beginner resistor question

Hello, I am trying to build an ihome, and I am having issues when it comes to finding the correct resistance to use. I am trying to charge the ipod touch 3g using the data+ and - connections. here is a quote from the table:

To charge an iPhone 3G / iPod Touch 2nd gen, usb data- (25) should be at 2.8v, usb data+(27) should be at 2.0v. This can be done with a few simple resistors: 33k to +5v (23) and 22k to gnd(16) to obtain 2v and 33k to +5v and 47k to gnd to obtain 2.8v. This is a notification to the iphone that it is connected to the external charger and may drain amps from the usb.

I am not understanding why I would need resistors at ground AND the power source, and I am having trouble understanding how all of this will be connected. Here is a quote that simplified the hooblah in the first quote:

Has anyone used one of these to charge an iPod Nano (6th gen)? Based on reference material for the MintyBoost, I was thinking 5V at pin 23, Ground at 16, then 2.8V (2.7, in my case, and in the case of my 10W Apple wall charger) on pin 25, and 2.0V on pin 27. When I attach the iPod, the voltage at pin 27 drops to 0V (0.01V). Thinking maybe the MintyBoost information was just wrong, I sacrificed a USB cable, plugged it into the Apple charger, measured the voltages, and confirmed what I was producing. From there, I ran the Apple charger voltage through the PodGizmo via my sacrificed USB cable, and get the same behavior. Can the traces on the breakout not provide the current needed to charge, or could my breakout be faulty? (Hope it’s not the faulty one, because I desoldered the connector, and was soldering wires to the pins when I broke off a pin and trashed it - I’d just like to know if I should bother ordering another one of these…). My project kind of depends on being able to charge the device…

Here is a picture of the podBreakout, I have a 3.3 volt connection using a logic converter if it would be easier to bump down the voltage from there, otherwise I have a 5v power source from an UNO r3:

So my main question is, how am I supposed to connect all of this so it charges the ipod, and how can I do it in an efficient way?

I am not understanding why I would need resistors at ground AND the power source, and I am having trouble understanding how all of this will be connected.

You need to make a [u]voltage divider[/u].

Ground is zero volts, so for example if you connect two equal value resistors in series between +5V and ground, there will be +2.5V at the junction between the two resistors.

Note that a voltage divider only works with a high impedance (high resistance) load. i.e. If you try to convert 12V to 5V with a voltage divider in order to power your IPod, the voltage would be less than calculated as soon as you connect the iPod..

Thank you, I have heard of voltage dividers before, but I don't think I fully understand how it would help me. I know I would have to use different resistor values to get to 2.8 and 2.0, but how would I find which will work? And where would I put the 2 volts, to actual pin (27)? Same for the 2.8 volts?

I found this video to help me understand voltage dividers more: http://afrotechmods.com/tutorials/2011/11/28/voltage-divider-tutorial/

A resistive voltage divider will (with a high impedance load) be carrying the same current (to a good approximation) in both upper and lower resistors, therefore the voltage ratio between the upper and lower is in direct proportion to the resistance ratio.

For instance:

+5V -- 2k --x---1k -- GND

will have the voltages in ratio 2:1, meaning x is at 1/3 of 5V (= 1.67V)

Or put another way

5 - Vx        2k
--------- =  -----
Vx - 0        1k

The rest is simple algebra.

Thanks, I'm understanding now. but is this with Vx being the output voltage? And will i be using the podbreakouts ground, or the ground from the arduino?

They should be the same ground surely? Anyway the USB ground is the one to choose for a USB device....

Learn more about the apple charging

learn.adafruit.com/minty-boost/icharging http://www.epanorama.net/blog/2010/08/18/apple-charger-secrets/

That last link was exactly what I needed, they had a schematic that showed what I needed I know. I just needed to know about USB connections more. Thanks for all of the help.

Another question, are voltage dividers required or just recommended?

needmorecowbell:
That last link was exactly what I needed, they had a schematic that showed what I needed I know. I just needed to know about USB connections more. Thanks for all of the help.

Another question, are voltage dividers required or just recommended?

In this application, charging a Apple device, it’s just a hardware hack needed to solve the hardware method that Apple used to make it harder to use ‘generic’ chargers, leaving their proprietary chargers at a marketing advantage.

In most all normal USB devices the D+ and D- signals are just used to pass USB messages back and forth between USB master and slave device and they require no voltage to be applied to these two signals.

Lefty

If you read the entire thread, I am asking how to use voltage dividers, or resistors in general, to help solve a problem about an iPod. The iPod is the situation needing fixing, resistors are the continuing theme of the posts. I'm sorry if its not relevant enough for this forum- if it needs closing or to be moved, feel free. Otherwise, I would like to continue to learn and better myself, using this forum as a vector.

EDIT: ignore this, a post must have been moderated.

Thanks lefty, that's what I was thinking. So I'm guessing I can send the right voltage message by any means necessary, I just need to send the right thing? Meaning division of voltage is optional?

End result is the data pins need to have correct voltage to signal the charge, you can use other way to generate the voltage but resistors voltage dividers is the cheaper.

retrolefty:
In this application, charging a Apple device, it’s just a hardware hack needed to solve the hardware method that Apple used to make it harder to use ‘generic’ chargers, leaving their proprietary chargers at a marketing advantage.

In most all normal USB devices the D+ and D- signals are just used to pass USB messages back and forth between USB master and slave device and they require no voltage to be applied to these two signals.

That would be an unfair judgement of Apple’s behavior. Not that Apple doesn’t like building proprietary chargers, but the data signals do come into play in the authoritative device charging spec. It’s a lot of reading, but: http://www.usb.org/developers/devclass_docs/BCv1.2_011912.zip

It’s worth noting that Apple was building chargers before there was a spec published (Dec, 2010).