Flyback Inverter for Fluorescent Lamp: Part 1, Making Repairs

A friend of mine approached me a while ago asking for some help. The fluorescent lamp assembly for his VW Westfalia camper was dead and, knowing I knew more about electronic devices than he did, figured it was worth challenging me with it.  I was actually happy to do so. Being involved with DC power conversion of a variety of forms I was always a bit curious to learn about how fluorescent lamp assemblies that were powered from low voltage DC worked anyway.

“My lamp does not work; can you look at it for me?”

“I suppose. Did it just stop working? Did you try anything to get it working again?”

“Well, it really never worked for me. I messed around with it a little but it did not help. I may have hooked it up backwards.”

“Why do you think you hooked it up backwards?”

“Well, it did not work so I tried reversing the power connections. That didn’t make it work however.”

“You really should not do that with electronic things!”

I took the lamp home and later when I had chance to look at it carefully I visually identified several problems. Like many other things I have repaired, a lot of the times it is not the device itself but rather a previous owner unintentionally inflicts unnecessary damage on it when attempting to make repairs.  In my friend’s partial defense, someone previously had already made unsuccessful attempts at trying to make it work again, unwittingly making things worse.

Referring to Figure 1 I unanchored the inverter circuit board from the back of the lamp assembly for closer inspection. It was immediately obvious there were problems that would keep it from working:

• The connectors for the wiring to the fluorescent tube were not making contact.
• A portion of a circuit board trace where the power feeds in was blown away.

Figure 1: Fluorescent lamp inverter board had obvious problems

Clearly someone had let the smoke out of it that made it work!  After making repairs to these problems I then tried powering it up using a power supply with a current limit to keep things safe. As I expected I was not going to get off that easy. The power supply went right up to its current limit setting. The lamp still did not work. 

The next step was to probe around the circuit board with a DMM.  With the abuse this lamp assembly has been subjected to I suspected the switching transistor would be damaged and sure enough it was measuring shorted. However, after removing it, it seemed to check out good. Probing around on the board again, a diode adjacent to the transistor measured shorted as well. Upon its removal it fell in half as a result of being overheated. I found where the rest of the smoke that makes it work had come out!  I replaced the diode, reinstalled the transistor and remounted the circuit board. Upon applying power again the result was a bit different as shown in Figure 2. I managed to reinstall all the smoke back into it again!

Figure 2: Fluorescent lamp assembly back in working order

While I had a general idea of how it works, now that I had the fluorescent lamp assembly working again I had take the opportunity to make some measurements and study the finer aspects of how it works, which I will cover, coming up in part 2. Stay tuned!