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From: squishy@casbah.acns.nwu.edu (Shishin Yamada)
Newsgroups: sci.electronics
Subject: Re: 12V car --> 3V cd player
Summary: Reply
Keywords: CD, cd, cd player, car, power supply
Message-ID: <2382@casbah.acns.nwu.edu>
Date: 1 Jan 91 15:07:20 GMT
References: <1990Dec31.081024.16910@yenta.alb.nm.us> <2374@casbah.acns.nwu.edu> <%BS^BG*@rpi.edu>
Organization: Northwestern University
Lines: 39

In article <%BS^BG*@rpi.edu> wrf@mab.ecse.rpi.edu (Wm Randolph Franklin) writes:
>In article <2374@casbah.acns.nwu.edu> squishy@casbah.acns.nwu.edu (Shishin Yamada) writes:
>>
>>+12V   Fuse    L1              1 _________ 2        D1   D2   D3
>>O-------O~O---UUUUU---+------+---|       |-----+---->|--->|--->|----O  +3V
>>                     +|      |   | 7805  |     | 
>>                     ===    ===  |_______|    === C3
>>Ground                | C1   | C2    |3        |
>>O---------------------+------+-------+---------+--------------------O Ground
>>
>>	Where, L1 is say 1 uH, C1 is say 2200uF, C2 is 0.33uF, C3 is 0.1uF,
>>D1 D2 and D3 are silicon rectifying diodes (like 1N4001). All components
>>should be rated at least to 16 volts.
>
>What does C2 do since it's so much smaller than C1?  My guess it that it
>also has a smaller  inductance and  so traps hi  freqs better.   Is this
>right?  Thanks.
>

You hit the nose right on the button! C1 is large to filter out very slow
changes in the power supply (like smoothing out power ripples in typical AC
to DC converters).  C2 is for filtering out high frequencies that are
typical of cars. C2 is required since the DC regulator is far from the
power source (battery). It helps to filter out high frequencies from both
sides of the 7805. By the way, I only wrote down 0.33uF and 0.1uF because
they are typical values. 0.33uF was given in the chip data book, and 0.1uF
is common when using digital circuits (notably TTL) to block spikes.

These higher frequencies are typically from the ignition
coil as it fires the spark plugs. Finally, C3 is also there for high
frquencies developed in the right hand side of the circuit. It is common
practice with any digital circuits to place 0.1uF caps around the power
supply as close as possible to logic chips (especially TTL), because they
help to stabilize the power supply during changes of the logic gate's state.

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Shishin "Squish" Yamada               * squishy @ casbah.acns.nwu.edu
Electrical Engineering (class of '91) * Northwestern University
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