[Elecraft] To Fuse or Not To Fuse...

[Mike Morrow]

Ronald Evans wrote:

>I routinely operate my K1 and K2 from a 12V, 7ah gel cell without inline
>fuses in the power cord.
>
>Questions:  Are such fuses advisable?

Yes, in my opinion, at least as far as the K1 is concerned.

Some seem to feel that, if there's a series reverse polarity diode in the
power path, a fuse is NOT necessary.  I'll give one example where that can
cause a problem:  Suppose you transmit for a while into an unmatched or
disconnected antenna.  The RF voltage at the final stage can build high
enough to destroy the protective zener diode that's in the output circuit.
If it shorts, then you'll have a high DC current flow that could destroy the
reverse polarity protection diode or the PCB traces in the DC power path.  A
fuse will prevent that, as well as provide protection for any number of
similar power circuit faults.

> If so, should I fuse both power leads?

I think that would be mandatory only if you were operating while attached to
a vehicular DC system, where a bad ground connection from the negative
terminal of the battery to the car body could cause the battery to seek
ground through the negative cable, case, and antenna cable shield of your
radio.  Otherwise, I wouldn't bother.  Of course, it doesn't hurt to have
both leads fused.

For my portable cables for my QRP rigs, I prefer to fuse the negative lead.
That's because the greatest amount of conductor that is exposed on the
coaxial power connector that plugs in to the radio is connected to the
negative terminal of the battery.  If, when unconnected to the radio, the
exposed negative conductor of the connector should come into contact with
the positive terminal of the battery, a fuse will help only if it's in the
negative lead.

 What would be the correct size fuse for both the K1 and the K2?

I use a 2 amp fuse on my K1.

>What sorts of dire disasters could result if I do NOT fuse the power
>lead(s)?


See above.

73,
Mike / KK5F