Resistors

High voltage resistors have to meet special requirements. In particular, the following aspects are important:

High voltage resistors often must have very high resistance values (in the GOhm range). These values are hard to find at the well known electronics stores.
The resistor must not arc over. This means that is has to be just long enough or well insulated, e.g. by immersion in oil.
The resistor itself must be able to withstand the high voltage. Common 1/4W carbon film resistors, e.g., are only suitable for up to 500V. Although it takes a lot more (some kV) to arc over, they start to change their resistance value above that voltage. Breakdown is nothing but a rather dramatic and in this case irreversible change in resistance!
The resistor must be constructed to dissipate the heat generated by current flow and voltage drop. E.g., at 100kV across a 1GOhm resistor we already have 10W power dissipation. This requires a certain physical size or special means for cooling.

Commercial high voltage resistors can be found in TV sets and old tube (valve) devices. The picture below shows some examples.

Commercial high voltage resistors. The shortest one (3) has only 2MOhm and is good for some kV at most, above that it becomes too hot. The slightly longer one (2) below has 200MOhm and can be used at up to 10kV. The long resistor at the bottom edge (1) is quite a rare find. It also has 220MOhm, but is nearly 20cm long and can be used for up to 30kV. The remaining pieces (4) and (5) are focus potentiometers from TVs. They have a very high resistance and are intended for 5-10kV. Large Version

There are basically two easy way of home-brewing a hv resistor. The first possibility is to put lots of resistors (of identical type) in series. Thus, the resistance and voltage ratings add up. A chain of N equal resistors will have N times the resistance and N times the voltage rating of a single one, while the accuracy equals that of a single resistor. For example, to make a 1GOhm 50kV resistor for measuring high voltages, you could make a chain of 100 pieces 10MOhm/500V metal film resistors (1% accuracy).

The second possibility involves an insulating hose or pipe, filled with more or less destilled water. Such a resistor is easy to build and can be constructed to take quite high voltage and power. Using de-ionized water and adding a calculated amount of salt (NaCl) or copper sulphate (CuSO4) allows to vary the resitance over a wide range. However, it´s difficult to meet (and keep constant) a prescribed resistance value. Also, this type is not suitable for high DC currents, as this leads to dissociation of the water into hydrogen and oxygen, which not only produces unwanted bubbles but may present a risk of explosion.

The value of such a "bulk" resistor can be calculated using the following formula, where the specific resistance depends on the material used (in this case, the substance and it´s concentration in the solution). The graph below can be used to determine the specific resistance of NaCl or CuSO4 solutions.

Specific resistance of NaCl and CuSO4 solutions.
Data taken from "CRC Handbook of Chemistry and Physics (62nd edition).

Print this graph: PostScript PDF

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