Charging Two Vostro Laptops

Question

I have two Dell Vostro laptops - a 7500 and 5402 -- both purchased in the last 3 months (Dec '20 and Feb '21). I'd like to carry only one power brick when I travel.

The 7500 is labeled 19.5V and 4.62 A / 6.7 A. The 7500 has a charger/brick labeled 130W, 19.5V, and 6.7A, and positive polarity.

The 5402 is labeled 19.5V and 2.31 A / 3.34 A. The 5402 has a charger/brick labeled 45 W, 19.5 V, and 2.31 A, and negative polarity.

Both laptop chargers have identical connectors / tips.

Both ALSO have a Type-C port which the manuals (for both laptops) say can handle charging. In fact, I have a Dell docking station, with a Type-C port, which successfully charges both laptops.

If it weren't for the opposite polarity, I believe I could use the larger charger for both laptops. (As the 5402 would dictate the "pull" and only take what was needed -- 2.31 A or 3.34 A.) But alas, I don't think that's an option - due to the polarity issue. (I think.)

Questions, in order of priority:

1) Does polarity matter for the power connector? I'm relatively confident it does.

2) Does polarity matter for the Type-C port? Can I purchase a separate 19.5V / 6.7 A power brick with a Type-C connector / tip to charge both these laptops?

3) Less important: why the reversed polarity for two laptops within the same family? Consumers already need to put up with different connectors / tips. Is there some electrical engineering reason for this? (Or engineers just over-engineering, for good job security?)

4) Even less important, I also have a Dell Latitude 7490 (19.5 V / 4.62 A, different connector) (yeah, I know, lots of Dells) which charges successfully over the Type-C docking station connection. Do I have any hope of getting one charger for all three laptops and charge via the Type-C port? (Now I'm just being greedy.)

Thanks.

jphughan · Accepted Answer

@phil1313 The last time someone was concerned about polarity, they missed that Dell had changed the design of the polarity diagram in such a way that the two styles were conveying exactly the same information, just with a slightly different appearance. I would take a closer look at those diagrams. I suspect you will find that in both cases, the outside is negative and inside is positive. The question of which polarity appears on the left vs. right is irrelevant. The important question is which side of the diagram has the break in the outside ring and therefore which side relates to the inside polarity. THAT was swapped at some point on Dell's adapters.

Dell has used the same 7.4 mm barrel tip design since at least 2003, and every single Dell laptop I've seen that accepts that connector has worked with any functioning Dell power adapter that uses it, so long as the wattage is at least the amount the system is designed for. Using a higher wattage power source with a system that doesn't require it is completely fine. I've used a 130W Dell AC adapter with systems designed for 45W, 65W, 90W, and 130W for years. I've used new 130W Dell AC adapters with old systems, and old 130W adapters with new systems.

The only variation to the barrel style adapters has been the introduction of the 4.5 mm slim tip barrel for some thin and light systems. But even there, Dell makes a dongle to allow you to use a 7.4 mm power supply with a 4.5 mm connector. (There is no dongle to go the opposite direction.)

You definitely don't have any polarity concerns with USB-C. That connector is an industry standard. You can even use third party USB-C chargers with Dell laptops (and Dell USB-C chargers with third party products). However, some Dell laptops will only draw up to 65W from non-Dell USB-C sources, which can be a problem if the system is designed for 130W. That particular scenario is an additional complication because 130W is above the 100W max of the USB PD spec. Dell did something proprietary on some of their docks, systems, and a single AC adapter to push 130W over USB-C, but you won't find that implemented on third party products. (And annoyingly, Dell's own 130W USB-C charger doesn't support some common voltage levels used in other USB PD products, so it's not as useful as it could be for charging other third party products, otherwise it would have been a nice universal charger.)

For the reasons I just described, your only option for a single power source to use with all systems may be Dell's own 130W USB-C charger, assuming your Vostro will use it as a 130W source. Some earlier Dell 130W systems like early XPS 15s don't support Dell's own 130W USB-C charger even though they will pull 130W over USB-C from Dell docks -- no idea why. Dell also has some docks that can push 130W over USB-C, namely the WD19 180W (not the WD19 130W that only passes 90W through to the system) and the WD19TB if your systems have Thunderbolt and your setup would benefit from using it.

