending back 2.5a, the meter will read 0a as the 2 wires will cancel each other out.
You would have to split the lead to access each wire.

 

[HCHTech]

I too was once upon a time very interested in technology and always tried to do various projects, some of which worked, others ended up as disaster.

When I was 12 or 13 (so, early 70s), my science teacher let me plan, source parts for an build what was essentially a bank of capacitors (helpfully donated by the local TV repair shop) which stored an insane amount of power, and allowed me to discharge it all at once through a single strand of copper wire. This instantly vaporized the wire in a little puff of purple smoke, and sounded like a 12-Gauge. It was the hit of the science fair and I could have died about 19 ways building and testing that contraption. I used a knife switch to detonate it, and I had to keep getting bigger and bigger ones because it kept welding them shut.

 

[NviGate Systems]

The simple way to measure amperage is to use a clamp meter, I say this having never tried it, so...

Well I just looked this up and it doesn't work because you have both live and negative wires in the lead.

You can only measure 1 wire at a time so while the laptop may be taking 2.5a from the charger it is also sending back 2.5a, the meter will read 0a as the 2 wires will cancel each other out.
You would have to split the lead to access each wire.

For most laptop charger that use a 3 pin or two pin AC cord, these can be easily split, and a cheap clamp meter will work, albeit on the AC side, but still better than nothing.

 

[wavey]

For most laptop charger that use a 3 pin or two pin AC cord, these can be easily split, and a cheap clamp meter will work, albeit on the AC side, but still better than nothing.

True but it's D/C going into the laptop.
Either way, if a genuine charger works without issue there's no problem.

 

[Philippe]

Just a simple idea: it should work using a cheap AC wattmeter. Plug the meter in the wall socket, plug the laptop AC adapter on the meter, turn on the laptop and read the power drawn...