Power indicator LED in High Voltage Circuit

Well there's no picture of the actual unit, but I'm betting it's either an actual 115V bulb or a package deal since the listed rating is 105-125V.

The Neon Bulb requires a Series Resistor to Limit current to it.
For an NE-2 bulb, a 1/4 watt resistor with a value between 22k and 100k is suitable.

If I Remember correctly:
On an AC Supply, Both Probes in the bulb create light
On DC, only one side Lights

alec_t said:

Would a neon be happy with a DC supply? I don't see why not, but I've never tried that.

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Yes, and the Negative lead will glow. When a Neon bulb is used with AC at for example 50 / 60 Hz while both elements appear to glow they are actually alternating at the line frequency.

Also as MrAl points out, when using a Neon bulb a series resistor is needed. When the gas in the bulb ionizes the bulb effectively becomes a short, the same principal used by early VR (Voltage Regulator) valves (tubes). A typical NE2 lamp has a breakdown voltage AC of 65V and DC of 90 V. The current is about .3 mA (300 uA). The breakdown voltages are approximate. Just remember when using a Neon bulb the series resistor is essential. Leave out the series resistor and bad things happen.

Ron

Hi again,

Ron:
Thanks for mentioning the series resistor again, as that is very important to the life of the bulb and makes a big difference if it works or burns out within 24 hours.

Long long time ago i thought all neon bulbs where the same, so i used a typical value resistor, but then i found out that some can only take light current while others can take a higher current. I also found that there are different suffixes to the "NE-2" base part number, like different letters.

For example, one bulb might require a 100k resistor while another bulb (different part number) could put up with a 30k resistor and be brighter because of it. But use that 30k on the bulb that requires 100k and say goodbye to the little neon bulb.

If i remember right, there is a slight difference in size too. The bigger envelopes can take more current and glow brighter while the slightly smaller envelopes can only take the lighter current (100k resistor on 120vac) and dont glow as bright.

Probably the best bet is to look up the part number or find out from the place where you buy the bulb, that why they can recommend the right resistor size for a given voltage like 120vac, then you can adjust for a different voltage if needed.

Reguardless of the size of the NE-2, I always find it better to use a Higher value resistor on these Neon Bulbs.
Than they last longer.

Usually Brightness is not all that important anyway.

Gary

A neon with the correct resistor is often salvagable from a defunct domestic appliance.

alec_t said:

A neon with the correct resistor is often salvagable from a defunct domestic appliance.

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My freezer is at least 30 years old (it still works perfectly) and its neon bulb indicator has not worked for about 20 years.
Maybe it burnt out, wore out or leaked its neon gas out.

Nixie tubes were neon, weren't they?

audioguru said:

My freezer is at least 30 years old (it still works perfectly) and its neon bulb indicator has not worked for about 20 years.
Maybe it burnt out, wore out or leaked its neon gas out.

Nixie tubes were neon, weren't they?

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Off the top of my head I don't remember, nixies where many years ago.

About 53 years ago I made some Christmas gifts in a project that used 10 Neon bulbs, a 90V battery, some resistors and capacitors making a chaser circuit. I cast them in clear plastic. They operated for months.
But the first one BLEW UP because I forgot to make a vent hole for the battery gasses.
I told everybody to get rid of them.

Hi again,

chemelec:
That's a good idea too if you dont need the brightness why sweat it right?

Alec:
Yes that's a good idea too if the neon still works. My problem with neon's is when i got switches with neons in them in some appliance or something, after a couple years they neons would burn out and it's very difficult to replace them inside the small switches. I end up replacing the whole switch.

audioguru:
Wow that's great that it still works after 30 years. I would not mind as much about the neon as long as the freezer still works. Maybe you can replace it, maybe with a new LED and resistor.
I also made a project with two neons when i was very young, with only two neons. The two would blink, one turned on and the other turned off, then vice versa, so it was like a multivibrator but without transistors. Only used a couple caps and a couple resistors and two neons, run off of 120vac, not sure if it had any diode but probably did. This was a really long time ago too
The neons are interesting because of their negative resistance which allows them to be used in an oscillator circuit without any transistors or anything else like that.

audioguru said:

My freezer is at least 30 years old (it still works perfectly) and its neon bulb indicator has not worked for about 20 years.
Maybe it burnt out, wore out or leaked its neon gas out.

Nixie tubes were neon, weren't they?

Click to expand...

Yes, Nixie Tubes were / are neon. They use a mesh grid for the positive (+) and the numbers are actually just the negative (-) side. They use a series resistor on the + side since only a single digit should be on at any given time. Heck, don't care for orange? Change the gas.

Years ago the old fluorescent tubes that used a "starter" used a small neon bulb with a capacitor in them. These were the old ballast style with a starter that had a pair of bayonet lugs to twist in place. When I was a kid I would take the starter apart and add a resistor. The cap would slowly charge till the gas in the tube ionized at which point the tube would discharge the cap. Little neon blinking light. While boring by today's standards it was amusing in 1959 or so.

