On Dec 5, 4:28*pm, No Spam wrote:
On Sat, 04 Dec 2010 11:44:55 -0800, Richard Clark wrote:
On Sat, 4 Dec 2010 18:22:21 +0000 (UTC), No Spam
wrote:

I used a carbon watercleaner cartridge to build my term resisor :-)

! ! ! !

This demands more discussion.

73's
Richard Clark, KB7QHC

Sure!
I have dremmel-tooled open a used dried-out water filter cartridge from
my fridge that contained a hollow carbon shaft about 10" long and 3" in
diameter with a 3/4 inch hollow inside diameter. The end to end
resistance was about 600ohms. The carbon shaft is a bit soft and crumbles
under the pressure of tools. This was how I destroyed the first unit I
played with in trying to make it 480ohms by trimming the ends.

Using another filter, I encapsulated the carbon shaft inside a sewer
tube. (Denser PVC and hopfully more impervious to heat) I attached
conductors to the ends using flat washers with springs and left vent
holes in the PVC end caps when I attached them to push it all together. I
did not attempt to trim the resistance by shaving the length. The modules
finished resistance measured ~590ohms.

Since this WAS a used water filter, I ran some HV through it to heat it
up a bit to see if it would vent gas or smoke. The resistance decreased a
bit as it heated but returned to near 600ohms when it cooled. I kept the
power to less then 100w using my bench HV power supply. After about 1
minute, the PVC started to feel warm but no smoke was seen though it did
stink a bit.

The whole PVC module I made sweeps out flat from 1Mhz (bottom of my test
gear) to ~40Mhz where I think my wiring started having some effects. I
had to use springs on the ends to push the contact washers up against the
carbon shaft and suspect I need to come up with something better here.
Also, the pressure of the springs and end caps varied the resistance
about 20 ohms.

I have run about 400w avg for several minutes into the load under voice
and so-far the damn thing is still hanging on the T2FD antenna and loads
just fine. In usage, the resistance varied about 50ohms upward in the
last several months of tracking it. I hope to remove the resistor shortly
and inspect if there is any chemical reaction with the steel washers and
the carbon rod ends. I need some better way to make this contact and also
some way to increase the surface area of the contact to stabilize and
perhaps reduce the resistance. I'm thinking copper pipe endcaps heated up
and cooled over the ends either on the outer diameter or on the inside
hollow part of the carbon shaft.

/****/
Another idea I just had is perhaps having someone with a ceramics oven
paint on the endcaps with ceramic glaze. It would look like a giant
ceramic resistor! *But then again, I would guess that as the resistor
heated up, it would crack. * Never mind...
/****/

I now have another filter module to dissect. I hope to come up with a
better way to mount contacts to the carbon shaft and some way to assure a
nice airflow around and hopefully through it to allow for higher power.

In retrospect, with the amount of time and work I've put into this
resistor project, I would have been better off purchasing the buxcomm
resistors for $25. (I have one) * But then again, I had fun playing with
this too! If anything, ideally, I hope to come up with something that
weighs half the weight of the buxcomm resistor so my T2FD antenna looks
more graceful under the burden of the resistor. :-)

73!

A better way to attach it may be to electroplate Cu on the ends of it.
Did this with some battery cores one time. Think I used CuSO4 solution
as the electrolyte.

Jimmie