What voltage readings should I get from my 60 watt Magsafe if I check the pins with a multimeter? My readings are all below 1 volt (I checked all combinations of pins), but I don't know if this is correct since there is no load. Also what pins should I check? My computer is a MacBook Pro 2011 early model.
A good description can be found at Teardown and exploration of Apple's Magsafe connector
Below is taken verbatim from the above link.
The charger startup process
When the Magsafe connector is plugged into a Mac, a lot more happens than you might expect. I believe the following steps take place:
- The charger provides a very low current (about 100 µA) 6 volt signal on the power pins (3 volts for Magsafe 2).
- When the Magsafe connector is plugged into the Mac, the Mac applies a resistive load (e.g. 39.41KΩ), pulling the power input low to about 1.7 volts.
- The charger detects the power input has been pulled low, but not too low. (A short or a significant load will not enable the charger.) After exactly one second, the charger switches to full voltage (14.85 to 20 volts depending on model and wattage). There's a 16-bit microprocessor inside the charger to control this and other charger functions.
- The Mac detects the full voltage on the power input and reads the charger ID using the 1-Wire protocol.
If the Mac is happy with the charger ID, it switches the power input to the internal power conversion circuit and starts using the input power. The Mac switches on the appropriate LED on the connector using the 1-Wire protocol.
This process explains why there is a delay of a second after you connect the charger before the light turns on and the computer indicates the battery is charging. It also explains why if you measure the charger output with a voltmeter, you don't find much voltage.
Mixing answers from @David Anderson and @Brandon :
The inner large pins are V+ (16.5 VDC). Measuring with no load will give 6.86 VDC; the full 16.5 V is provided to the proper load.
The outer large pins are Ground.
The tiny center pin is a Charge Control pin that assists with changing the LED color, and also assists with switching the adapter off. No power will be output from any of the other pins until contact is made with the center Charge Control pin
(Source : ifixit forum)