Energy Saving Fluorescent Globe Circuit Repair

The "energy saving" light globes, based on a small fluorescent tube and intended for replacement of incandescent globes, have a quite substantial electronic power conversion circuit. This circuit generates a high voltage, around 600V, at a frequency of several tens of kilohertz. It consists of a full-wave rectifier, filter, oscillator and transformer.

My interest in these came about when two rather costly 12V 20W globes failed. These are being used in a remote 12V solar battery powered bush cottage. The first failure was an intermittent fault as a result of a poor solder connection and was easily fixed. The second was a total and apparently random failure.

The circuit for the electronics is very simple and elegant:

The rectifier circuit simply allows the globe to be used without concern for polarity (although it drops over 1V and hence wastes about 15% of the power). A transformer based line input EMI filter stops switching noise getting back to the power source. The oscillator uses a simple transformer feedback and provides the voltage boost to the fluorescent tube (shown at the top). The filaments to the globe are connected by a twisted wire arrangement to the circuit. The 750R resistor ensures that the oscillator starts up reliably.

The two transistors were completely dead. My first thought was that the high-voltage winding had shorted to the other windings, but a megger test didn't indicate that to be the case. By a most amazing stroke of fortune I has two 2SC882 transistors in my box of parts ratted from various dead and obsolete electronic equipment (these transistors and any possible replacements are now obsolete). Simply replacing these got the circuit to work.

The CRO trace at the collector of one of the transistors showed the following:

The vertical scale is 5V per division. This was measured using an 11.6V power source. The transistors have a 30V peak-to-peak absolute maximum collector-emitter voltage rating. It can be seen that the peak voltage reaches over 27V, and this is likely to be higher with a 13V maximum voltage source.

An attempt to replace the transistors with another type having very similar characteristics but a higher maximum
collector-emitter voltage rating didn't work. The circuit oscillated but only delivered about 20% of the power.

I don't hold out much hope for the lifetime of my globes.

First created 17 September
Last Modified 17 September 2007
© Ken Sarkies 2007