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HVPaul

Fri Aug 28 2009, 05:58PM

Registered Member #2321 Joined: Fri Aug 28 2009, 05:13PM
Location: Boston, MA
Posts: 34

Hi All,

I'm long-time lurker first time poster.

I'm interested in building an inverter or boost circuit of some kind to power a 90 - 100v
tubular neon indicator lights. Now this is very application-specific so the search I've done
hasn't turned up a solution for this type of case scenario.

This inverter is to be used to drive neon lights as power and HD indicator lights in a PC computer case instead of LEDs. My idea is to have common HV source driven either by the 5 or 12 volt rail boosted up to 90 or 100 volts DC and then use a transistor controlled by the motherboard's HDD pins to switch the neon light. Power neon will be driven directly off the HV.

I have found what seems to be a perfect circuit snippet for this purpose but I'm trying to fill in the blanks and find out in general how safe this solution would be. After all, computers are sensitive devices and i wouldn't want 100v transients entering the 5/12v rails.

Here's a snippet that intend to use. Of course adjusting cap and inductor values for the target
voltage.

I probably will use a 555 to drive the transistor and switch the output stage with another.

What are you thoughts? I'm very much a newbie when it comes to designing circuits so it'll probably end up in ashes :)

Here's another one that is a complete nixie tube power package. I've yet to figure out how to adjust its output voltage down
and what parts I'll need to control the output stage for switching the HDD neon.
Nixie supply

Any ideas guys?

I'm feeling kinda dumb suspecting that solution is staring me right in the face :)

Myke

Fri Aug 28 2009, 09:41PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Is it one of those neon indicator lamps or one of those tubular CCFLs? If it's a CCFL, I would use a current driven royer. For driving a normal NE-2 type indicator, a buck boost converter using an inductor and a 555 should be fine. If you fear those transits on the voltage rails, you can put a zener diode across the rails with a current limiting resistor. Don't forget caps to smooth the supply voltage as well (something like 220uF and a .1uF cap)

klugesmith

Fri Aug 28 2009, 09:49PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Welcome to the forum.

A plain boost converter 12V:90V will serve you well, and not put high voltage spikes on the supply rail.
Google can direct you to plenty of design notes.

For any design you need to start by specifying the minimum and maximum load current. If there's a guaranteed minimum (such as your power indicator or even a dummy load resistor), you can design for continuous current (in the inductor) mode. Then no need for voltage regulation -- a simple fixed-duty-cycle 555 circuit can drive your high voltage transistor for a fixed voltage boost ratio. Get a second HV transistor of the same type to turn your HDD indicator on and off. Can do fine with parts like

It might be even simpler to use a separate inductor and boost transistor for each lamp. Simply place the neon lamp in parallel with the transistor (can skip the diode, capacitor, and current limiting resistor on HV side). Use non-continuous current mode, with average output current proportional to transistor on-time and rep rate. The two switchers can share a timer for base(gate) drive, with one also needing an enable signal from HDD activity. Could use a photo-isolator if you're feeling nooby and really want to play it safe.

jonny5

Sat Aug 29 2009, 05:41AM

Registered Member #1807 Joined: Tue Nov 11 2008, 07:36AM
Location: San Luis Obispo, California
Posts: 19

One disadvantage of simple boost converters: no inherent way to stop current flow from the input to output. If your boost experiences a shorted output, your computer 5V/12V rail is shorted as well (through the inductor and diode). A fuse would be a good idea. Flyback and buck-boost topologies are far more complicated, but the current-controlled versions can often tolerate a fault (i.e. output short circuit) indefinitely. You could always use a separate supply (wall-wart type?) for the boost, possibly saving your computer supply from a worst-case scenario.

Proud Mary

Sat Aug 29 2009, 11:36AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I'm all for an easy life, Paul, so if I lived in the USA, with a 110V mains supply, I'd simply knock that down to 90V. So long as a circuit is properly thought through, then the fewer the components, and the less likely to fail over time.

GeordieBoy

Sat Aug 29 2009, 12:07PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

> It might be even simpler to use a separate inductor and boost transistor for each lamp. Simply place the neon lamp in parallel with the transistor (can skip the diode, capacitor, and current limiting resistor on HV side). Use non-continuous current mode, with average output current proportional to transistor on-time and rep rate....

That is exactly how I would do it myself. Let the neon bulb act as the clamp for the flyback pulse from the boost inductor. Then it will effectively set the output voltage from the converter equal to the voltage drop of the ionised neon gas.

-Richie,

teravolt

Sat Aug 29 2009, 04:36PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

Im with Harry use a small stepdown transformer and make a power supply then use a mosfet to switch the voltage through the neon. you may also want to use a opto isolator to seperate your computer from the circuit. I'd be carefull about what you atach to your computer supply because any failure in your circuit could leed to computer death

Steve Conner

Sat Aug 29 2009, 05:27PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

For switching, check out the MPSA42, it's a high voltage TO-92 transistor. I've seen them used in nixie clocks and the like.

To supply a neon lamp you normally want at least 150V, and a series resistor to limit the current. Neons are like LEDs but worse: they have a negative resistance, so the ballast resistor is compulsory.

I once built a 9V to 150V boost converter using the MC34063 with a high voltage switching transistor added on, in this case the MJE340.

You may want to optoisolate this whole mess from your motherboard's HDD activity line.

Heck, if you used optoisolators, why not just run the HDD activity light off the mains too? Neons meant for AC can look funny on DC anyway, because only one of the electrodes will glow, as Klugesmith demonstrated with his mechanical rectifier in another thread. And the MPSA42 is rated 300V, so it could switch 120V line.

HVPaul

Sat Aug 29 2009, 06:20PM

Registered Member #2321 Joined: Fri Aug 28 2009, 05:13PM
Location: Boston, MA
Posts: 34

Wow, thanks for your insights guys!

I had no idea this would be so complicated. Seems like isolating option is
the best solution for this problem.

GeordieBoy: what would such circuit look like?

GeordieBoy

Sat Aug 29 2009, 09:21PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

> GeordieBoy: what would such circuit look like?

Just like a normal boost converter, but with the neon bulb connected directly across the boost switch. No boost diode or output smoothing capacitor required.

Operation is as follows: When the boost switch is closed the current in the boost inductor ramps up, when the switch is opened this current transfers to the neon lamp and ramps down. You might need an RCD snubber across the switch to prevent spikes if the neon gas takes a while to ionise.

One seperate converter for each neon lamp.

-Richie,

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