Re: reverse polarity protection (ubitx.net)

Ralph Mowery
 

Around 1970 I repaired the CB radios for extra spending money.  Many of the mobile 12 volt rigs  had a fuse in line before the radio and a diode across the line inside the radio across the line.  I repaired about a dozen or more where the diode had shorted and blew the fuse, but the radio was ok.  The thing is that you use a fuse that is as low in amp ratings as you can.  Many of the fuses were around 3 to 5 amps and the diode was rated for 1 amp.   I did see one or two where someone had put a 20 amp fuse in it and the diode burnt open and the rig was beyond reasonable repair. 

de ku4pt


On Mon, Aug 6, 2018 at 11:07 AM, Jonathan Washington <kd5cfx@...> wrote:
Thanks, Allison.

Yes, I believe fuse-first, then diode is what MVS Sarma's solution does, once the reverse polarity of the jack is accounted for.  The wiring diagram on this group's assembly page, however, seems to have no particular dependency.  I guess the fuse would probably go first, assuming it reacts faster than the diode, but the µBITX would be exposed to the reverse voltage before the fuse blew, and the diode would blow too.

But my understanding of this sort of stuff isn't terribly extensive, so someone probably needs to verify my assessment.  Any suggestions?


Jonathan, KD5CFX

On Mon, Aug 6, 2018 at 11:01 AM, ajparent1/KB1GMX <kb1gmx@...> wrote:
A while ago (months?) I stated that fuse first then diode is the preferred way.

You want the diode to block reverse voltage and the fuse to blow to keep from burning
the diode open and destroying the radio.

That needs to be get out to everyone.

Allison


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