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Carbon Film Resistors: Methane Cracked on Ceramic Rods, Helixed and Varnished (Bell Labs 1951)
Ceramic rods tumble in a hot, gas-tight rotating furnace while methane cracks on them and leaves a thin, hard film of carbon. The coated rods get conducting ends, a helix ground through the film to bring them near their value, a light rub to the exact value, and a coat of baking varnish.
Result: carbon film resistors adjusted to as close as ±0.5 %. The helix alone can raise a rod's resistance by any factor from about 10 to about 8000.
Source: Grisdale, Pfister and van Roosbroeck, Pyrolytic Film Resistors, Bell System Technical Journal 30 (1951). Unknown: the ceramic of the rods, the end caps and leads, and which baking varnish; the paper does not give them. Picture: Wikimedia Commons.
Avanzado
2 days
Instrucciones
1
1
Load the furnace
Load the furnace
Load cylindrical ceramic rods into the gas-tight refractory core of a rotating (or rocking) batch furnace, with silica sand to support the rods and keep them from damage as they tumble. Unknown: which ceramic.
Materiales para este paso:
Núcleo aislante de resistencia1 batchHerramientas necesarias:
Horno rotatorio por lotes
Arena de cuarzo2
2
Heat under nitrogen
Heat under nitrogen
Turn the core and bring it to temperature, usually about 975 to 1300 °C, flushing it with nitrogen before any hydrocarbon goes in.
Herramientas necesarias:
Horno rotatorio por lotes
Nitrógeno3
3
Crack the methane
Crack the methane
When the temperature is steady, let in the coating gas and keep it flowing for the time needed. The film grows at a steady rate: at 1000 °C and 30 % methane the paper runs from 0.2 to 10 hours. Longer gives a thicker film of lower resistance; temperature sets the rate more than anything else, and more methane deposits faster.
Materiales para este paso:
Metano1 flowHerramientas necesarias:
Horno rotatorio por lotes4
4
Cool under nitrogen
Cool under nitrogen
Shut off the methane and let the core cool while oxygen-free nitrogen keeps flowing. The films come out clean, even and firmly stuck.
Herramientas necesarias:
Horno rotatorio por lotesNitrógeno5
5
Terminate the ends
Terminate the ends
Paint the ends of each coated rod with a water suspension of colloidal graphite (Aquadag), then burnish and bake it; this suits all but low-value resistors. Fit the end caps with their leads. Unknown: the caps and leads, which the paper does not describe. Ours: an agate burnisher and an oven.
Materiales para este paso:
Grafito coloidal1 as required
Casquillos terminales de resistencia con hilos2 per resistorHerramientas necesarias:
Bruñidor de ágata
Horno6
6
Grind the helix
Grind the helix
Grind a helical groove through the film with a water-cooled, metal-bonded diamond wheel, the blank floating against the wheel so the groove stays even. Pick the pitch to land slightly below the wanted value (330 Ω, 10 kΩ or any other). The groove must be smooth: cracks into the carbon ribbon bring noise and drift.
Herramientas necesarias:
Máquina de corte helicoidal de resistencias7
7
Rub to value and varnish
Rub to value and varnish
Turn the resistor while measuring it continuously, and rub its whole surface lightly with a cotton pad wet with an organic solvent until it reaches its value. Leave it just below tolerance, then give it one or more coats of baking varnish, which raises the resistance by generally less than 1 %. Unknown: which varnish and its baking. Ours: acetone as the solvent, a Wheatstone bridge to measure.
Materiales para este paso:
Barniz de horno1 as requiredHerramientas necesarias:
Puente de Wheatstone
Algodón hidrófilo
AcetonaHornoMateriales
5- Marcador de posición
- 1 flowMarcador de posición
- 1 as requiredMarcador de posición
- 2 per resistorMarcador de posición
- 1 as requiredMarcador de posición
Herramientas requeridas
9- Marcador de posición
- Marcador de posición
- Marcador de posición
- Marcador de posición
- Marcador de posición
- Marcador de posición
- Marcador de posición
- Marcador de posición
- Marcador de posición
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