Spec says 5V output but actual output 9V (Phone Charger )!!!

[TechJunkie97]

Hi, I have an old Nokia phone charger rated 5V 350 mA however the output voltage is 9V with no load. The output voltage changes with load, for examples powering 2 LEDs changes output voltage to 7.8 V. I am pretty sure that it would become 5V under suitable load.
I was wondering whether it is a bad design. All they need to do was to add a voltage regulator. Perhaps it is complex than that.

[IanB]

Such chargers are usually designed to be combined with a circuit inside the phone which does the final regulation and battery charge control. So it doesn't matter to the phone that the charger behaves this way because phone and charger have been designed to work together as a system.

[BMack]

It's pretty common for chargers and external power supplies to work like this.

[amspire]

Hi, I have an old Nokia phone charger rated 5V 350 mA however the output voltage is 9V with no load. The output voltage changes with load, for examples powering 2 LEDs changes output voltage to 7.8 V. I am pretty sure that it would become 5V under suitable load.
I was wondering whether it is a bad design. All they need to do was to add a voltage regulator. Perhaps it is complex than that.

Not necessarily a bad design. If they had a charger IC that could accept , say, 5-12V, then they could get away with little regulation in the charger.

However if was the fact that every phone company was using different charger connectors and voltages that motivated the EU to legislate for all phones to use USB charging. Apple has been the main stand-out, but at least they have been forced to release a microUSB adapter. USB does require properly regulated 5V.

 

[TechJunkie97]

Interesting! I was thinking that it is better to put the regulating circuitry in the external adapter for freeing up some space inside the phone and for better heat dissipation. But modern smartphones are more constrained by costs than space and now I realize that it is safer to regulate inside the phone as it won't be a big problem if someone connects the wrong power supply.

[TechJunkie97]

evb149, I agree with you. I am very new to electronics and was getting familiar with diodes. So I was testing voltage drop of a LED with different currents and collecting data with Arduino and MATLAB. Now, I need use something else for power supply.

[IanB]

Strictly speaking you are only meant to plug such a charger into the device (phone) as intended. These things are not sold as general purpose power supplies, they are sold as specific purpose Nokia phone chargers (or whatever).

If you do anything other than plug it into a Nokia phone you are misusing it and you do so at your own risk. There are no grounds for complaint if you do otherwise.

[alm]

I have never really seen a AC-DC charging power adapter that in clear print specifies the MODEL of device it is intended to charge.  Rather they all look generic and if anything will only say "Nokia" and "5VDC" output and with some particular value of output current listed along with the usual safety agency logos and input voltage range.

I think the manual of the phone might have specified a specific charger. But I imagine that Nokia just had a common specification for that particular barrel connector, so any Nokia-branded charger with that connector would likely be compatible. And that specification might very well have had a very loose voltage spec (e.g. max 12 V open circuit, min 4.5 V full load). Without looking at the Nokia design documents, this is hard to tell. A phone charger without micro-USB connector is either very old or from Apple .

Actually it is also a big risk factor I have been troubled by for years -- many equipments come with GENERIC ACDC PSUs which are IN NO WAY labeled with the model or even the brand of equipment they are designed to power.  Only their rated output voltage sometimes, the current capacity sometimes, the polarity sometimes, and that is all the useful information.  Nor is the MODEL of the charger usually listed in the MANUAL of the associated product's documentation / or on its casing.

Agreed. And for bonus point, sometimes they screw up the polarity that is printed on the case. I was recently messing with an old SOHO network router that obviously no longer had its original power supply. The label stated center negative, but the sleeve was connected to the ground plane . No bridge rectifier so polarity does not matter either.

[Zero999]

It is pretty miserable though that they do not label the open circuit voltage rather than the "under load" voltage.
The way it is if you trust it to be a regulated supply (and who would think that an isolated SMPS based one isn't properly regulated) and you plug its output (circular plug or worse micro-USB!) into some other device that REQUIRES the supply be a peak +5.0 VDC you'll damage your device by plugging a "properly rated" +5VDC power supply into it!
Garbage!

Yes, not all switched mode power supplies are regulated. It's quite common for small, cheap switched modes to just be a blocking oscillator or CFL type circuit driving the primary side of the transformer, with no feedback. It simplifies design greatly.

One could also argue that phone manufactures should all design their phones to be resistant to this kind of abuse, especially the more expensive models.

[TechJunkie97]

Well, I conclude that they are pretty useless for general purposes. If you want to use a voltage regulator to step down the voltage to lower value, 5V in my case, it will work up to some mA then the actual voltage would drop to 5V, therefore, the regulator would cause the voltage to drop below 5V (due to forward voltage of regulator).

[Richard Crowley]

Well, I conclude that they are pretty useless for general purposes.

1. They weren't designed for "general purposes"
2. If you are "re-purposing" some piece of gear it is YOUR responsibility to evaluate it properly and apply it appropriately. 
3. You are ill-served by making unwarranted assumptions going in and then being disappointed.

[ejeffrey]

Interesting! I was thinking that it is better to put the regulating circuitry in the external adapter for freeing up some space inside the phone and for better heat dissipation. But modern smartphones are more constrained by costs than space and now I realize that it is safer to regulate inside the phone as it won't be a big problem if someone connects the wrong power supply.

Another factor is that you have voltage drop on the wires connecting the wall wart to the device.  So you still need a regulator at the load if you want an accurate voltage.  Also, if you are Nokia, you produce many products over several years, each with their own supply requirements.  You want to use the same style of power adapter for all of them.

