Retro 12V fluorescent light driver with pre-heat

Let's take a look at a faulty automotive fluorescent light. So the front cover unclips in the front. Quite tricky to get off. And once it is off, we have a circuit board that has been painted white for some reason. Maybe by the original manufacturers just to make it blend in or maybe it's just degraded in the surface. But that looks like a coating. And we've got the four pin fluorescent lamp going into this. So when I power this up at the moment, it is uh showing a lot of heat on this inductor here. And also this transistor, I'm guessing that's folded under here. So, I'm going to take the circuit board out uh and reverse engineer it. Just basically I'm going to have to drill these rivets out from the back. Um other things worth note in here. There is a little uh central place for a H hallogen light presumably just for perhaps just other functionality. But anyway, I'm going to take the circuit board out now and uh take a picture of the back. Hopefully, it's not painted as well. And we can reverse engineer it and see what is likely to be the issue and resume. And as you can see, it is now working. The transistor had failed, but I've put in a completely different transistor. There was also a very dodgy capacitor. I'll show you that on the schematic so you can uh assess why you think that went wrong. But anyway, let's take a look at the circuit board. And we'll start with the component side uh which is covered in paint. That's okay. I used a cotton bud with a tiny quantity of uh acetone on it to just basically dust off the top of these components so I could read the values. But uh we have a bit of filtering the mains coming on here. We've got a fuse holder, but there's a PTC thermister, a electronic self-resetting fuse in it. And there's also another one of those here that's one of the hottest components in the board along with this capacitor. But um that is used they just use that as a self-resetting fuse so that if things go wrong it can have another go at working I guess. Uh we've got filled capacitors. We've got a very odd arrangement here with this capacitor and these two xener diodes. More filled capacitors. A horrible little shunt diode which shunts out the power supply if it's connected the wrong polarity. Um and then the usual uh capacitor and resistors associated with a 555 and then a drive to go out to the uh originally uh NPN transistor. There's a transformer. Very simple, just two windings. All the there's no feedback from it. It's just purely fixed frequency from this 555. And this thing here, that's the lamp holder with its four connections. Okay, let's take a look at the back of the circuit board. And here is the back. I'll let you take a snapshot of that if you wish. Uh but we can see a lot more if we go straight to the schematic. And here is the schematic. So let's start at the very beginning. 12vt supply comes in. This is for vehicle use. Uh it goes through this uh overcurren protection device. There's the diode that just literally shunts out the power supply if you connect the wrong priority. It's not uncommon. It's actually more efficient a way than putting in series and dropping voltage across that. Then we've got a small decoupling well filter capacitor uh inductor and then the main filter capacitor 220 microfarad. Then it creates a new supply for the uh 555 I should write that 555 such a standard chip ridiculous so readily available. I shall also write in the last pin number there. Oh, actually no. I can see another pin number that I need to write in which is uh let me just grab that. And it was pin number seven. So this power supply, this decoup power supply feeds this 555. And it's a very standard circuitry. It uses two resistors for the charge and discharge and it uses a low value capacitor to give high frequency output and pin three. What's really odd is that pin five is referred to as the control pin. There's internal there's a voltage divider inside the chip and normally just a little capacitor on the output of this but in this case they've got a 7. 5 vol xener diode plus 2. 4 volt xener diode in series with a capacitor. I don't know why they've done that. That's strange. If you know, let me know in the comments down below. Then it drives the transistor. It drives it with a capacitor for a high current turn on of the uh transistor and then a resistor here which then provides the sustained current once it's had that boom on. Across the transistor are a diode and a capacitor. And it's worth mentioning that this capacitor is rated 100 nanofarad. It had dropped to about 40 nanofarad and it was bulging slight. I don't know if you can see that slight if I catch the light off. There's a slight doming of the surfaces because it's got hot and I stuck in a standard 100

nanofarad 400 volt capacitor in that and it actually gets quite warm. The two components that get the most warm are uh this capacitor here and [snorts] uh this resistor here because this uh transistor is pulsing the primary side of the transformer. The transformer steps voltage up and then applies across the outside the tube. Across the inside of these, these are heater electrodes. And to make a tube start reliably and also to make it much easier on the tube than just cold starting it like a cold cathode tube, you have to run a bit of current through the heaters first. So what happens is that when you initially power up, uh current flows through this uh thermister here and this 3. 3 nanophar capacitor. It's high frequency. So it passes a significant amount of current. The electrodes heat up then that goes high resistance cuz it it's heating up as well and then it goes open circuit and that then strikes the whole voltage across the tube and makes it light. Um the transistor here was a D44 H11. Oh, I was searching for that. It's definitely documenting the internet. But though it's used to be a very standard transistor, it's just kind of like just gone obsolete. And I did consider that I could have got one from AliExpress, but what you actually going to get from AliExpress when you order that. You just don't really know. They'll probably substitute something and maybe even put new numbers on it. In the end, I just put in an STP 36 NFO6L, which is a standard logic level MOSFET. Um, and it works fine and it runs cool about 20° C above ambient temperature, which seems okay. Now, let me bring the light back in and show you the startup sequence. It's worth mentioning that the original transistor was just kind of like folded underneath the circuit board with a bit of captain tape wrapped around it for insulation. So, I did the same with the one I put in instead. Now, when I turn this on, it's not going to light up instantly. It's going to preheat the electrodes at the end. I want I'll turn the light off and we could maybe uh see this. So, I'll just This is going to be dazzling, isn't it? Are you ready? 1 2 3 on preheating. And then it strikes a tube. And that's it. Um, let's tame that down a bit. It is tamed. I'll put it down here. So, that's it. It's working again. Um, I didn't put in the original transistor. I put in a MOSFET, which is a completely different type of transistor, but it seems to be working. I'm not sure how reliable it will be with that, but ultimately, if it did give problems, you know, you could try changing to something closer to the original transistor. Although I was struggling to find something that was a direct match. But there we have it. Um interesting circuitry particularly that capacitor and the little xeno diodes. That was very strange. But that's it. It is working again.