21 November 2015

CCFL laptop inverter board for Nixie/valve testing - 1

A WORD OF CAUTION. This experiment involves dangerous high voltage AC. I take no responsibility for whatsoever damage may result in reproducing what is described below: if you don't know how to deal with HV, just read and learn.

Surfing around and reading about Nixie testing I came across a text that describes a way to check these tubes in a wireless fashion. With such a device, that is battery operated, it is possible to test Nixies found at flea markets so that you don't buy broken items.

The device produces high voltage AC through an antenna and when put close to the Nixie glass the gas inside lights up. There are commercial products starting from 12€, but a DIY solution is possible too.

The laptop CCFL (screen backlight) driver circuit fits this purpose. You need an older laptop with CCFL and not LED backlight. So all those laptops pre-2008 should do (do your homework first and check its specs before taking it apart).

The circuit usually sits in the lower part of the screen. How to get there probably will be covered in a future post. Hint: you don't need to take apart the whole laptop, just the screen. Otherwise you unscrew a lot for nothing.

Possibly do this job on a laptop you can power up, so that you can reverse engineer the connections. The driver has a least 4 inputs: broad range Vcc (like 8 to 20V), GND, enable and luminosity. The latter two signals usually take up to 3V, some are analog other digital, but in general if you pull both "high" you'll turn on the booster at max strength, that is what we need.

Here is the circuit powered with my own bench PSU and an extra battery for 3V lighting up the original CCFL:

The frequency counter can pick up the 56.1 kHz without any physical connection to the booster: radiated energy is enough! The yellow crocodile brings 3V DC to both Enable and Luminosity pins.

What happens if the CCFL is disconnected? This booster controller chip has a cut-off feature that stops HV if there is no current flow after few seconds. Anyway even without a wire going to the CCFL the frequency counter picks up the signal (at 80 kHz) and a good Nixie lights up too!

Next ... a video showing the effect on a B5092 Nixie.