Oil Burner Ignition Transformers

Oil burner ignition transformers (obits) can be found in oil and gas heatings, where they are used to produce an arc which ignites the flame. they are directly connected to the mains (230V primary winding in Europe), while the secondary winding gives 10kV(eff) between the two high voltage leads, and 5kV(eff) between each one and the iron core. This is called a center-tapped secondary (mid-point grounded). The iron core is connected to the green/yellow earth lead and must be connected to the PE (protection earth) contact of the socket.

The photo shows a typical unit, which is often used in Germany and probably other European countries. The electrical connections are on the down side and can't be seen on this picture. There are usually three leads, one with three wires for the mains connection, and two thickly insulated ones for the high voltage output.

Obits are current limited to about 25mA between the HV leads, i.e. they may be short-circuited on the secondary side. However, the short-circuit time must be kept below 1 minute, and the obit must cool down for 2 minute before the next short-circuit period.

It is possible to parallel several identical units (on the primary as well as on the secondary side), to obtain a higher current output. You have to pay attention to correct phase when connecting the HV leads. This is checked by bringing the leads to be connected near each other. If no arc can be jumps between them, they may safely be connected. Otherwise, either the primary or secondary leads must be swapped.

Power supply with 9 obits in parallel, giving an output current of more than 200mA. The obits are mounted inside the wooden case.

It is also possible to cascade obits for higher voltage, but it is not possible to use the straight forward method paralleling the primaries while connecting the secondaries in series. This shorts out the secondaries, as they are all "midpoint grounded". The trick to be used instead is illustrated in the circuit diagram below. Note that only one transformer is actually connected to the mains. All others are fed by one half of their secondaries, while the other half feeds the next in turn. This way, every obit adds only 5kV to the output voltage.

Cascaded Obits. The case or iron core serves as the midpoint connection (which is usually grounded). As the midpoints here are at successively higher voltages, the obits must be well insulated against ground and each other.

Note that the cores (and primary windings) of all but the middle obit are on high voltage, and must not be touched!

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