i know its more electronics but.

[shanky887614]

am i right in thinking to get 3v on d+ on a usb

i could use a voltage divider between pos,d+ and neg/ground

so basically Vout = Vin×(R2/(R1+R2))

bassically im just looking for clarification on the theory because eventually ill need say

+ 5v
D+ 2.8v
D- 2v
grnd

i havent taken an electronics course in my life so i was looking for your opinion's on this.


bassically long story short, if there is no voltage on the two data pins your device will only pull 500ma from the charger, by using resistors you can increase tthat value

 

[HappyHippyDad]

I've just reported your post Shanky as they were the only words on the page that I understood.

 

[shanky887614]

I've just reported your post Shanky as they were the only words on the page that I understood.

basically the outer pins on a usb stick are power and the middle two are data pins


i wanted some clarification if a voltage divider (bassically two resistors in series) would work to reduce the voltage on the data pins


https://learn.sparkfun.com/tutorials/voltage-dividers


this will explain about usb charging

https://learn.adafruit.com/minty-boost/icharging

 

[tazz]

As you stated, the outer voltage pins are 5v & gnd.....the data pins have a varied voltage compared to the data, out/in. So only the 5v & gnd can be used for supplying 500mA. Sourcing votage from data will damage host computer

 

[shanky887614]

As you stated, the outer voltage pins are 5v & gnd.....the data pins have a varied voltage compared to the data, out/in. So only the 5v & gnd can be used for supplying 500mA. Sourcing votage from data will damage host computer

not quite, bassically the standerd spec is 500ma output

your phone chargers etc support upto 2a output.

the voltage on the data pins tells your device what the supply is rated at

for example if i put 3v on d+ and 2 on d- it would tell an idevice it can pull 1amp or whatever


and this is for power from a dedicated supply feeding a usb port and it will be pulling current though +5 and grnd not the data

the data lines in this case will act like a 0-10 switch to tell the device how much current is safe to pull

 

[telectrix]

this makes more sense.:

Pink Floyd - "Brain Damage" / "Eclipse" - YouTube

 

[tazz]

Usb D+ and D- are data input output ports, this signal is varied for data, including voltage comparators, video, audio or any other universal serial data, hence the name. The only constant is the 5v and gnd

 

[tazz]

Now if you need 3v, then an option would be a voltage divider, or current limiter depending on if you need to charge or supply the equipment...but only using 5v & gnd

 

[shanky887614]

Now if you need 3v, then an option would be a voltage divider, or current limiter depending on if you need to charge or supply the equipment...but only using 5v & gnd

thanks taz

 

[HandySparks]

Now if you need 3v, then an option would be a voltage divider, or current limiter depending on if you need to charge or supply the equipment...but only using 5v & gnd

If you read shanky's second link in post 3, you'll understand that if you apply particular voltages to the data pins of some Apple gadgets, it will set the charging current that they draw from the 5V usb supply.

The same article also defines the resistors required for the voltage dividers, which answers the original question.

https://learn.adafruit.com/minty-boost/icharging

 

[shanky887614]

If you read shanky's second link in post 3, you'll understand that if you apply particular voltages to the data pins of some Apple gadgets, it will set the charging current that they draw from the 5V usb supply.

The same article also defines the resistors required for the voltage dividers, which answers the original question.

https://learn.adafruit.com/minty-boost/icharging

the thing is, it has been a standard to do this in the uk since 2012 and china etc since 08

Samsung and others use there own values

 

[tazz]

From this you can see, the data is being manipulated by basic voltage comparisons ...weather this is good or bad, is above me...but certainly would be careful with expensive phones or computers.

 

[shanky887614]

From this you can see, the data is being manipulated by basic voltage comparisons ...weather this is good or bad, is above me...but certainly would be careful with expensive phones or computers.

bassically i researched how it works and tested it using an extension cable from a samsung usb charger.

i checked first for resistance between the data lines and +/- and there was some resistance with it turned off.

anyway i next checked the D+ to N voltage this was 1.2v

D- to n 2was 1.2v.

next i tested between + to D+ this gave me 4v.

now i have already worked out i can use a 69.8k and 22k resistor to get close enough to 1.2v for it to work


im not taking my phone charger apart so i dont think ill find out the exact resistors they use

 

[Lucien Nunes]

I don't know the voltage / charge rate pairs but the principle sounds right. However, your resistor values sound very high. You need not only to get the ratio correct but the total value must be low enough that the divider passes sufficient current that whatever current the sense circuit in the phone pulls out of the tap, does not upset the voltage too much. The impedance at the tap is the parallel of the two limbs, 16.7k for your choices. If the data input has say 5kΩ (I don't know the real figures top of head) it will swamp your divider and throw the voltage out. Check for typical values and make your divider suitably stiff!

To the naysayers, it works like this: If the host device is a real host that can send and receive data, then that's what it uses the data pair for. But if it's only a dumb PSU, rather than having it send data to describe its capabilities, it falls back on an alternative use for the data pair to send simple voltage identifiers that the client device can decode.

Other things have used this system, e.g. in the good old days of video cassettes, the high-end mini-DV cassette had a memory chip built in that the recorder could use to store a shot log etc, which also served to identify the type of cassette to the recorder. Cheap cassettes didn't have the memory chip, so to identify the tape type they could be fitted with specific values of resistors across the same contacts that the recorder would also detect using analogue electronics instead of digital.