Reverse Polarity Protection Diode Question

[Shadetree Mechanic]

The other day I was wondering. ( That could be dangerous, right?) Why are the diodes put in parallel to the incoming power? This blows the fuse, but something else will get damaged before the fuse disconnects the power.

Why not put the diode in series?

 

[144inBama]

I believe it's more for prevention of interferance from the radio than actually protecting the device. It's been a few years since I piddled with that type of stuff though.

 

[brandon7861]

That reverse polarity diode has saved me a few radios! Saved my Kenwood TK-705D twice.

The reason it is not used in series is because of the voltage drop. Those transistors are already working at a low impedance, so losing almost a volt when all you have is 12 or 13 to start with is not acceptable when you can sacrafice the diode IF someone screws up (like I have several times).

When that diode shorts, it's peak current capacity is enough to blow any fuse before the diode gets hot. The problems occur when people like me skip the fuse. Use a proper fuse and that diode will take several hits and be fine.

Edit: the rest of the circuit never sees a negative voltage greater than the forward voltage drop of the diode (unless there is no fuse and the diode gets smoked, done that lol). Most transistors can handle that just fine.

 

[nomadradio]

There are two basic ways to protect a circuit. Turn it off by opening a switch, or blow the fuse by purposely shorting the power source.

Putting a diode in series with your load will do the job the first way, with the drawback mentioned above. North Star used a series protection diode in their mobile radios decades ago. People disliked the slight loss of peak wattage those radios could deliver. And they used a diode that was almost big enough. We had a policy that any of those radios we serviced got converted to a shunt protector. Saw too many overheated factory protection diodes.

The latter method is commonly called the "crowbar" method, named after the tool used to short across the subway track's third rail and shut it down before your coworker is electrocuted.

The reverse-protection diode is a classic crowbar, but like any other it needs a current limiter like a fuse or circuit breaker to do its job properly.

And if it gets tripped with a "no-blow" fuse in line, you get smoke.

73

 

[sp5it]

I quit using fuses since I bought quality power supply. Shortcut protection kicks faster than any fuse.
Mike