Flats Light - final chapter

[The Admiral]

Back in March (https://stargazerslounge.com/topic/265275-low-cost-light-source-for-flats/ ) I posted about a flats lights I was constructing. Unfortunately, its light output was so great that it was really totally unusable. I tried using some thin translucent plastic to reduce the light intensity, but to no avail. I figured that the only effective way to do it was to reduce the light intensity from the LEDs directly, but this was a so called 'non-dimmable' light fitting, so there was only one thing for it, take a look inside!

The light was a very simple design, comprising an annular LED-tape with 60 LEDs, edge lighting some diffusing medium. Below is a photo of the light taken apart.

There appeared to be no integral current limiting resistors, so it was expected that the current would rapidly increase with applied voltage. The light was so cheap and the LED lamp driver that came with it weighed so little I half-expected it to be a primitive affair with no isolation from the mains whatsoever. But surprise surprise, it actually contained a miniature transformer, and the circuit appeared to be quite sophisticated.

 

I connected up the LEDs of the light to a power supply and slowly increased the voltage. The unit started to light at 29.1v, drawing a current of ~30µA. At 30.9v it was drawing ~500µA, and by 31.9v it was pretty bright, and I estimate the current to have been about 5-10mA.

The next thing was to determine what series resistor to use to limit the light output. I mounted the light on my 'scope with my camera at the other end, and using the 32v adapter I let the camera determine the exposure for a few different series resistors. Interestingly, a plot of exposure time versus resistor value gave essentially a straight line! (OK, I know that there are only 3 points, but this is a far cry from just 2!)

 

I decided on a suitable value (10kΩ) and attached an interface box to the back of the light, to house the resistor and power supply socket.

When I first tested the light, it didn't work. It would have helped if the red wire from the PS actually meant positive, rather than negative . Anyway, all sorted, and here's the finished item.

 

Ian

[SteveNickolls]

Thanks for this informative thread. I particularly like your method of holding the flats light to the objective end of the tube. Good work.

Cheers,
Steve

[The Admiral]

Thanks Steve, but I can't take full credit for the method of attachment, as it's similar to the Baader solar filter arrangement.

Ian