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Advice to get 100v fixed dc output from unregulated 120v input

Nirmal Padwal

New Member
I am posting my question though I am unsure in which section this question should go.

For my college project, I need 80V dc voltage to operate a neon bulb. For which, I have been able to bring 240V AC mains supply down to 120V voltage fluctuating between 100V and 120V. I now desire to stabilize this voltage to around 110V and then use a voltage divider circuit to get 80V operate the neon bulb.

I am unsure as to what voltage regulator I should use to stabilize the 110V. I am currently thinking of TL783. But I am concerned if a heat sink will successfully get rid of all the waste heat. Is there any other alternative to TL783?
 

Pommie

Well-Known Member
Most Helpful Member
As neons use very little current, you could use a resistor and zener to get the required voltage.

Mike.
Edit, the neon may act like a zener so only a series resistor would be required.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Your entire premise is wrong - you don't need 80V to operate a neon, and you don't need DC either.

The sensible way to do it, and the way it's always done, is a simple series resistor directly from 240V AC mains - because it's AC both poles of the neon light up, giving double the brightness, and an extended life for the neon bulb.

You can even buy them ready built, so front panel mounted indicators housing a neon and resistor for direct connection to 240V AC - look at pretty well every guitar amp ever made :D

I appreciate that it's a college project, and we don't know what your actual instructions were, but if it's anything like like what you said? - either the tutor is a complete idiot, or it's a test to see if you fall for it. You wouldn't power a neon in anything like the way you suggested.
 

crutschow

Well-Known Member
Most Helpful Member
What is the purpose of the neon bulb?
 

ronsimpson

Well-Known Member
Most Helpful Member
You wouldn't power a neon in anything like the way you suggested.
Neon bulbs have a "fire voltage" and a "hold voltage". Each bulb is different but lets say it takes 80V to turn on the bulb but only 60V to keep it going. When you use a regulated bench power supply the bulb will pull no power up to 80V then it will pull a huge amount of power and load down the supply. (do not power a Neon bulb directly from a power supply)
Normally you use a power source (ac or dc) that is well above the fire voltage of all bulbs. Then a resistor is used to limit the current to the uA or mA level. 100V will probably fire the bulb. The current is then set by; 100-60=40V across the resistor.

There is nothing wrong with using 240vac and a large resistor.

If you job is to measure the fire voltage; then you should use the (bulb + resistor). You will turn up the supply slowly, when the bulb turns on read the voltage of the supply not the voltage across the bulb. (because the bulb voltage dropped from 80 to 60 faster than you can measure ..... <uS)
 
Neon bulbs have a "fire voltage" and a "hold voltage". Each bulb is different but lets say it takes 80V to turn on the bulb but only 60V to keep it going. When you use a regulated bench power supply the bulb will pull no power up to 80V then it will pull a huge amount of power and load down the supply. (do not power a Neon bulb directly from a power supply)
Normally you use a power source (ac or dc) that is well above the fire voltage of all bulbs. Then a resistor is used to limit the current to the uA or mA level. 100V will probably fire the bulb. The current is then set by; 100-60=40V across the resistor.

There is nothing wrong with using 240vac and a large resistor.

If you job is to measure the fire voltage; then you should use the (bulb + resistor). You will turn up the supply slowly, when the bulb turns on read the voltage of the supply not the voltage across the bulb. (because the bulb voltage dropped from 80 to 60 faster than you can measure ..... <uS)
Actually, I focused on what did he want and didn't care about what suggestion is good for him.
 

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