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HVPaul

Fri May 28 2010, 07:22PM

Registered Member #2321 Joined: Fri Aug 28 2009, 05:13PM
Location: Boston, MA
Posts: 34

I had a chance to take apart a cheap 3.6 volt Nokia charger and was pretty surprised by what I found.

Primary winding is very skimpy. It's connected to the mains via a single capacitor to drop the current.

The secondary, I kid you not, is a simple bridge rectifier, that's IT!
I imagine this kind of design is common in inexpensive consumer electronics.

The international 110-220 version of a similar Nokia adapter showed more sophistication.

I'd imagine it's not the best design to cut the current to primary with just a single cap. Or is it okay to weather most storms?

mikeselectricstuff

Fri May 28 2010, 09:40PM

Registered Member #311 Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253

As long as it's suitably fused I can't see a problem...

Conundrum

Fri May 28 2010, 10:23PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

eeeek...

The problem is, if that capacitor fails the adaptor and attached phone is going to fry and possibly ignite the battery if left long enough.

EDIT:- does not apply if it is X2 rated, however with the recent rash of counterfeit parts I have my doubts if any markings on imported parts are genuine.

I've taken apart a few failed LED lights of the bi-pin variety and these use a single capacitor in series with the mains (not even X2 rated!) to limit the current so this seems to be a common technique.

klugesmith

Fri May 28 2010, 11:43PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Is the capacitor covered with national safety-agency symbols, and does it have an obvious X or Y or XY rating label?

Most consumer electronics have mains-connected capacitors (usually for EMI control), whose failure could cause fire and/or electric shock. Safety approval for the product requires that the capacitors meet very conservative standards to earn the X, Y, or XY rating. I bet there are plenty of counterfeits, hard to trace after an offbrand cellphone charger burns down somebody's house.

Here is an example where nominal 250 volt capacitor with X2 rating is operated at 1700V in a commercial product:

radiotech

Sat May 29 2010, 02:47AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Before transistor radios small sets using tubes had an Expensive B battery and a cheap flashlight cell to power anode/filament. Imported B Battery eliminators hasd a selanium rectifier, electrolytic filter and a paper line series capacitor to drop voltage. Still later,
capacitors were used as filament voltage dropping devices in very small portable tube TV sets. Using capacitors to drop AC voltage keeps heat down.

HVPaul

Thu Jun 03 2010, 06:23PM

Registered Member #2321 Joined: Fri Aug 28 2009, 05:13PM
Location: Boston, MA
Posts: 34

mikeselectricstuff wrote ...

As long as it's suitably fused I can't see a problem...

Nope, no fuse at all :) If that cap fails the appliance is gone.

I'm a bit worried that iPhone wall charger uses similar technique considering the size of the thing.

radiotech

Thu Jun 03 2010, 08:30PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

IF you are worried do the following:

Connect a resistor across the output and adjust the resistor to produce the exact AC voltage that the phone battery has (in DC volts). Next calculate how many mA AC is flowing. Now short out the output wires by connecting an AC mA meter across them. Slowly increase the line input voltage with a variac and find out what AC voltage causes the calculated current to flow.

Lets say the capacitor is 10 ufd. at 120 volts 60 Hz approx 270 mA AC flows short circuited. A 3.6 volt cellphone battery that is being charged at 270 mA. has a rectified load of 13 ohms. the capacitor has a reactance of 270 ohms. Pythagorus tells us that with the battery shorted out 270 mA still flows

Conundrum

Fri Jun 04 2010, 06:00PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

The Iphone charger is probably a very small SMPS..

however the latest technology is piezoelectric transformers

supposedly with these transformers you could at last integrate the charger into your notebook PC, no unwieldy brick required

the cool part is that the Transoner is failsafe, even if it cracks due to impact etc it just stops working, Overheating these just causes the output to shut down but this self corrects when the overload or whatever is removed.

-A

radiotech

Fri Jun 04 2010, 06:54PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

" the Transoner is failsafe, even if it cracks due to impact etc it just stops working, Overheating these just causes the output to shut down but this self corrects"

Gee we've come a long way. Seems piezo used in radio transmitter
crystals, especially the singe tube kind could break the crystal if it was zapped by a snivet. And Piezo tweeters: We had those blow all the time even though Motorola swore they were foolproof.

Failsafe in a computer is very good though; those things sometimes, rarely, give trouble.

Steve Conner

Sat Jun 05 2010, 11:22AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Conundrum wrote ...

supposedly with these transformers you could at last integrate the charger into your notebook PC, no unwieldy brick required

Compaq did that back in the 90s! They had a laptop with a figure-of-8 mains inlet on the back. OK, it wasn't particularly slim or sexy, but no laptop was back then.

Mains circuitry can only be miniaturized so far, because of creepage and clearance requirements mandated by law, and the size of the EMI filtering components, again mandated by law.

As far as I know, Nokia phones use a nominally 3.6V unregulated supply for charging. The phone has charge controller circuitry inside. If you plug some crazy voltage into it, it'll display a "Not charging" warning message. I've had Nokias that came with an ordinary transformer-based wall cube, as well as a switchmode one, which could indeed be very crude, because the regulation doesn't have to be good.

The Iphone wall charger has a USB outlet on it, doesn't it? So it'll probably be 5V DC, regulated. That suggests it ought to be a proper SMPS.

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