Parameters defining the charging rate

[Electroenthusiast]

I was charging my cellphone from a wall wart, and through the USB cable, and through a low rated wall wart. What i find was that the speed of charging was different with different wall warts, and one charging cable doesn't seem to charge so fast when i connect it to the PC.

On what does the charging time depend on? Does using a cheap cable increase the charging time? If so? How? Since voltmeter always show zero resistance.

 

[Nigel Goodwin]

Yes. But, i think all AC to DC convertors have a zener diode at the load side to regulate the output voltage, or to provide a DC voltage across it. I did check the circuitry about both, and found that both had diodes at the load side.

Please do explain it if that is not the case.

They certainly don't, and it would be VERY, VERY rare for a 'wall wart' type PSU to have such a zener.

An unregulated PSU is simply a mains transformer, a bridge rectifier, and a reservoir capacitor - that's it - nothing else.

A regulated one will be a lot more complicated, with a switch mode circuit, and the regulation done in the feedback loop, most commonly using a TL341 or similar.

 

[rjenkinsgb]

One common type of SMPSU feedback for voltage regulation uses an optocoupler with the LED side fed via a zener or voltage reference and a resistor.

That allows the opto isolator output to switch or swing between fully off and fully on at a fairly precise output voltage, controlling or modulating the "live" input side of the supply.

Example - there is a zener connected in the secondary side, it it does not directly control or limit the output, it just provides a voltage offset so the output is greater than the LED forward voltage.

 

[ChrisP58]

Well, i believe that the adapters use 7805 ICs to limit the output voltage. When you think of the normal old adapters, do they deliver constant voltage, or constant power? Because this would be important to know how they deliver current to the devices.

So, in that case, no matter the charging time (in old adapters), the power consumed will be constant for a constant power delivered adapter, regardless of the changeability of the electronic device.

Actually, most wallwart adapters us a TL431 at the output to sense the voltage and drive an optocoupler to control the primary side of the flyback converter to deliver the set voltage. Much more efficient.

 

[Electroenthusiast]

I see. But, why can't a zener diode be used in (place of) bridge rectifiers? I might have forgotten something from my school.

Still confused with the ratings on these adapters. Some say 5V 3A, while some other say 15W. Aren't both the same and be used interchangeably? Especially if they are not fast charging ones.

 

[rjenkinsgb]

5V 3A and 15W are equivalent ratings.


???
Zener diode act like a normal diode "forward" and break down at their rated voltage in reverse.

If the zener voltage was lower than the AC voltage peak, they would short the AC power and explode; if more than the highest ever peak voltage they would act no differently to the normal diodes.


Zeners are very inefficient as actual voltage regulators. They must be fed via a resistor of a value that supplies more than the highest ever load current, then the zener dissipates whatever power is not used by the load.

They are OK as simple reference voltage for an active circuit, as then the current is typically very low.


It's a bit like using a pressure relief valve to regulate a fluid system, and wasting everything the source provides that is not used by the load system.

In both cases, an active regulator of some sort that controls the current/flow to maintain the correct voltage/pressure at the load is far, far better.

 

[Electroenthusiast]

Is there any way in which we can understand whether a PSU is a regulated one or an unregulated one?

 

[Nigel Goodwin]

Is there any way in which we can understand whether a PSU is a regulated one or an unregulated one?

Measure the output voltage - or simply feel the weight, unregulated ones will almost always be traditional transformers, and far heavier.

 

[Electroenthusiast]

Interesting, Nigel. But, why wont they just add zener diode at the load side and make that regulated too. Just pondering.
So, that does mean that SMPS is always regulated. How?

 

[Nigel Goodwin]

Interesting, Nigel. But, why wont they just add zener diode at the load side and make that regulated too. Just pondering.
So, that does mean that SMPS is always regulated. How?

You can't just 'add a zener diode at the output', making then regulated would have considerably increased their price, and there was no requirement for them to be regulated. If you needed regulated supplies, there were available, but at higher cost.

SMPSU's are regulated by their feedback, it's what makes them efficient, small, and light.

 

[danadak]

Is there any way in which we can understand whether a PSU is a regulated one or an unregulated one?

If its unregulated then load changes = relatively large Vout changes. Regulated ones
would see small changes in Vout. Of course the amount of change is controlled by fdbk
loop dynamics/response/gain.

Also a unregulated supply typically has no minimum load requirements whereas a
regulated supply typically often has. There are regulated designs than dont have min
load requirements, all dependent on design architecture.

Wall warts come in a variety of flavors as far as internals.......

Another way of telling, look at output spectrum with a scope, FFT, with R load. Not
unusual to see clocking and SMPS reference spectrum (ramp, saw).


