Budgeting a Wallwart

It's down to the current rating of the transformer and other components.

USB outlet PSUs are only that: 5V power supplies, not technically "chargers" - that's a leftover name from early cellphones etc. that each needed a dedicated external charger, before USB charging was standardised.

With any USB-charged device, the actual charge control and battery monitoring & protection is in the phone / tablet / device itself.


The only [usually] guaranteed rating for a USB-A socket is 5V and 500mA, so most things are compatible with that.

Faster charging relies and a higher rated PSU and a combination of resistors or connections on the USB socket "data" pins, which are not used for data on a power-only unit, to indicate that the power source has a higher current rating.

Newer standards like USB-C have intelligent controllers and a data link in the cable, so the device can ask the power unit what it is capable of, voltage and current wise. USB "PD" (power delivery) devices can control the voltage on the main power output terminals to different levels rather than just 5V; 9 & 12V are common, 15 & 20V in some so they can charge such as laptops at up to 60W or even 100W out.

The higher voltage is only enabled if the connected device requests that specific voltage.

Thanks for the reply, rjenkinsgb

But, what does happen if the PSU is higher rated, but it wont rely on "Data" pins or a specific PD devices?
Think of a cheap make which doesn't have sophisticated controllers for it's USB socket. I mean, the impedance of the phone will be constant, but the current throught it will be different. Would this burn the phone (or any such connected device).

For basic USB A power units, 5V is 5V and that is it.

It's down to the connected device electronics and "intelligence" for smartphones to try different currents and see what works or causes the power supply voltage to drop too much.

There is nothing complex or involved with the data line connections - for a dedicated power unit with high current rating, the D+ & D- pins are just bridged together in the power unit. There is no communications, just a short or link between pins.

For any smart device such as a phone or tablet, charging is controlled by the CPU in the device, including selecting what rate to charge at and monitoring the power source to ensure it supports that.

A cheap cable between a high current PSU and a fast-charge phone will prevent fast charging, as the voltage will drop too low because of cable resistance when fast charge is tested, and the phone will then only use slow charge.

Electroenthusiast said:

I mean, the impedance of the phone will be constant, but the current throught it will be different. Would this burn the phone (or any such connected device).

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It's not really meaningful to talk about the impedance of the charging input of a phone.

The concept of impedance implies a linear system, where more voltage results in more current. A phone being charged isn't like that at all. The are systems to control the current, and what they control it to depends on a lot of things such as state of charge of the battery, temperature of the battery, temperature of the charger components, and what current the system thinks it's allowed to take from the supply.

Thanks both of you. So, does this also mean that charging the phone overnight will not hamper the device?
I had once seen a timer, which shuts down the power after a specific time, used while charging the phone overnight.

If the CPU in the phone technically controls the current through it, that does mean that there is no timers essential.

Can anyone also post the sample circuit diagrams used in the phone, and also in chargers.

The USB power unit ("charger") is a basic 5V output switched-mode PSU, there are dozens of variations in the internal designs.

This is one of the more highly integrated versions, which means the overall circuit is relatively simple.

Remember they are just 5V power sources with no "charge control" facility - that's why you can use any USB outlet and you do not need a dedicated device for each type of phone.


This is an example of a mobile phone battery controller - the IC also has many other power-related functions; see page 28 of the data for more info on the charge subsystem:


In general terms, leaving lithium cells held at full charge is not a good idea, it causes them to degrade far faster than if they average around half charge.

Some newer phones "learn" charging times if you always take them off charge at the same time of day, and will only charge to 80% to start with, then top up to 100% for the predicted time you will take it off charge.

What arrangements are used for surge protection, and line voltage leaks through the wallwart?
I mean, how will mobile be sure that the mains voltage is not feeding through the terminals?

Electroenthusiast said:

I mean, how will mobile be sure that the mains voltage is not feeding through the terminals?

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They are transformer coupled, with electronics driving the transformer.

Usually, if anything goes wrong, they just stop working. Nothing will come out of the transformer without an active drive signal, the same as with any isolated switch-mode PSU.

