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Old 07-02-2020, 04:53 AM   #1
Dannyx
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Unhappy Handheld vacuum cleaner kills power adapters (With schematic)

Good day folks. A friend of mine asked me to take a look at this handheld/upright vacuum cleaner which supposedly has power adapter issues and would no longer charge. Upon first inspection, its original 14v/400mA adapter was dead - the thermal fuse in the transformer's primary windings was open. I went out of my way to replace it and indeed brought the adapter back to life. I gave it back, it worked fine for a few days, then the chap calls me again and tells me the vac no longer charges again......

Ok, so this time I asked him to bring in the motor part as well since I thought something must be drawing excess current or is shorted and that causes the adapter to overheat and blow the thermal fuse, which was open again, by the way. I opened up the vac today and here's what it's inside....pretty basic, not that I expected more, but nothing looks out of the ordinary either. The battery pack was replaced at one point after it would no longer hold a charge, as it happens with NiMhs, and indeed it works now, but it doesn't charge back for some reason.

I doodled a schematic myself just out of curiosity and to better understand how it works and how it's supposed to charge: when you close the power switch on the handle, current from the battery goes straight to the motor - easy enough. When the charger is plugged in, current starts flowing through R2 and D1 to the battery. When discharged, the battery's resistance to GND will be low, so LED1 also comes on...I THINK. Not sure what happens with Q1 and those zenners on the right there...looks like some sort of regulator, but I can't understand how it works....

Another thing I don't get is SW2+ and SW2- points: there's a set of 4 metal contacts on the bottom of the motor part - these mate up with a set of contacts on the floor adapter/dock. Two send power to the roller brushes and two connect the second power switch on the top of the handle, though this last part I'm only speculating, since I don't have the dock part too, so I don't know if there's additional components in it too, though I doubt it. I imagine when the base of Q2 is pulled low, Q2 conducts (since it's PNP A1270) and closes the relay. To do this, I was expecting the switch on the top handle to simply close a path to GND across those two metal plates, but SW2+ goes to the adapter input...what gives ? :|

What I'll try is to drain the battery and charge it up with my bench supply in place of the power adapter to see what the current draw is. Could've just been a bad adapter, BUT here's the catch: the owner also tried it with a 12v SMPS adapter while I was fixing the original one, which although didn't match the voltage precisely, was good enough for a test and I think what happened was the vac didn't charge and also shut down that one too at some point, pointing to overcurrent. It only worked after I patched up the original one for a little while, until it died again....

Any help with this issue is appreciated, as always. Cheers guys. Thanks
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Old 07-02-2020, 09:06 AM   #2
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Default Re: Handheld vacuum cleaner kills power adapters (With schematic)

I've seen a bunch of power bricks for charging that are specially tailored for the device. Swapping batteries for higher capacity units with lower internal impedance could have done it in.

The lithium ion rechargeable vacuum I have - the wall wart must be current limited. If not, you could destroy everything...

So if you patch up the original one, try to sneak in a resistor in series with its output, it'll make it take longer to charge and waste energy but hopefully no call backs? Heh...

Another thing I learned from poorly designed electronics: never use the device when plugged into the charger. Some devices automatically disable itself when plugged in. Some just leak a little magic smoke each time you do it and eventually all the magic smoke leaks out...

btw where's schematic?

and you seem to be working on a lot of rechargeable vacuum cleaners???

Last edited by eccerr0r; 07-02-2020 at 09:08 AM..
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Old 07-02-2020, 09:18 AM   #3
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Default Re: Handheld vacuum cleaner kills power adapters (With schematic)

so where's the schematic?

maybe the guy tries to use it while charging?
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Old 07-02-2020, 10:08 AM   #4
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Red face Re: Handheld vacuum cleaner kills power adapters (With schematic)

Quote:
Originally Posted by stj View Post
so where's the schematic?
Quote:
Originally Posted by eccerr0r View Post
btw where's schematic?
Yeah, epic fail....I think what happened is I had the upload window open and loaded up the pics in the "slots" but forgot to click upload

I took the board and battery with me home today so I can provide some shots of the board at least - I took some at work too, along with the schematic, where I wrote the post, but they got lost along the way like I said. The reason I took it home is because I wanted to discharge the battery some and then charge it up on my bench supply....don't have one of those at work - I know...professionalism at its finest >_> The schematic shall be posted tomorrow. The vac is a Zepter LMG-310 BTW.

