Spec says 5V output but actual output 9V (Phone Charger )!!! - Page 1
Forum section: electronics / beginners

author: TechJunkie97
author_rank: Contributor
timestamp: July 17, 2017, 01:15:58 am
content: 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.

author: IanB
author_rank: Super Contributor
timestamp: July 17, 2017, 01:20:06 am
content: 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.

author: BMack
author_rank: Regular Contributor
timestamp: July 17, 2017, 01:27:46 am
content: It's pretty common for chargers and external power supplies to work like this.

author: amspire
author_rank: Super Contributor
timestamp: July 17, 2017, 01:34:30 am
content: Quote from: jaskiratubhi on July 17, 2017, 01:15:58 amNot 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.

author: TechJunkie97
author_rank: Contributor
timestamp: July 17, 2017, 01:36:35 am
content: 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.

author: TechJunkie97
author_rank: Contributor
timestamp: July 17, 2017, 01:42:52 am
content: 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.

author: IanB
author_rank: Super Contributor
timestamp: July 17, 2017, 01:54:45 am
content: 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.

author: alm
author_rank: Super Contributor
timestamp: July 17, 2017, 02:20:45 am
content: Quote from: evb149 on July 17, 2017, 02:11:24 amI 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.Quote from: evb149 on July 17, 2017, 02:11:24 amAgreed. 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.

author: Zero999
author_rank: Super Contributor
timestamp: July 17, 2017, 07:47:11 am
content: Quote from: evb149 on July 17, 2017, 01:32:49 amYes, 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.

author: TechJunkie97
author_rank: Contributor
timestamp: July 18, 2017, 02:13:46 am
content: 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).

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 18, 2017, 03:08:08 am
content: Quote from: jaskiratubhi on July 18, 2017, 02:13:46 am1. 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.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 18, 2017, 03:41:22 am
content: Quote from: jaskiratubhi on July 17, 2017, 01:36:35 amAnother 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.

author: boffin
author_rank: Unknown
timestamp: July 18, 2017, 03:53:23 am
content: Worth having a read of this thread:https://www.eevblog.com/forum/projects/how-smallest-form-240vac-to-9vdc-200ma/msg988115/#msg988115One of the cellphone standards for charging over USB actually allows for raising of the voltage up to 9 volts

author: BMack
author_rank: Regular Contributor
timestamp: July 19, 2017, 06:11:08 am
content: Quote from: ejeffrey on July 18, 2017, 03:41:22 amYup, 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.

author: Brumby
author_rank: Unknown
timestamp: July 19, 2017, 01:36:18 pm
content: 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.

author: Audioguru
author_rank: Super Contributor
timestamp: July 19, 2017, 01:42:11 pm
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: July 19, 2017, 02:22:55 pm
content: Quote from: Audioguru on July 19, 2017, 01:42:11 pmWhat sort of wall wart is it? One with a low frequency transformer or an SMPS?

author: Refrigerator
author_rank: Super Contributor
timestamp: July 19, 2017, 05:05:41 pm
content: 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.

author: rdl
author_rank: Super Contributor
timestamp: July 19, 2017, 05:58:26 pm
content: 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

author: Gyro
author_rank: Super Contributor
timestamp: July 19, 2017, 06:17:35 pm
content: Quote from: Refrigerator on July 19, 2017, 05:05:41 pmIt'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!).

author: Zero999
author_rank: Super Contributor
timestamp: July 20, 2017, 08:12:08 am
content: Quote from: Gyro on July 19, 2017, 06:17:35 pmIt 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.