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Subject: Car power supply tip/question
From: Joris van den Heuvel (
Date: 2003-06-19

First a tip.

I've found that plugging the charger in only briefly switches on the JBR,
while Rockbox DOES load. Waiting a minute or so before reconnecting the
charger will charge the batteries with Rockbox. If you would build a simple
timer that does the following: switch power on for a few seconds - switch
off for 1 minute - switch back on forever - would be a nice workaround for
the power-on-and-original-archos-firmware-starts-charging problem. A 4093
CMOS ic, a few RCs and a logic level P-MOSFET will do this.

Now a question.

I'm currently connecting the JBR in my car. My main problem is the supply
voltage. putting a 10.5 volt regulator on it will not work properly, because
a 10.5 volt regulator (an LM317) will require at least a 13 volts input.
With the engine running, no problem. But a stationary car battery will
supply only 12 volts, 11.5 in practise. This means an output voltage of 9
volts, which means slower charging.

From the schematic and the datasheet of the battery charger IC in the JBR I
can't see why I can't connect the JBR directly to my car battery supply
voltage. The charger regulator is a switching type so it shouldn't run any
hotter from a higher input voltage. Am I correct in assuming this - is there
anyone who has tried to hook it up to 12-15 volts? Another thing: the AC/DC
adaptor supplies 14.5 volts under no-load condition. So when not charging,
the input can take those kinds of voltages without damage.


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Page was last modified "Jan 10 2012" The Rockbox Crew