Power supply was set at 14v and no current limit: red LED comes on and the whole thing draws around 300-350mA. After around 10 minutes, it dropped down to 160mA. Voltage on the battery terminals was 12.4v (still charging). Q1 and R1 get warm to the touch. 12v on the input is not enough, which explains why the brick he tried using didn't work - 0 current on the supply at 12v. 15v goes a bit too high for my taste, at 500mA. LED turned off after around 30 minutes, with a current draw of 140mA and 12.1v on the pack.

Quote:
Originally Posted by eccerr0r View Post
I've seen a bunch of power bricks for charging that are specially tailored for the device. Swapping batteries for higher capacity units with lower internal impedance could have done it in.
Yes, I encountered something close to this on a Braun epilator back at my old shop: its original charger was dead and since it was a bitch to crack open, being all waterproof and sh!t, my colleague and I decided the easiest and cheapest option was to replace it with a similar one. I think it was 6v and 400mA, so I dug through our box of AC adapters and found one that was 6v at 1A or something like that, which I thought would be perfect. Before testing it, I wanted to check the epilator itself just to make sure it can still charge, so I used our bench supply to hit it with 6v and the current shot WAY up before I even hit the 6v mark, so a 1A charger would've definitely killed it - there was definitely some current limiting going on inside the brick itself. Not the case here, since the wall-wart is a simple transformer.

Quote:
Originally Posted by eccerr0r View Post
Another thing I learned from poorly designed electronics: never use the device when plugged into the charger. Some devices automatically disable itself when plugged in. Some just leak a little magic smoke each time you do it and eventually all the magic smoke leaks out...
Quote:
Originally Posted by stj View Post
maybe the guy tries to use it while charging?
No - you can't: the power switch itself toggles the battery between the charger and the motor, as you'll see in the schematic when I get around to uploading it tomorrow.

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and you seem to be working on a lot of rechargeable vacuum cleaners???
Not exactly. You probably refer to my old thread about one of our super-skilled guys here replacing cells inside them with with LiIons directly, but me personally haven't touched a vacuum like this till now - this is just a favor for a friend, not a ticket for the joint. That's his job
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Old 07-02-2020, 10:35 AM   #5
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Default Re: Handheld vacuum cleaner kills power adapters (With schematic)

Some people run the batteries totally dead - to the point some cells in the pack get reverse voltage across them. There might be weak cells in the pack. Those cells can act like a short and so the initial charging current is very high.
I don't see a fix here, even adding a limiting resistor just makes a ton of heat.
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Old 07-02-2020, 11:00 AM   #6
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Default Re: Handheld vacuum cleaner kills power adapters (With schematic)

The date code on the pack would suggest it's from 2015. I'm not sure how you'd check for dead cells - would the faulty one have less voltage on it ?
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Old 07-02-2020, 12:13 PM   #7
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Default Re: Handheld vacuum cleaner kills power adapters (With schematic)

cut the shrink off the pack and look for leaks and differing cell voltages.
maybe even test each cell seperatly
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Old 07-02-2020, 12:52 PM   #8
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Default Re: Handheld vacuum cleaner kills power adapters (With schematic)

I was even considering doing a (proper) Li-ion upgrade, provided the owner agrees....I was thinking a 3s2p pack....maybe ? Not sure how many amps that motor draws on startup and when running freely...
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Old 07-03-2020, 04:22 AM   #9
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Red face Re: Handheld vacuum cleaner kills power adapters (With schematic)

Here's the schematic at last.

I connected the motor today and it does run.....for a while. The pack voltage quickly drops to around 4-5v but it still spins rather nicely. It seems to charge fine even with a 15v brick I had laying around the shop, since I don't have a bench supply like I said, but I'm not sure if this run time is considered acceptable or satisfactory to the end user, hence why I was thinking of upgrading it to LiIon which they were happy to hear about, apparently. 15v makes Q1 and R1 even hotter, of course, since they appear to do some sort of regulation.
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Old 07-03-2020, 04:28 AM   #10
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Default Re: Handheld vacuum cleaner kills power adapters (With schematic)

what is the run-current if you block airflow?
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Old 07-03-2020, 04:32 AM   #11
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Default Re: Handheld vacuum cleaner kills power adapters (With schematic)

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what is the run-current if you block airflow?
No idea...haven't measured it and it's got connectors with plugs on them, so I'd have to really go out of my way to get my meter in there right now (I have a clamp meter at home) though you're right: I need to know that if I want to switch to Li-Ion...size is also a factor. The enclosure in the back can hold approx 9x6cm worth of stuff....

