Mintyboost resistor values.

[jackjack]

I just had two questions regarding the resistors required for Apple compatibility.

If 2v is required for the D+ and D- pins for 500mA charging, and the way to alter voltage at those pins with resistors can be calculated by V(out) = V(in) * (r1/(r1+r2)), then doing some algebra, we get 2=5 (r1/r1+r2), or 2/5 = r1/r1+r2. Solving, we essentially get r1=2 and r2=3, in essence, r2 = 1.5x r1.

My question is:
1. If r2 is 75k, then shouldn't r1 be 50k and not 49.9k?
2. Does that mean theoretically, we can use a 15k r2 and 10k r1?

Just for interest, I did get a 50k resistor (which were much easier to find than 49.9k) to try it out and can confirm that I could get an iPhone 4 charging.

[adafruit_support_bill]

For 1% tolerance resistors, the closest standard values are 49.4 and 51.1.
The disadvantage of using lower values such as 15K & 10K would be a higher no-load current.

[jackjack]

Thanks for the info. I'm a novice with electronics and didn't even know there was such a thing as standard resistor values.

Regarding alternate resistor values with the same ratio, if lower resistance leads to higher no-load current, then using higher resistance would further reduce any no-load current?

Or is 75k+49.9k the limit and any higher would result in 0v on the D+/- pins?

Thanks.

[adafruit_support_bill]

A higher resistance would lower the no-load current. But, since we don't know what is inside the phone, we don't know how the higher resistance will affect the sensing circuits. It might work. You are unlikely to damage anything experimenting with higher values. The values for the resistors in the kit were determined by tearing apart an official Apple charger.

[soccerdude71]

I just had two questions regarding the resistors required for Apple compatibility.

If 2v is required for the D+ and D- pins for 500mA charging, and the way to alter voltage at those pins with resistors can be calculated by V(out) = V(in) * (r1/(r1+r2)), then doing some algebra, we get 2=5 (r1/r1+r2), or 2/5 = r1/r1+r2. Solving, we essentially get r1=2 and r2=3, in essence, r2 = 1.5x r1.

My question is:
1. If r2 is 75k, then shouldn't r1 be 50k and not 49.9k?
2. Does that mean theoretically, we can use a 15k r2 and 10k r1?

Just for interest, I did get a 50k resistor (which were much easier to find than 49.9k) to try it out and can confirm that I could get an iPhone 4 charging.

Jackjack,

Are you using yours with an iphone 4? If so, Have you had any luck?


G

[jackjack]

Hello G,

Yes, I am using my mintyboost with an iPhone 4. I have tried using 50k 1% resistors as well as the 49.9k 1% provided with the kit. Both works.

I switched to a 4AA holder though and with 4x Enerloop batteries, I get 1.3x full iPhone 4 charges.

For those interested, prior to building the mintyboost, I bought a Tekkeon mp1550. Using the exact same batteries (4xEnerloop), the Tekkeon can only charge the iPhone 4 to 70%. It's also a lot warmer.

[soccerdude71]

Jackjack,


i have tried this and I am still not able to get my iphone4 to charge. I bought a new MB kit and still the same. I tried 4 different iphone 4s and same banned. What happens is when I plug in the phone, it flashes charging for a sec, then changes to just plugged in status. If i try unplugging it and plugging it repeatedly, I can get it to start charging, but it only does it for a minute or so... Very odd... any ideas??

[adafruit_support_bill]

@soccerdude - What happens to the output voltage when you connect the phone? If the output voltage drops, so will the signal voltages on the data lines.

[soccerdude71]

A_S,

Without the phone plugged in: 4.96v

With it plugged in: 4.95v

(Those values were measured on the USB output pins 1 and 4)

Data lines w/out load:

pin 2: 1.9v

Pin 3: 1.97v

with phone plugged in:

Both data lines:

pin 2: 1.76v

Pin 3: 1.82v

[adafruit_support_bill]

Both data lines:

pin 2: 1.76v

Pin 3: 1.82v

I think that's the problem. The phone see's them drop significantly below 2v, so it stops charging.

But these voltages are derived by a simple voltage divider from the main output voltage, so that must be dropping too. This is usually due to weak batteries, but sometimes can be caused by a phone that tries to pull more than 500mA from the charger.

[soccerdude71]

Well, I can see how that may be the problem. However, the phone pulling to much current seems unlikely, since I tried 4 of them. What might be a good resistor to put in R1 and R3 to bring the voltage high enough to compensate?

In regards to the batteries, I have a 3.7v 6600mAh and I tried it with a 1300mAh as well with the same results. Both batteries are new and fully functional.


G

[adafruit_support_bill]

Using the cells you have, you should have no trouble supplying enough current. If I understand correctly, this is your second MintyBoost?

[soccerdude71]

yes, this is my second MB.

[adafruit_support_bill]

What is your input voltage when the data line voltage drops?

[jackjack]

Hey Soccerdude,

Sorry to hear that your MB somehow isn't working out for you. I know the situation is a bit different since you seem to be using a nice juicy Lipo to power your MB, but just to put the info out there, as I had mentioned previously, I'm using 4xEnerloop AA Metal Hydride rechargables. They are supposedly 2000mAh each.

These are measurements I'm getting on mine (Fluke 115):
Each cell is providing 1.281v.
Measurement between Pins 1 & 4 at USB = 4.988v
Measurement between Pins 1 & 2 or 3 at USB = 1.990v, 1.993v

Could you try to solder the original 2 cell battery holder and pop two *definitely* new AAs in and see what you get? That way, if on the off chance that it works, then you know somethings wrong with the Lipo either in the battery itself or in the charging part. And for the sake of completeness (aka a$$ coverage =P) - I'm a novice, but I'm assuming if you do try, make sure the Lipo is not still connected... currents, charge etc. might not flow the way you want.

Good luck.