"Chris Jones" wrote in message
...
Dave wrote:


"Chris Jones" wrote in message
...
Dave wrote:

I saw somewhere that you could use an NE-2 bulb between the antenna
"hot" lead and ground as a static discharge device for a receiver, but
can't find
an NE-2 and don't know anything about neon bulbs. Anybody know if a
standard neon bulb rated at 125 VAC could be used in this manner? Or
how I
could test it? I'm thinking about charging up a 50V electrolytic
capacitor
and hitting the bulb with that, to see if it discharges the cap. I
have
a .22uF 630V mylar cap between the antenna and the input to the tuner
because the 35V disc I had there got blown, so I know that static can
be
a
problem with my setup even though it is grounded at two points.
Would welcome any ideas anyone has on how to make this work...

Thanks,

Dave

Your "standard neon bulb rated at 125 VAC" probably contains a series
resistor of maybe 100kOhms, and so will be not much use for protecting
anything, because any current would develop too much voltage across the
series resistor. If you open it up and remove the resistor then it will
be
able to shunt larger currents, though it still may not be ideal for
protecting receivers. (The main advantage of the neon as a protection
device is very low capacitance which could be important on the higher
frequency bands, but another advantage would be that it would introduce
practically no intermodulation even in very strong signal conditions,
but
the breakdown voltage is probably so high that it may not protect
solid -
state receivers very well, as someone else already mentioned. You can
buy a ceramic cased gas discharge surge arrestor, they are popular for
telephone circuits. e.g.:
http://www.epcos.com/inf/100/ds/ec350xx0810.pdf
)

You can probably make the bare neon bulb flicker by charging up
something
with static electricity (e.g. rub a balloon on your head or on a jumper)
and then hold this near the bulb so you can hear crackling.

Chris

Hey Chris,

Thanks for the input. I am wondering why a neon bulb would include a
100K
resistor... To maybe lessen the current being driven through the bulb?
I'm
going to have to check that out. Still, I am thinking that a couple of
back-to-back diodes each with a 100K resistor in series would probably do
what I want. Going to try my hand at building a test-bed and give it a
shot. Will check out the gas discdharge surge arrestor though. Sounds
much simpler, and likely more reliable.

Appreciate your feeback.

Dave

I think that the diodes with resistors in series will probably not be
ideal.


I was intendending to connect the diodes in parallel, such as you describe
below. Sorry for the lack of clarity...


If you want to just discharge small steady currents of static electricity
being picked up by your antenna then all you need is a 100k (or 10k)
resistor from the antenna to ground.

If you want to protect against voltage spikes (e.g. caused by distant
lightning), (practically nothing will stop direct lightning) then your
diodes, connected in parallel, and one pointing in each direction, will
offer some protection, but in that case you must leave out the series
resistors because the series resistors will stop the diodes from
performing
any useful function.

Why would the resistors disable the diodes? I expected they would merely
limit the current through the diodes...

If you are likely to experience strong radio signals
that could produce more than 0.2V on your antenna (and I would guess that
the answer is likely to be yes), then maybe the diodes will introduce
intermodulation (a form of interference) into your signal. In this case,
a
number of diodes in series, in each direction, will allow larger RF
signals
to pass without excessive distortion.

\|/
| Antenna
|
*-----\/\/\/\------. 100k resistor
| |
*--||--||--||---* 3x diodes, one direction
| |
*---||--||--||--* 3x diodes, other direction
| |
'--||--To Rx ___ Earth
Cap _
.

For very high frequencies, small diodes such as 1N4148 may be needed,
though
for lower frequencies you may be able to get away with using larger, more
robust diodes. At higher frequencies, big diodes may have too much
capacitance.

I am working in the HF region, 2-30 MHz.


This won't give all that much protection against powerful surges, so if
your
radio is especially valuable then a commercial protection device might be
a
good idea, and in any case unplugging the antenna when not in use would be
sensible.

Oh yes, I do unplug it when not in use. Definetly.


Instead of the above circuit using the diodes in parallel, you could also
try using a pair of zener diodes in series, one pointing in each
direction.


I was thinking about Zener diodes, and trying to figure how they might be
applied. My only hesitation there is the power dissipation capabilities of
Zeners, compared to standard diodes.

Chris


Thank you very much for the input. I appreciate the chance to bounce ideas
off of other people and hear their responses.

Dave