Didn't really understand. I was once using a cheap wallwart (mostly switch mode PSU) to supply power to my cordless phone. And, eventually after few days of continuous use, the wall wart started leaking the mains voltage through it.

And, the phone receiver got burnt, and i had to throw away the phone within few days.

So, rjenkinsgb . This seems to be complicated to understand. How will the controller circuit withstand mains voltage?
Any diagrams for this?

A figure of merit in power supply design is cost per watt. A 2 watt power supply uses more mass (in the transformer, switching device and output rectification and filtering than a 500 ma wall wart. Since the lower power wall warts have a price advantage compared to the higher powered wall warts the higher powered wall warts would have an exceedingly low profit margin if priced as a lower power wall wart.

Electroenthusiast said:

Didn't really understand. I was once using a cheap wallwart (mostly switch mode PSU) to supply power to my cordless phone. And, eventually after few days of continuous use, the wall wart started leaking the mains voltage through it.

And, the phone receiver got burnt, and i had to throw away the phone within few days.

So, rjenkinsgb . This seems to be complicated to understand. How will the controller circuit withstand mains voltage?
Any diagrams for this?

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It's meaningless - we can't answer when we've no idea what you did, or what you were using.

A 'proper' wall wart can't 'leak the mains through it' (although that's not really a 'thing' anyway) - did you have a big pool of 'mains' on the floor?, or why would you imagine mains 'leakage'?.

I can only imagine it was a cheap (and completely useless) Chinese wall wart, and was putting out too much voltage - and that was what damaged your phone. Presumably it wasn't a proper phone charger?, which I'd like to think would be better designed and better built.

Electroenthusiast said:

How will the controller circuit withstand mains voltage?
Any diagrams for this?

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Already given - see the "Topswitch" example in post #7

The only connection between the high and low voltage sides is via a high frequency transformer.

Dick, yes, i do understand that many of the consumer electronics dealers provide lower rated (usually 500mA) for their applications.

Nigel Goodwin I dont know what exactly happened then, i used a multimeter to check the voltage at the pins of the wall wart, and i found that to be reading mains at AC dial of multimeter.

rjenkinsgb Thanks. Will go through it in detail.

Electroenthusiast said:

Nigel Goodwin I dont know what exactly happened then, i used a multimeter to check the voltage at the pins of the wall wart, and i found that to be reading mains at AC dial of multimeter.

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Sorry, but that's pretty meaningless, digital multimeters often give completely spurious readings when used in 'unusual' ways.

Did you take it apart?, or just throw it away?.

rjenkinsgb said:

The only connection between the high and low voltage sides is via a high frequency transformer.

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And via noise-suppression caps. Non-hospital-grade power supplies will show ~½ line voltage leakage on their outputs

For The Popcorn said:

And via noise-suppression caps. Non-hospital-grade power supplies will show ~½ line voltage leakage on their outputs

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To ground, yes - but not across the output + / -, which is what the OP was concerned about, that could damage a phone being charged.

I agree, up to a point. There are several comments in this thread that flat-out that there cannot be "high voltage" on a wall wart output. This simply is not true and in some situations, it is possible to get a mild shock when touching a switching power supply output connector.

One area where this can be important is with USB-powered soldering irons. If plugged into a wall wart, the tip may be at ~½ line voltage.

For The Popcorn said:

I agree, up to a point. There are several comments in this thread that flat-out that there cannot be "high voltage" on a wall wart output.

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I think you've missed the point , the comments have referred to a 'high voltage that could damage a cell phone', not the tiny leakage current through the safety components - and also, referring to the output, not the output to earth.

For The Popcorn said:

This simply is not true and in some situations, it is possible to get a mild shock when touching a switching power supply output connector.

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Or with non-SM PSU's as well, it's not been SM which causes that minor effect, it's the safety components in a Class II device - which are often present in non-SM devices as well.

I support For The Popcorn 's views here. I have felt the same. The output voltage (of the faulty wall wart) was mains voltage, and i was also able to feel the tingle when i touched the pinout. To be frank, this also meant that there was an issue with the china made cheap wall wart.

I can't completely explain this because this was years ago. I don't know whether it is still there out there in between the spares or whether i threw it out.