|Newton Car Charging Adaptor|
For those interested in powering/charging their Newton device from a 12V car, motorcycle or boat battery, Apple supplied a small adaptor for the purpose which contains a simple but accurate regulator.
It cannot be stressed enough that powering your MessagePad or Emate from a power source that is not properly regulated or current limited could be fatal for it.
Always check that the voltage cannot exceed 7.5V under all conditons and observe the correct polarity for the plug that connects with the Newton - the inner pin is positive.
The best thing to use is one of the original Apple mains power supplies. The 7W or 9W versions will power all the Newton models.
The adaptor with its red bayonet locking ring attached and the Newton power plug.
The construction is rather nice for this kind of accessory with a sprung center pin, decent cable clamp and strain relief, a car type blade fuse and an LED to show the unit is receiving power from the vehicle. A bayonet locking adaptor is provided for those vehicles with a matching socket.
With half of the housing removed, the circuit board is exposed. The 3A fuse is the violet object between the board and the cable clamp.
The circuit board folds open to reveal its foil side, the LED and the sprung center pin.
Here is a small scan of the card that was included with the adaptor:
I have disassembled the unit and determined the circuit - its the basic configuration for the LM317 regulator chip used in this design.
1% resistors in the voltage setting potential divider ensure a repeatable output voltage.
A datasheet for the LM317 can be found here:
I have tested the adaptor on the bench and it delivers a no-load output voltage of 6.99V for inputs of 9V to 15V (car batteries are normally charged at 14.5V)
With a 400mA load the output drops to 6.75V
I would have preferred that the regulator was attached to a small heatsink but, while it does get hot, its within the parameters for the device.
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