There is one thing I noticed: when I cup my hand over the nozzle and the thing starts to struggle a bit, the voltage across the pack itself actually increases a bit, by around 1v, which is unusual - I was expecting it to draw even MORE power...

One of the cells in the pack does get ever so slightly warmer than the rest after conducting this experiment.

Last edited by Dannyx; 07-03-2020 at 04:38 AM..
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Old 07-03-2020, 07:12 AM   #12
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Default Re: Handheld vacuum cleaner kills power adapters (With schematic)

maybe one cell has a high resistance and shorts under load?
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Old 07-03-2020, 07:32 AM   #13
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Default Re: Handheld vacuum cleaner kills power adapters (With schematic)

Found a russian forum post with a battery pack from a Zepter Vac - is your enclosure the same as in the pictures http://www.magicnet.ee/ru/forum/obsh...vuiu-tekhniku/ post #8 and 12

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Old 07-03-2020, 07:33 AM   #14
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Default Re: Handheld vacuum cleaner kills power adapters (With schematic)

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Found a russian forum post with a battery pack from a Zepter Vac - is your enclosure the same as in the pictures http://www.magicnet.ee/ru/forum/obsh...vuiu-tekhniku/ post #8 and 12
Yes, the vacuum itself is identical - LMG-310.
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Old 07-03-2020, 02:46 PM   #15
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Default Re: Handheld vacuum cleaner kills power adapters (With schematic)

I just realised I don't know something about the operation of BMS and LiIons: is a (cheap) BMS taking care of the charge process as well ? Say I just get a 3s4p pack off Ali, do I have to worry about how I charge it ? Sure, a pre-made pack already has a DC barrel plug attached to its BMS inside the shrink-wrap, but some sellers suggest a CC adapter is required and suggest selling one to you, of course. I see a fat current sense resistor on all BMSs, no matter how cheap, so I reckon it does SOME sort of current control.

I was hoping I could get away with charging it with a trash-picked 12v adapter everybody is likely to have around.....trying to give these chaps a relatively cheap solution by keeping parts cost relatively low while still providing decent results.

A colleague of mine did this to his cordless drill which had dead NiMhs in its pack and he used a "standard" 12v 5a power brick to charge it and from what I gather, it still works fine to this day......

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Old 07-03-2020, 04:07 PM   #16
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Default Re: Handheld vacuum cleaner kills power adapters (With schematic)

Fairly easy to check for shorted cells in a pack:
1. Charge battery as much as possible with a constant current charger.
2. disconnect and rest 5 minutes and measure voltage
3. If voltage is significantly less than # cells * ~ 1.4 volts, you probably have a shorted cell...

For your 9 cell pack, if it measures less than about 12+ volts freshly charged, you probably have a shorted cell somewhere.
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Old 07-03-2020, 05:31 PM   #17
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Default Re: Handheld vacuum cleaner kills power adapters (With schematic)

Since the cells are connected in series, you can check the Voltage drops of each cell while charging, that will give a clue as to which one is bad.
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Old 07-04-2020, 02:26 AM   #18
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Thumbs up Re: Handheld vacuum cleaner kills power adapters (With schematic)

Quote:
Originally Posted by eccerr0r View Post
1. Charge battery as much as possible with a constant current charger.
I guess a bench supply would be adequate for this. That is, connect it to the pack directly, not via the vac's board.

Quote:
Originally Posted by eccerr0r View Post
For your 9 cell pack, if it measures less than about 12+ volts freshly charged, you probably have a shorted cell somewhere.
I think it tops out at around 12.1v or thereabouts, so maybe it's good after all and that's just the normal run time.
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Old 07-04-2020, 03:20 AM   #19
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Default Re: Handheld vacuum cleaner kills power adapters (With schematic)

most "BMS" boards are just charge protection, they wont keep the cells balanced.
(so you cant throw some random shite-cells together!)
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