Ron

I have Both the Older, Larger NE-2's and the Newer Smaller NE-2's.
On 115 VAC, Both light up with Reasonably good Brightness with a 100K resistor.
Even Good with a 150K.

Super Bright with just a 22K, But I doubt it would last Long!

And Both will also light with a 470K, But about 1/3 of the 100K brightness.

One way to use an LED without a lot of wasted power, and to show that both the filament and the plate voltages are present, would be to drive the LED from the filament voltage. But put a small NPN transistor in series with the LED and it's resistor. Drive the base of the transistor from the plate supply. Since you need very little base current, the base resistor can be in the meg ohm range.

Hi,
I saw many power line (AC 220V ~ 240V, 50Hz) indicators using neon bulb + series resistor since donkey years ago. But in the past 10 years also saw increasingly using discrete red LED + a series resistor (around 100K ohm, 1/4W). I like to know if it is better to use LED instead of neon bulbs now ? Because of cost, life span, brightness, or others reasons ? Are neon bulbs used as power supply indicators will be taken over by LEDs in days to come ? Technically I also like to know why LED with its natural low reversed breakdown voltage (I think it is about few volts) can withstand the high AC voltage when it is being reversed biased (during the non-conducting half cycle of AC power). Many thanks!

I also like to know why LED with its natural low reversed breakdown voltage (I think it is about few volts) can withstand the high AC voltage when it is being reversed biased (during the non-conducting half cycle of AC power).

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It doesn't have to withstand it; there's another diode (could be a LED) in anti-parallel. Each diode only sees the Vf of the other diode.

Most LEDs will stand the reverse current. The actual breakdown voltage is often a lot higher than the rating, so the majority of the power is dissipated during the reverse half cycle, but it usually doesn't blow the LED.

Many devices either have a rectifier for the LED. If the current limiting device is a resistor, there is often a diode in series to block the reverse voltage.

Often the LED is connected to the low voltage supply that is there for other functions within the appliance.

Hi,

It is better to use an LED these days than a neon unless you have some special requirement that an LED can not be used for.

The LED will last longer if biased properly and also it will still light up if the voltage goes down a little. Some neon's dont like to light up with lower than standard line voltage so they can be a pain sometimes.

There are however several ways to drive the LED. The different ways can be classified based on the number of diodes used with it.

First we have the single diode in anti parallel with single series resistor. This works, but requires the resistor power rating to be twice the power that is actually needed if two diodes are used, so the resistor has to be bigger because the resistor conducts even when there is no light being produced.

The two diodes version uses one diode in series and one in anti parallel. The series diode lowers the power rating requirement of the series resistor. The anti parallel helps reduce the risk of the LED getting a reverse bias because of leakage or capacitance of the series diode. The anti parallel diode may be left out but then there is more risk of blowing the LED.

The four diodes version supplies more power to the LED and helps to eliminate flicker. This is basically a bridge rectifier with a series resistor to limit current.

Then we have the zero diodes version. This requires two LEDs however, connected in anti parallel. The series resistor always conducts, but when it conducts it always produces light in one of the LEDs.

Sometimes capacitance is included to help filter the DC and so supply a smoother current to the LED which reduces motion flicker.

Also, sometimes a series capacitor is used to help reduce power consumption and get higher brightness. This is a bit more complicated and is usually not needed in an indicator if the LED is a high brightness type.

I still puzzle how a single LED with a single current limiting resistor (100K, 0.25W) can work under high stress at mains voltage (240V 50Hz). On the CREE 5mm LED datasheet, the reversed breakdown voltage is only 5 volt, and during the non-conducting half cycle (no light producing), the LED receives reversed voltage as high as 339V (peak values of RMS 240V), without collapsing and without any diode protection.....only two components, one LED and one resistor. Fyi, I tested before a LED (only one time) in reversed biased at 15V from a desktop power supply unit and it was gone instantly!

If the LED does not blow up with the very high peak reverse voltage then the power in the poor little 100k, 1/4W resistor is half of 230V squared/100k= 0.265W so the resistor will be extremely hot.

Modern resistors are made with an insulating tube covered with a carbon film. The film is cut to make it a long helix. The cut arcs at a voltage higher than about 250V. Higher voltage resistors are available.

steven7890 said:

I still puzzle how a single LED with a single current limiting resistor (100K, 0.25W) can work under high stress at mains voltage (240V 50Hz). On the CREE 5mm LED datasheet, the reversed breakdown voltage is only 5 volt, and during the non-conducting half cycle (no light producing), the LED receives reversed voltage as high as 339V (peak values of RMS 240V), without collapsing and without any diode protection.....only two components, one LED and one resistor. Fyi, I tested before a LED (only one time) in reversed biased at 15V from a desktop power supply unit and it was gone instantly!

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Hi,

You do not run the LED with only a resistor, you have to use also at least one diode with it. The diode protects the LED. Two diodes even better.