Good regulation has *never* been a common feature of wall adapters.  Back in the days before switch mode converters, they were usually not regulated at all -- just a transformer, rectifier, and a little bit of filtering -- no regulation at all.  Most switch mode converters I have seen have some regulation -- if nothing else so that they can be universal voltage.  However, not all converters are universal or regulated, and those that are may have a minimum load current before they regulate.

[boffin]

Worth having a read of this thread: https://www.eevblog.com/forum/projects/how-smallest-form-240vac-to-9vdc-200ma/msg988115/#msg988115

One of the cellphone standards for charging over USB actually allows for raising of the voltage up to 9 volts

[BMack]

Interesting! I was thinking that it is better to put the regulating circuitry in the external adapter for freeing up some space inside the phone and for better heat dissipation. But modern smartphones are more constrained by costs than space and now I realize that it is safer to regulate inside the phone as it won't be a big problem if someone connects the wrong power supply.

Another factor is that you have voltage drop on the wires connecting the wall wart to the device.  So you still need a regulator at the load if you want an accurate voltage.  Also, if you are Nokia, you produce many products over several years, each with their own supply requirements.  You want to use the same style of power adapter for all of them.

Good regulation has *never* been a common feature of wall adapters.  Back in the days before switch mode converters, they were usually not regulated at all -- just a transformer, rectifier, and a little bit of filtering -- no regulation at all. Most switch mode converters I have seen have some regulation -- if nothing else so that they can be universal voltage.  However, not all converters are universal or regulated, and those that are may have a minimum load current before they regulate.

Yup, I've seen several wall adapters that are literally just a board, transformer, four diodes and two caps...with two wires going to the plug and two going to the wire with the connector. Off the top of my head, Motorola multi-channel two-way radios (Chargers) used by security and service guys in hospitals/universities/etc. All the regulation is in the dock base.

[Brumby]

There is a fundamental flaw in any discussion that attempts to put the responsibility for regulation on the plug pack ... it's just too risky.

I would find it FAR more comforting to know my product was designed and built to be tolerant of variations in charger supply voltage.  That means regulation in the device.

Regulation in the charger is just a bonus.

[Audioguru]

I am making an LED blinking circuit and found a little wall-wart listed at 12VDC/300mA which is perfect for CD4xxx Cmos ICs, right? No way! It produces 20VDC with no load (the Cmos ICs produce no load). When a bunch of LEDs turn on then the voltage drops to 18VDC and the current is 60mA so I will use a 78L12 (12V) voltage regulator for the Cmos ICs.

Most ordinary wall-warts that use a low frequency transformer do not have voltage regulation.

[Zero999]

I am making an LED blinking circuit and found a little wall-wart listed at 12VDC/300mA which is perfect for CD4xxx Cmos ICs, right? No way! It produces 20VDC with no load (the Cmos ICs produce no load). When a bunch of LEDs turn on then the voltage drops to 18VDC and the current is 60mA so I will use a 78L12 (12V) voltage regulator for the Cmos ICs.

Most ordinary wall-warts that use a low frequency transformer do not have voltage regulation.

What sort of wall wart is it? One with a low frequency transformer or an SMPS?

[Refrigerator]

What you have is an old style power brick. The output is not regulated but seems perfectly reasonable.
I have a couple bricks like that laying around.
There's basically a transformer, a bridge rectifier and a smoothing capacitor in it.
The output voltage sags under load because the transformer is small and has a high ESR.

[rdl]

I bought a couple of AC adapters, switch mode and supposedly regulated, from one of those internet based suppliers popular with the "maker" crowd. At only 80% of its rated current the output voltage was down nearly a volt from the 5 volts it promised.

Not long after that I ran across the Phihong PMS03 series. These are regulated and hold up well even when pushed to their rated maximum current. Noise is not too bad either. If you don't need a lot of power but want decent quality at a fair price, they might be worth a look.

http://www.mouser.com/Power/Power-Supplies/Plug-In-AC-Adapters/_/N-brvy9Zscv7?P=1z0spsy&Keyword=Phihong&FS=True

[Gyro]

What you have is an old style power brick. The output is not regulated but seems perfectly reasonable.
I have a couple bricks like that laying around.
There's basically a transformer, a bridge rectifier and a smoothing capacitor in it.
The output voltage sags under load because the transformer is small and has a high ESR.

It's worth keeping a few old bricks around for 'quiet' applications (sensitive analogue, Vrefs etc), where switching noise and Y-cap leakage could be a problem. I'm sure even the OP's brick would be ok with an 5V LDO regulator as long as he only pulls a couple of [Edit:hundred] mA.

Transformer bricks are getting much harder to find these days (although I did find some decent new UK 7.5VDC 1.5A ones on ebay the other day for £1.79 including postage!).

[Zero999]

What you have is an old style power brick. The output is not regulated but seems perfectly reasonable.
I have a couple bricks like that laying around.
There's basically a transformer, a bridge rectifier and a smoothing capacitor in it.
The output voltage sags under load because the transformer is small and has a high ESR.

It's worth keeping a few old bricks around for 'quiet' applications (sensitive analogue, Vrefs etc), where switching noise and Y-cap leakage could be a problem.

It should be fine if it's a transformer + rectifier, but it's quite likely the filter capacitor won't be large enough or it might be crappy, with a high ESR, so add an extra 16V 1000µF capacitor to the input off the regulator.

If the wall wart is switched mode, then this could be more problematic. The ripple rejection ratio of linear regulators tends to be poor at higher frequencies. An LRC filter at the input might be necessary to remove the high frequency content before the linear regulator.