Build A Nixie Tube Clock

J Aldridge writes: "People are still using Nixie tubes. Their warm glow seems to be the digital equivalent of the warm sound of vacuum amplifiers. One person has constructed a tube wristwatch."

Comments

TV and monitor power supplies

[wowbagger]

Actually, all modern power supplies ARE isolated from the power line - that's the core of a switching power supply.

First, the incoming AC is rectified to DC and filtered. This gives you 300VDC on about 10 to 1000 microfarads of capacitance - enough to kill you dead if it goes through your chest. That part of the circuit is directly connected to the power line.

Then the DC is chopped by a transistor and fed into a transformer (this is the "switching" part of the power supply). This chopping is done at between 60 kHz and 1 MHz (depending upon the power supply). The other side of the transformer (the secondary) is completely isolated from the power line. This voltage is then rectified, filtered, and supplied to the rest of the device. The voltage is measured, and fed back to the switching controller, which drives the switching transistor through either an optocoupler or pulse transformer, closing the feedback loop and regulating the voltage.

If you look at a modern switcher, you will usually see the "hot" side is completely isolated - even unto having sections of the PCB cut to prevent arc-over.

So, the bulk of the power supply, and the rest of the chassis, are NOT electrically connected to the power line. In fact, less of the system is connected to the power line than in an old style line frequency tranformer design. (Now, there were some old designs that rectified the power line and used that directly. THOSE designs were "hot" chassis designs, and were widowmakers...)

That said, you shouldn't go poking around inside a monitor or TV unless you know what you are doing. The anode voltage on the CRT is between 12kV and 25kV (although the source impedance is high enough that it won't kill you). However, the deflection circuits that sweep the electron beam across the CRT are at about 400V and have enough energy to knock you on your ass.

I've worked on TVs, monitors, radio transmitters (including tube based 250 watt repeaters (3kV at .1 amp, never work alone, one hand in your back pocket)) I design these things for a living.