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100 megaohm resistors

Why

Because for high-voltage measurements, and high-voltage work in general, high-value resistors are necessary. In addition, these resistors have to be made from humidity-nonabsorbing materials and have to be of sufficient length to avoid arc-overs.

How

A large amount of 10 MΩ 5% 1206 SMD resistors was obtained as a source material.

A jig was made from a wooden board with a groove cut in, so the resistors can be placed in and then soldered individually together. The jig was found to be functional but suboptimal.

Another version of the jig was made from a wooden board with a protective layer of polyimide (Kapton?) tape, and a pair of steel sheet ribbons, polyimide tape coated on sides. This one was found to be much more satisfying, due to better planarity of the underlying base and better side-grip of the resistors.

Ten resistors were lined up in the jig, then soldered together carefully, butt-to-butt. The flux residues were removed with trichloroethylene.

As the resistor bar is very fragile (the weakest part, where the failure happens most often, is the metallization of the ceramic substrate, it tends to separate from the ceramics under even minor load), some sort of mechanical support is necessary. A polypropylene plastic welding rod was chosen for its hydrophobic properties (won't attract moisture). The material was softened with a hot air from a propane torch and flattened between two steel boards. It was then softened again and a groove was impressed with the edge of a steel board; the board thickness virtually exactly matches the resistor width.

A small hole was drilled on each end of the plastic rod. Wires were pulled through the holes and twisted over the ends of the rod, so the mechanical loads caused by pulling the wires during installation and operation are absorbed by the plastic rod and not by the resistor bar.

Resistor bar was then soldered on the ends of the wires, and the ends of the rods with the wire wraps and contacts were secured with pieces of heat-shrink tubing.

(The opposite order of installation, with wires soldered to the resistor bar and then pulled through the holes and tightened is also possible and may be more comfortable in some situations.)

Caveats

The metallization at the ends of the resistor can separate from the ceramics under even moderate loads. In such case it is advised to replace the affected resistor, as the reattachment connection is unlikely to be well-defined, and the mechanical integrity is almost guaranteed to be low. The cost of SMD resistors in bulk is very low.

When soldering the resistor bar, heat each joint well (so the solder soaks into the joint, which can be inspected later from the bottom side) but limit the total heating time as the metallization can dissolve in molten solder and the joint will then be weak or nonfunctional.

Images


Old version of jig

Soldering jig

Soldering jig, detail

Washing the resistor bars

Resistor bars, top

Resistor bars, bottom

Polypropylene rods

Polypropylene rods, softening and flattening

Groove impressing

Groove

Groove with resistor bar

Rod with wire holes

Rod with attached wires

Rod with attached wires and soldered resistor bar

Finished resistor

Support bars, resistor bars, finished resistor

Six finished resistors

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