Hi JIM.

28-Dec-02 23:13:00, JIM HOLSONBACK wrote to JASEN BETTS


 JH> Hello, Jasen.
-=>> JASEN BETTS wrote to JIM HOLSONBACK <=-

 JH>> Transformer still OK - has 15-0-15 VAC at the secondary.

 JB>> anything connected to the centre tap?
 JB>> (a transformer like that is typically used for a push-pull rectifier)
 JB>> (especially if you want about 14V out of it rather than about 30V )

 JH>> I'll post diagram later.  There are two small parts there which
 JH>> are clearly cooked.  The bulb think looks to be OK.  The secondary
 JH>> side just has those two small parts (now blown), the bulb thing,
 JH>> and a 4000 mfd capacitor.

 JH> OK, here's diagram of the secondary side of the transformer.  I moved
 JH> the Center Tap part over to the right, for clarity.  Trafo is 15-0-15
 JH> VAC.

 JH> SCHEMATIC - "AUTO Stereo Home Adapter - Internal Circuit Breaker
 JH>           12-14VDC 3A continuous, 5A switching"
             7

 JH>                  +12V  Pos Out                    Neg Out
 JH>                            |                          |
 JH>                            |                          |

                        thermal cut-out/
                        circuit breaker
                     whatever you want to call it

 JH>                     'MYSTERY BULB '                   |
 JH>                            |        4000 mfd          |
 JH>                            |       +     -            |
 JH>                            |---------| |--------------|
 JH>                            |                          |
 JH>                            |                          |
 JH>               |-----MP-----|----MP-----|              |

a pair of diodes      __|\|__      __|/|__
                        |/|          |\|
 JH>               |                        |              |
 JH>               |                        |              |
 JH>               |                        |              |
 JH>             15VAC                     15VAC         CenterTap


 JH> The "MP" are those mystery parts, now both burned up, and
completely
 JH> into two pieces.  I don't think they could have been 12V 3A bead-shaped
 JH> diodes, because they are only about the same size a maybe 1/4 Watt
 JH> resistors,

  they make 10A diodees smaller than that.

with a diode the heat comes from the junction and flows down the metal
leads, they don't heat up in the muiddle like resistors so they don't need
the bulk. (often just long leads is enough.)

 JH> and I think this unit was built before the time of
 JH> bead-shaped diodes..

A 3A 1N5408 is smaller than a 1/4W resistor (not smaller than a 1/8 w
though.

 JH> I don't understand this circuit at all, but afaict, that
"mystery bulb"
 JH> is the only thing in there which could possibly be a "rectifier".

It couldn't be. not insttalled after the reservoir capacitor like that.

 JH>  I
 JH> don't see anything which could be called a 'circuit breaker', unless it
 JH> is those two (now blown) mystery parts.

see the above mangled circuit diagram.

you've got a push/pull rectifier feeding a capacitor with a thermal cutout
on the +ve output

During one half of the mains cycle one of the diodes conducts to charge the
capacitor during the other half the other diode conducts

 -=> Bye <=-

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