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From: Kevin McMurtrie
Subject: Re: Car Battery Supply
User-Agent: MT-NewsWatcher/3.2 (PPC Mac OS X)
Date: Wed, 15 Jan 2003 03:13:22 GMT
NNTP-Posting-Date: Tue, 14 Jan 2003 19:13:22 PST
In article ,
>"Sean Burns" wrote in message
>> Hi All,
>> Has anyone got a good example schematic of how to power a digital circuit
>> from +12V car power supply?
>> I am of the understanding that the supply is quite crap - what voltage can
>> it drop to and what kind of artefacts can be seen on the supply? How do I
>> protect against these artefacts?
>Yeh the car line voltage does vary a bit. 12V when the engine is not running
>and 13.8 when running. And the voltage can drop quite a bit when the engine
>is being cranked. If your just powering a small 5V logic circuit then a 7805
>regulator will to fine. Put a 16V zener across the input, to stop any high
>voltage spikes. And a large 10,000uF cap across the input aswell to maintain
>the voltage to the regulator during engine cranking.
>A thyristor crowbar with fuse protection across the output would not hurt
>either, just in case the reg goes short.
The biggest problem I've seen with auto power is sharp negative surges.
If not well filtered, it sucks power from your circuit's capacitors in
the reverse direction of the normal power flow. It can cause burnouts
or crazy behavior.
There needs to be a 1 - 5 Ohm resistor or inductor before the power
supply filtering capacitor. The current of the surges can be very high.
Headlights, engine fan, climate control fan, and the tail lights
generate spikes in the 20A to 60A range. The alternator can produce a
small amount of 4KHz ripple at tens of amps. It can blow apart
capacitors, especially compact low ESR types.
(A 16V zerner diode clamp wouldn't be a bit of help. It would pop as
soon as the car is started.)
1 Ohm -----------
+ --/\/\/\/\/---+-----| regulator |---+--->
| ----------- |
1000µF --- | ---
35V --- | ---
| | |
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