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G8MNY  > TECH     10.02.19 10:30l 105 Lines 4715 Bytes #4 (0) @ WW
BID : 58641_GB7CIP
Subj: Car 12V Switch Mode Adaptors
Sent: 190210/0925Z @:GB7CIP.#32.GBR.EURO #:58641 [Caterham Surrey GBR]

By G8MNY                                      (Update Jan 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

12V adapters for your Walkman, Mobile Phone, PDA, GPS, Digital Camera etc. are
often based on the many versions of the 1.5A 34063 8 pin DIL switch mode
regulator IC in this circuit using several PCB layouts in different sized plugs

I have seen what happed to a GPS (not mine) when the 12V to 5V adaptor failed!
There was no worthwhile fusing/zener to protected the kit & hence the whole
thing is a right off, after the GPS internal 3.3V regulator then failed
(overheated), fusing the CPU to a short circuit with 12V!

12V  Fuse
Caro-oĿ  >+ to gadget
Plug               R1   R2      ||(        
                              ||( L1    __
           Ĵ    Ŀ  ||(       \_/==
             85  R3       +   LED
              )  K34063            ===   
             14          C3  
             ))>Ĵ        R4
         +                     __       
      C1===C1       C2===        /_\D1     
gnd-     47uF                           
gnd>- to gadget

Typical values
       R          C        D      Fuse        IC PIN  Function
1   1 - 4.7R    47uF     1N4001    2A           178   Current drain & sense-
2   1 - 4k7   n47 - 1n                           2    Switched Output
3  560 - 1k   4u7 - 220u                         3    Osc Timing C
4  330 - 1k                                      4    Ground
                                                 5    Voltage NFB
                                                 6    Current limit +ref
R2/3 sets the output voltage.
R1 sets the switching current limit.

Output cap C3 has ripple on it due to pulse currents. These give this sort of
spiky ripple..

       _   _     Due to the small capacitor value. its self inductance,
Mean DC  ~-_ ~-_  & ESR, you always get this waveform with the sharp rise
                   QRM edge on the gadget lead of these adaptors.  

Problems with this simple circuit are:-
   1/ No output over voltage protection. (important on costly gadgets!)
   2/ No input idiot diode.
   3/ High output ripple & QRM?
   4/ High input ripple & QRM?
   5/ Direct QRM magnetic radiation from L1 if bobbin type.
   6/ Too high a fuse rating?
   7/ Only 1 ground spring connected in car plug earth.

If you have space in the plug, try some of these embellishments...

12V  Fuse                                                     __
Caro-occccĿ  (((()>+ to gadget
Plug      ====          R1   R2     ||(       =====    ~~
           L2                     ||(L1  __  L3      FB
                 Ĵ    Ŀ ||(    \_/==       
        __        85  R3    +  LED    +  
        /_\         )  K34063          ===      C4=== 
       D2         14      C3            D3
                   )))Ĵ     R4        __
               +                   __             /_\'
              ===C1      C2===       /_\                 FB
gndĿ         47uF                                 __
gnd>- to gadget
Typical values
No.      R           C         D        L      Fuse
 1     1 - 4R7     47uF     1N4001              1A
 2     1 - 4k7   n47 - 1n   1N4001   5 turns
 3   560 - 1k     220uF      Zener   3 turns
 4   560 - 1k     100uF

 1/  Add an output zener just a bit higher than the no load voltage.
 2/  Add input idiot & back EMF diode D2.
 3a/ Add series L3 of a few turns on a 6mm ferrite bead feeding new C4.
 3b/ Add ferrite beads on both output leads.
 4/  Add series input L2, several turns on a 6mm ferrite bead.
 5/  If L1 is bobbin, try a ferrite ring & listern to MW QRM for comparison.
 6/  Change the fuse to one just higher than the maximum drain.
 7/  Tin & wire up the 2nd plug ground earth spring.

After you have done these mods, & carefully put it back together, tested it
(not on your gadget at first!). If it is working OK, you should find the
adaptor is much safer than before on your precious gadget. And any nearby QRM
is much reduced, as is the possibility of malfunction, in an RFI environment.

Why don't U send an interesting bul?

73 de John G8MNY @ GB7CIP

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