Alternatively, if all of your systems have a 7.4 mm barrel connector, then just use a Dell 130W AC adapter with a 7.4 mm barrel connector. But then you don't get the benefit of carrying a charger that could be used with a variety of other devices as would be the case with a USB-C charger.

ieee488 · Answer

Where are you getting your information about negative and positive polarity?

jphughan · Answer

@phil1313 Happy to help. Not sure what's up with those polarity diagrams then. Curious to see them if you feel like posting a picture of the diagrams on each barrel-style power adapter you're working with.

As for the dongle, here is the part I was referring to. Be aware though that I've seen this part go bad in such a way that when using the dongle, the power adapter will be deemed "unrecognized", even though the power adapter works fine with other systems and the the original system properly recognizes other power supplies that don't need the dongle. When a Dell system can't recognize a power adapter, it draws as little power as possible from it as a safety mechanism, which means no battery charging and pretty slow performance. Evidently the internal wiring of that dongle broke in some way that created that problem. Just letting you know so that you avoid potentially tossing a perfectly good power supply someday!

phil1313 · Answer

@ieee488I'm reading the polarity symbols off the charger bricks. There don't appear to be polarity symbols on the laptops themselves however.

@jphughanThanks for all your great information. Tons to work with there. As for the polarity symbols, I'm tracking the differences in the mirrored versions of the symbols. It didn't seem right to me that the two chargers would be reverse, so I double-checked them before posting last night, and I just checked them again. They aren't the standard symbols (mirrored or not), with two rings plus a center pin (rather than a ring and a center pin), but from what I can tell one has a negative center pin and the other has a positive center pin.

Regardless, I can tell from your response that you know this topic very well. Definitely appreciate your input. If you've used different chargers on different Dell laptops, then I'm confident of the road ahead.

The two Vostros must use the 4.5 mm barrel connector then, based upon what you said, and the Latitude uses the 7.4 mm. Haven't measured them, but those numbers look approximately right. I didn't know about the dongle/adapter, so that's good to know. If I'm reading your post correctly, I could use the Latitude 130W charger, with the dongle for the Vostros, and charge all three laptops that way. Or try the USB-C charger and see if that works. For the record, I have the WD19 180W docking station, so that should be pushing the full 130W to the Latitude and the Vostro 7500 then.

Again, thanks for all the information.

ieee488 · Answer

I would love to see the polarity diagrams that put @phil1313  in such a frenzy. I have swapped Dell chargers without an issue for 20 years.   .

phil1313 · Answer

@ieee488 @jphughan

Thanks, both, for such good information and fast responses. It's reassuring to know that you've both had success swapping Dell chargers.

I'll send some pics of polarity symbols later, and perhaps you can correct me on my interpretation. I always like learning new things.

And @jphughan, noted regarding dongle. Thanks!

jphughan · Answer

@phil1313  As I suspected, those diagrams have a different style but convey the exact same information.  On both diagrams, there is a solid line running from the positive indicator to the center of the diagram, indicating that the inside is positive and the outside is negative.  That said, I will admit that the 45W adapter's diagram is very odd.  It has a full ring on the outside of the 'C-shaped' ring, and the solid line from the positive side runs through that outer ring -- all of which makes the break in the C-shaped ring on the inside utterly pointless.  The design found on the 130W adapter where the C-shaped ring is on the outside and the break exists to allow a line to pass through it cleanly is much more common (and easier to understand).

phil1313 · Answer

@ieee488 @jphughan

Here are the two polarity symbols. Seeing them again, I'm second guessing myself.

From what I found online (and that's always true, right?): since the "opening" (making a C shape) on the inner ring of the 45W symbol faces the negative, the polarity of the center pin is negative in that case. If it were in fact positive, the line from the + sign would connect to the inner ring and then inner ring would be solid. This would indicate both the inner ring and the center pin are positive. But, I don't know for sure now.

45W Charger Brick

130W Charger Brick

phil1313 · Answer

@jphughanThanks for the info.  Also appreciate your timely responses and well-written posts.

Vostro

Charging Two Vostro Laptops

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