Pencil leads as RF resistances

[Malcolm T]

I,m looking for a low value resistance of 18 ohms and took out the centre of a pencil. The total length measures 27 Ohms, half way down i get 18 Ohms !.
My question is has anyone successfully made resistors from pencil leads for low power RF applications ?, and how many watts / heat could it take as a dummy load ?.

[GMB]

Your problem may be making reliable contacts for it. You also need to seal it to avoid changes due to absorbing water vapour.

[G3PIJpeter]

I remember a 'Technical Topics' (G3VA) article about this in a 1980s Radcom. Cannot be more definite than that.

Peter G3PIJ

[Malcolm T]

Thank you for reply s. I shell have a look around for G3VA info.

[emeritus]

I believe prisoners of war sometimes made resistors from pencils for their clandestine radios. I remember some years ago measuring a range of different grades of pencil, expecting the harder grades to have higher resistances than the softer grades, but found no significant differences.

You also need to seal it to avoid changes due to absorbing water vapour.

I wouldn't think so, a) the resistance is much less than water and b) they are soaked in wax. Back to the OPs question, why not? If it is real SHF RF you would need a tapered hole to fit it in to present a varying characteristic impedance for a good match. What is your maximum frequency used?

[G6Tanuki]

Not sure about just how much RF power a pencil-lead could handle; I would be worried about its resistance changing if it got too hot. Maybe you could immerse it in oil [or water*] to cool it?

Pencil-lead rubbed on to a finely-sanded bit of Ebonite was used as a grid-leak resistor in some early home-built radios.

* No, I'm not mad - direct immersion of a bare resistance-wire in water is used un a lot of the little quick-heat hand-wash heaters you find in commercial/municipal toilets.

[paulsherwin]

I did once cobble together a thinwire Ethernet segment terminator from a pencil and half a patch lead in an emergency, and it worked. Not to be recommended though.

https://en.wikipedia.org/wiki/10BASE2

[Malcolm T]

Well only several watts at HF , about 5 at the max, so nothing stupidly high.
A resistor value i don't have, i have the other two but not the third for a pi layout.
The idea is to make a 3db attenuator. I will have to make a temp test and see if there is any significant change.

[joebog1]

quote:
Pencil-lead rubbed on to a finely-sanded bit of Ebonite was used as a grid-leak resistor in some early home-built radios.

My first ever piece of electronics was a single valve radio that dad helped me build. ALL the resistors were blotting paper with a line of carpenters pencil as each resistance. Lower resistance needed a thicker black line. This was shoe tacked to a baseboard, the tacks also serving as connections. I spent many hours with those horrible WWII headphones clamped to my head, listening to the local AM station ( 920 kilocycles, radio 2XL ). I still have the headphones, 2000 ohms impedance.
I was about 8 years old, and look what it did !!!! Im still making electronic stuff in my dotage.

NOTHING wrong with pencil lead!! I would be tempted to try it.


Joe

[fetteler]

Joe, that's a superb anecdote, absolutely superb.

Steve.

[Refugee]

I once tried to make a resistor out of a pencil.
First a passed several amps through the pencil until it got hot enough to free the "lead".
After sliding it out I pinched a replacement pencil leaving the empty one for the hapless owner to fruitlessly sharpen it away.
I never succeeded in making reliable connections to it.

[joebog1]

I would use a spring with an inside diameter slightly smaller than the lead. I would screw it onto the lead in reverse direction of the spring wind. Probably not the most reliable, but its where I would start.

To fetteler. Dad grew up in Warsaw before the first world war, and made his own "wirelesses" that way.



Joe

[joebog1]

So thinking about it for a while, instead of mouthing off, I decided to what results are like.

I have a few packs of "leads", in various grades and colours (I always carry a clutch pencil )
Here is what I measured with those I have.

There is a difference between B grade leads and 2H leads. B grade measures at 16.2 ohms per full length lead. 2H measures at 9.6 ohms for a full length. I also tested the three colours I still have, namely red, blue and yellow. They dont measure at all. the 1pF showing on my meter is due to proximity of the leads of the meter itself.

I did have great trouble making a decent connection with the leads. My leads are freshly made by me using high quality aligator clips and 40 amp silicon insulated wire, so previous comments made by several others remains true i.e. making connections will be difficult.

Joe

[Malcolm T]

Thanks everyone
I stuck the lead on the hot stove , connected to a digital multimeter and noticed no appreciable change .
So yes the connections will be a challenge , possibly also consider the very small solder tags with the internal teeth, a kind of internal finger locking washer style, problem is the fragility.
Nice info Joe and great story, marvelous what some people could come up with when things were in short supply or desperation circumstances .

What about surface mount resistors, they are so small that any "not quite at a right angle" trimming (laser) won't matter. Dead cheap too.

[G6ONEDave]

Perhaps 3 or 4 turns of tinned copper/fuse wire around each end of the lead, followed up by a coating of conductive paint over the wire wrap and onto the lead. Only use the paint at the ends where the wire is though and not the main body between the wires. The finished item will resemble those old thermistor/vdr types from years ago.

Dave

[CambridgeWorks]

Braid from miniature co-ax, bunches up, pushed on then pulled tight will grip the lead.
Rob

[Malcolm T]

Wow the suggestions are coming thick and fast , very interesting ,
This is for a small QRP attenuator Pi configuration 3db, 5 Wattts in 2.5 W out . Having never made one before i,m not sure of the RF path through these resistors ? which resistor will get hotter ?.
I,m working on the assumption that all resistors should be rated at 5 Watts. If i can goto smaller wattage ratings it will be a small unit and opens the doors to salvaging from old PCB boards and even surface mount ?.

[GMB]

i,m not sure of the RF path through these resistors ? which resistor will get hotter ?.

It's the same as the DC path - so simple ohms law.

The rought answer is the series element of the pi is the hottest at about 4/3W with the input parallel one just under 1W and about 0.5W at the output parallel one.