Up from the Ashes of Disaster, grow the Roses of Success!

I have a working fluorescent lamp inverter!

This time I built a H-bridge. I used bipolar transistors, which is going to add another 40mA or so to my current consumption; but at least it doesn’t need a higher voltage or P-channel power MOSFETs (which are about as rare as rocking-horse muck). Maybe I can do that for the Mark Two version — after I’ve built a little stereo amp with the remains of the TDA2030 jobbie. In the meantime, this bridge will do. And it’s inherently safe against both transistors on the same side turning on (barring a driver transistor failure).

H-bridge inverter

H-bridge inverter

The transformer is just a 12V / 1A one from my junk drawer (otherwise, I’d have chosen one with a centre tap and not had to faff about building bridges). This circuit runs at about 90Hz in order to try to squeeze a bit more power through without saturating the core. If you’ve ever opened up an eco-bulb, you’ll know that the supply gets rectified anyway; it’s really just a fancy switched-mode power supply.

One thing I didn’t do with the failed TDA2030 design was to ensure a 50% duty cycle. Of course it should be 50% anyway, barring any deviation from ideal op-amp behaviour — and indeed, was almost bang-on until I connected that pesky load (and I think you will agree that an extra couple of ohms output impedance is a deviation from ideal).

The easiest way to get a 50% duty cycle is to cheat a bit, and halve the frequency using a flip-flop. This also has the side benefit of offering two complementary outputs, which is just what my H-bridge needs. And, as luck would have it, there exists an IC with both a monostable/astable multivibrator and a flip-flop with complementary outputs: the HEF4047BE. It even helpfully provides the astable output from upstream of the flip-flop, in case you need it to do other things (like maybe run a step-up converter to turn on the top FETs in a H-bridge, for instance?)

One Response to “Up from the Ashes of Disaster, grow the Roses of Success!”

  1. AJS says:

    OK: in case it’s not obvious, here’s how it works. IC1 (4047) is configured as a free-running astable multivibrator with a period of about 5ms. An integral flip-flop effectively doubles the period to 10ms, and ensures an even duty cycle regardless of any asymmetry in the switching points; this ensures that there is no residual DC in the transformer winding, which would reduce efficiency. Pins 10 and 11 are the complementary outputs.

    When pin 10 is high and pin 11 is low, T3 and T4 (BC548) are on. T4 shunts away the current from R3, keeping T2 (BD437) off. T3 draws a current via R2 and the base of T1 (BD438), turning T1 on and the left-hand side of the transformer is at +12V. T7 and T8 are both off, so current flows through R8 into the base of T6, turning it on; while R6 keeps T5 off, so the right-hand side of the transformer is at 0V. The resistor values are chosen to ensure that all transistors are driven into saturation. No heatsink is required with a 9 watt bulb: the transistors remained cool to the touch even after an hour of running in a confined space (i.e., inside the base of the lamp).

    When pin 10 is low and pin 11 is high, the situation is reversed — and so is the current in the winding. Unless a transistor fails, there is no way for both T1 and T2 (or T5 and T6) to be turned on together.

    CR1-4 are there to protect T1-4 against back EMF in the transformer, in case the load becomes disconnected from the secondary, and also to ensure that the fuse (not shown here, but there is one in the distribution box into which the lamp is meant to plug) will blow if the supply connections are reversed.

    If you can get a 12-0-12V transformer with a suitable VA rating, then R1, R2, R4, R6, R7, R9, CR2, CR4, T1, T3, T5 and T7 can be omitted: connect the centre tap to +12V. Alternatively, you could replace T2 and T6 with power MOSFETs, with their gates driven directly from R5 and R10.