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8 Watt Fluorescent Lamp Inverter-1

by David Bradbury



Figure 1. 8 Watt Invertor

The circuit shown in Figure 1 has been designed to drive an 8Wfluorescent lamp from a 12V source, using an inexpensive inverter based on the ZTX652 transistor. The inverter will operate from  supplies in the range of 10V to 16.5V, attaining efficiencies up to 78% thus making it suitable for use in on-charge systems such as caravans / mobile homes/ RVs as well as periodically charged systems such as roadside lamps, camping lights or outhouse lights etc. Other features of the inverter are that it oscillates at an inaudible 20kHz and that it includes reverse polarity protection.

The 270W and 22W resistors bias a ZTX652 transistor into conduction, where the positive feedback given to the transistor byW1 drives it into saturation, thus applying the supply voltage across W2. This causes a magnetising current to build up in W2 until the transformer’s ferrite core saturates. When this happens, the base drive given to the transistor by W1 decays, causing the transistor to rapidly turn off.  Until the fluorescent tube strikes, the transformer is only loaded by the tube heater filaments which present only a minimal load. Thus when the transistor turns off the transformer ‘rings’ for half a cycle at a frequency governed by the windings’ inductance and the 0.068mF capacitor, reversing the magnetising current and turning the transistor on again. This ‘ring’ induces a high voltage pulse across the fluorescent tube which will cause it to strike once the heaters have warmed up.


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