Can this fluorescent inverter be tweaked?

[RandomJoe]

Below is a picture of the inverter circuit for a bargain-bin fluorescent "drop light" I picked up at Ace Hardware.  Sorry for the lousy photo, it appears my camera is no longer focusing well...

It is all of five parts: transformer, two caps, a resistor and the 3-pin chip says D880 and 4804.  There's a stylized "HC" above that.

The thing uses an F8T5 tube (came with a no-name daylight tube - I can't stand that color! - so I replaced it with a spare CW tube I had around) and with both the original tube and the new one the light only pulls 210mA or just under 3W when running.  It also, naturally, puts out relatively little light as compared to my other battery-operated fluorescents.

Is there a way to "tune up" this circuit so it will operate closer to the design 8W?  The "D880" is attached to the metal backing that runs behind the tube, and doesn't even get warm that I can tell, so I don't think it's a heat-limited issue.

I tried to do some searching on the D880 part, but am not familiar with the circuit so don't even know what it is, let alone where to start looking.  The only sites I found in Google were largely Oriental language sites...  (Of course, my searching talents are pretty poor anyway...!)

Worst case, the thing is usable as-is, just not terribly bright.  I imagine that means the bulb probably won't last too terribly long either...

[RandomJoe]

Hm, I may have finally figured the circuit out - funnily enough, a Google search on some more generic terms eventually dug up a thread from this site:

http://www.fieldlines.com/story/2007/11/8/235534/841

Which shows a circuit I had found a couple of other places that closely resembles what I have here - except the transformer is different design.  I also found a couple of other pages where people had reverse-engineered camping fluorescent lanterns and such that very closely resembled this.  All had a few more parts, but perhaps that's just "value engineering" at work on my bargain-bin lamp...

So, at this point I'm guessing the D880 part is just a transistor.  And from what I've read, efficiency is pretty lousy and not much point messing with this circuit.

Be nice to cram a better ballast into the base of this thing, but that PC board in the picture is all the room there is...  Might have to see what else I can find...

I am surprised, though - the circuit referred to in the above thread had some notes at the bottom that indicated the tube would last longer at lower power.  I thought that would cause faster darkening.  I still have lots to learn about fluorescent lamps!

[Norm]

They are continually advancing LED technology to

the point where it might someday surpass

fluorescents.

...but there is nothing wrong with experimenting

....who knows? you might someday come up with

a hybrid fluorescent and LED light ?

A friend of mine once scoffed when I told him

one day they would be able to record a TV program

....back in the days of wire recorders!

[gizmo]

I'll bet the D880 is a 2SD880. 3A,60V,30W

Glenn

[commanda]

I'll second Gizmo on the 2SD880.

Try decreasing the value of the resistor, to the next lowest preferred value, and re-measure the current draw.

Circuits like this are usually assymetrical in their waveform. The resultant DC offset applied to the tube causes it to blacken at one end and fail prematurely.

Amanda

[sPuDd]

I played with a few of these about 12 months ago.

Gizmo is right, its just a 2SD880 transistor.

The whole thing is a very simple self osc unit

which works OK in most cases. But as Commanda

said it will blacken the tube ends. Also I found

that it would get hot if run outside of about 12.5V

and eventually the transformer would short & melt.

I spose thats why they cost less than a new tube.

sPuDd..

[RandomJoe]

Well, thanks everyone!

I tried changing the resistor, that worked nicely.  It originally had a 5.6kohm, dropped it to 4.7k and only got another 20mA.  Went on down to a 2.2k and it's now running at 6W, 470mA.  The tube is much better looking, it originally had a fair bit of flicker evident at the ends, now I don't see any.

Have to see if I fry the thing now... ;-)  After just a few minutes running, the D880 is around 195 degF, the transformer nearly 150 degF!  Considerably warmer than before, and it's open to the air right now, not enclosed in the fixture.  Something tells me it won't run too long like that...

Okay, maybe about this long!  Didn't pop, but I just noticed the current was starting to climb, and the D880 spiked to around 230 degF!  Ah well...

Might just have to find a flame-proof location to set it (as opposed to in the middle of my desk!) and see how long it goes and how pretty the fireworks are when it dies... ;-)

[yuandrew]

Looks like you're getting a "thermal runaway" now.

I've messed around with another fluorescent light that had been posted here before as a $10.95 12 volt fluorescent light from Home Depot. I normally refer to it as the "AmerTac fluorescent light."

http://www.fieldlines.com/story/2007/6/28/16314/5486

One of the things I discovered was that there was a (normally hidden) rheostat on the circuit board that could be turned up to drive the tube a little harder. Stock, it drew maybe around 300ma.

Take the light apart (one screw on each end) but be careful not to lose the two metal tabs that the screws go into. The Rheostat is right next to the on/off switch behind a capicator. Turn it counter-clockwise (with the switch facing you) to increase the power and set it to anywhere between 500 to 600 ma. Don't crank it all the way up like I did to my first one though: I had it drawing close to 680ma at that point and while it did give off a lot of light for it's size, the inverter failed after about 30 mins with a strong "burned out electronics" smell. I'm only running my second one at 500 ma and it's still decently bright.

Schematic

http://members.misty.com/don/amtac1.gif

You may also want to replace the stock Daylight tube with something a little warmer. Look nearby in the lighting isle at Home Depot; you should find a Philips tube in a blister pack that would fit in the AmerTac light. It has a slightly warm tint to it.