Running CFLs from 12V

Introduction

One great thing about winter is the discounts on summer items as shops try to clear out to make way for the autumn/winter season. It was there that I got hold of a 15W Solar panel for 25 GBP (they usually retail for about 60 pounds). I originally thought of just holding it until next summer then selling it on but curiosity got the better of me. I ended up setting it up on the roof, attached to a 40Amp/hr battery (bargin at 12GBP) and charging circuit.

Now our roof is flat, so a great thing to do on summer days is have a barbeque up there. I figured that a great idea would be to use this power for lighting (as we usually have barbeques up there till the early morning).

Now while taking apart those energy saving bulbs (in my Recycling Ballasts article) I noticed that the first thing done when power enters the ballast, is that it is rectified to DC. This means that the energy saving bulbs should work with 325V DC, but might work on less. This article deals with how we could get our lights to work off 12V DC.

 DC to DC Converter

Well, the simplest way to step up voltage is with a transformer. The problem there is that we need alternating current (AC) for a transformer to work, while we have direct current (DC) available. So what we need to do is find a way to convert DC to AC.

Around this time I started expertimenting with 555 timers, and I decided that a 555 timer set up as an astable multivibrator circuit will work perfectly for my needs.

So I decided on the following setup:

DC IN ==> | 555 ASTABLE | == AC ~12V ==> | TRANSFORMER | ==> ~230V AC ==> | RECTIFIER | ==> DC OUT ~325V

DC 12V goes in, the 555 timer turns it into AC which is stepped up by the transformer to about 230V AC, this is then rectified back to DC.

As we will end up with DC anyway, we do not need to worry ourselves with what AC frequency we would use (so we don't have to match the European 50Hz frequency for example). This means the 555 timer circuit can really be built with any spare components.

Well, the only component I did not have was the transformer. It is funny how things change. A few years ago you could find a proper transformer in just about any piece of eqipment that was mains connected, especially consumer equipment.

Nowadays these are all replaced by solid state switched power supplies, which cannot be used in reverse (to get mains from low voltage), so I had to do something I never thought i'd have to do, buy a transformer (I used to have so many of them, I had to keep throwing them away. Everything had at least one transformer).

After much searching around (and I mean a lot of searching, most things listed as "transformers" are in fact switched power supplies which cannot work in reverse) I bought two toroidal transformers, which could provide 75W of power:

1st Circuit Attempt

This is the first circuit I designed for my converter:

 

This circuit unfortunately didn' t work as well as I had hoped. When under load (one 11W CFL) the voltage would drop to about 40V and the CFL would only intermittently flash. Not a bad first attempt, but not good either.

2nd Circuit Attempt

After doing some more research on the net, I came across this page, which gives a basic overview of a DC to AC inverter, so I decided to give it a go:

 

I had to make some modifications. The link specified that the emitter side of the transistor should go to negative 12V, but my one went to 0V, so I didn't expect the circuit to work well. Needless to say, it didn't. It did work better than the previous one, producing between 60V and 80V which caused theh CFL to light up dimly (see below), but still not good enough.

I had to find a better option.

3rd Circuit attempt

I left this project for a while, and took to improving my knowledge of electronics. It was during my study of transistors at this excellent site  , more specifically this page on amplifiers that I had the idea for my third circuit design. The 555 timer would control two transistors, one PNP and one NPN in what is known as an 'emitter-follower' configuation. The circuit looks like this:

 

The transistors I chose for this configuration is the TIP41A and TIP42A, because I had them already, they complement one another (hence why they both have TIPXXA names) and are rated at 6A continuous current draw, allowing us to draw 65W of usable power from the transformer (at 12V).

 

Thoughts

So far the biggest annoyance with this project is my lack of an oscilloscope. That would have been very useful to me. I have had a look around for oscilloscopes but they are all just so expensive :( 

References

http://www.jcmiras.net/jcm/item/88/

http://www.uoguelph.ca/~antoon/ (really REALLY good site. Ignore the 90's design, the stuff in it is top notch)