Regards, Dana.

 

[rjenkinsgb]

But, why wont they just add zener diode at the load side and make that regulated too. Just pondering.
So, that does mean that SMPS is always regulated. How?

The output IS regulated - it's the only part that is regulated!
See the example I posted in #62

The "Regulation" in a PSU or voltage regulator is set by wherever in the circuit there is feedback to the error amplifier, that controls it.

In the drawing in post #62, that feedback is from the output via the opto-isolator, to the control IC.


If a PSU just had a zener at the output, the PSU would permanently be running at full power, with the zener dumping it all as heat when nothing was connected, or dumping all the current the load was not using.

That would be an extremely wasteful and inefficient way of doing things; that's why I said that zeners are not often used for direct regulation at anything above trivial current levels.

Example:
Just for a 2A 5V USB power unit, it would need a zener at least this size:

https://uk.farnell.com/solid-state/1n2970b/diode-zener-10w-6-8v-do-4/dp/2101204

And a heatsink at least this big, in open air:

https://uk.farnell.com/fischer-elektronik/sk-81-50-sa/heat-sink-extruded/dp/1211718?st=5#c%20heatsink

to prevent it being so hot it you could not touch it without being burned instantly!

The type of zeners commonly used in electronic are rated at fractions of a watt - eg. 250mW or 400mW
Just twenty milliamps through a 12V zener would mean it was dissipating 240mW; that's still a lot of waste!
The currents are normally a fraction of what the could take, in real applications.

 

[Electroenthusiast]

Had been to some electronic dealers yesterday, checked for a wall wart with least ampere rating. All i could find was 2.1A to the least. I was told by the sales guy that i need not worry on overcharging, since the chargers (in phone) have the ability to cut off the PSU after fast charging.

 

[rjenkinsgb]

I was told by the sales guy that i need not worry on overcharging, since the chargers (in phone) have the ability to cut off the PSU after fast charging.

...Which you already been told numerous times on here! ...

"Overcharging" is not a problem - you would know if the battery was overcharged because the phone would have exploded or caught fire.

Holding the battery at or near full charge causes rapid ageing.
That is not overcharging, it's due to leaving it on charge for long periods after it has reached full charge, in a highly stressed state.

 

[Electroenthusiast]

I knew that from the previous posts, just wanted to let you all know on what i faced at the PSU dealer.

So, if it would cut off at 100%, then how would the phone always stay charged to 100% even after hours. Need to get into in-depth here.

 

[rjenkinsgb]

So, if it would cut off at 100%, then how would the phone always stay charged to 100% even after hours. Need to get into in-depth here.

Because it starts charging again if the battery is more than a certain amount below full charge. Either as soon as the voltage drops at all, or when it drops to eg. 98%

That's what I mean by it being held at full charge - it's never allowed to drop more than a tiny amount, as long as external power is connected.

Depending on the device, the display may never show the slight fluctuations - but a USB power meter in line with the cable should show the current fluctuating.

The harm caused by being held at full charge is why some phone makers now use a delayed charge system, if the phone is taken off charge at the same time every morning. They part charge immediately, then only charge again to 100% soon before the predicted disconnection time.

 

[Electroenthusiast]

Yes. This was similar mechanism used to control by the back emf of the Lithium Batteries. But, i feel that the advanced chargers nowadays don't rely on that anymore.

There were some chinese products which had timer in it to cut off the charging after a certain time period. I think that is not required nowadays since lot of the advanced ones have overcharging protection.

The phones nowadays do show the remaining charging time for full charge. That should help in finding the right setup and PSU for overnight charging.

 

[Roff]

I have a Drok USB digital multimeter. My old Samsung S7 phone will charge at 0.28 amps on my bedside clock charging port, 0.58 amps on a "standard"Samsung charger, and 1.62 amps on a Samsung adaptive fast charger. These measurements were taken with phone turned off.

 

[Electroenthusiast]

Roff Can you come back with the Ampere/Power ratings of all of those, or the make of those with online link?

 

[Electroenthusiast]

BTW, i found that from a friend that he was not having a fast charging PSU compatible cellular phone. So, he meant that his phone would explode if he uses a fast charging unit for it.

How does this work? How can these compatibility issues be dealt by the fast charging power supply manufacturers?

 

[Nigel Goodwin]

BTW, i found that from a friend that he was not having a fast charging PSU compatible cellular phone. So, he meant that his phone would explode if he uses a fast charging unit for it.

How does this work? How can these compatibility issues be dealt by the fast charging power supply manufacturers?

Look, this is the fourth page - and it's been explained endlessly to you - the PSU doesn't control the charging, the PHONE controls the charging, so there's no issues for the power supply manufacturer.