Schematic Circuits

Pocket Neon Glow Lamp Tester

The neon glow lamp tester is a staple in every electronics hobbyist’s lab. Here’s how to build one right in your own home.

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Neon glow lamp?

You may wonder what’s so cool about the “historical” neon glow lamp. For starters, it can serve as a key element in many types of circuits, and it’s easy to use as the basis for a number of projects. Fortunately, you can still purchase miniature NE-2 neon glow lamps from most online storefronts.

In principle, neon glow lamps are commonly recommended for high-voltage AC and DC applications. A standard-brightness neon glow lamp is constructed by mounting two electrodes within a small glass envelope, usually filled with a neon/argon gas mixture. When proper striking/starting voltage is applied, the gas ionizes and starts to glow, allowing a very small current to travel from one electrode to the other. After the ionization, only a lower voltage (usually 10–20 V below the striking voltage) is required to sustain the operation of the lamp. When the neon glow lamp is on, the voltage across its terminal is called operating voltage or burning voltage, which is fairly constant with lamp current variations. When the lamp is glowing but the current is low, only part of the negative electrode is glowing (normal glow) and the lamp behaves like a negative resistance (i.e., increasing the voltage will result in a decrease of the current). And when the lamp is glowing but the current is higher, the entire surface of the negative electrode is glowing (abnormal glow) and the lamp behaves like a normal resistance (i.e., increasing the voltage will also increase the current). The typical light output color of a clear glass neon glow lamp is in the orange-red range of 600–700 nm. Different colors (like green or yellow) are available through secondary emission by coating the inside surface of the glass envelope with phosphor.

After ionization, the neon glow lamp requires an appropriate external series resistor to limit the current flow. To calculate the value of that series resistor, just subtract the maintaining/sustaining voltage (90 V typical) from the input supply voltage, note the required current value (0.5–3.0 mA typical), and then use Ohm’s Law to calculate the series resistance value.

Neon glow lamp tester?

I’ve always been entranced by neon glow lamps, as they have many interesting electrical properties besides making a pleasant glow in the dark. For a quick test, a neon glow lamp can be connected to the mains voltage with a single current-limiting (light ballast) resistor. However, the “dangerous” test method is intended for experienced hobbyists only because it’s directly connected to the fatal mains and introduces the risk of electrical shock. A safe test method is the usage of a battery-powered neon glow lamp tester that provides a comfortable test socket to plug in the neon under test.


A pocket neon glow lamp tester basically consists of a battery-powered high-voltage generator self-contained in a small plastic box with test switch and test socket. While they are luring devices, they are also expensive, so from an experimenter on a budget’s viewpoint, I had to make my own. The schematic drawing of my pocket neon glow lamp tester, achieved with the help of a few cheap parts collected from my junk box, is shown below:

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What you see here is the simplest form of a one-transistor primitive self-oscillating inverter circuit configured to run on a 5-VDC supply. The key component of the circuit — the six-pin inverter transformer (TR1) — is, in fact, a 5-V SMPS transformer (also called a chopper transformer) lifted from a defunct smartphone USB wall charger’s circuit board. The chopper transformer is wired in reverse mode here, meaning that its original primary coil is treated as the secondary (output side) coil, and vice versa.

S1 in the circuit is the power on/off switch, while S2 is the test switch. The NE-2 neon glow lamp to be tested can be plugged directly into the test socket (NL1), as there’s a built-in ballast resistor R2. The 5-VDC input seems to be enough to generate near 500 Vp-p on the output with the neon glow lamp under test as the output load, and that “raw” output is available through the cinch connector X1. The observed current consumption of the tester is 300–350 mA at 5 VDC, and the frequency of oscillation hits 300 KHz. Note that the high-voltage output of the tester can be dangerous for those hypersensitive to electricity. I can feel a bit of tingling (not an electric shock) when I touch the raw output terminals.

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Tester construction hints

Even though the tester circuitry seems extremely simple, handling the chopper transformer (our inverter transformer) is not very easy, as it’s the trickiest part of the tester construction. You can use literally any 5-V chopper transformer from any USB charger circuit board if it’s a type with a total of six pins — one primary coil, one auxiliary coil, and one secondary coil (refer to the next image). Usually, you can see the primary (high resistance) and auxiliary (low resistance) coils on one side, while the secondary coil (low resistance) is on the other side of the chopper transformer. The actual “mileage” might vary largely from transformer to transformer, so remember to monitor the high-voltage output with an oscilloscope (not with a cheap digital multimeter) immediately after the construction (i.e., before jumping to the first neon glow lamp test). Quite often, it should become necessary to change the value of the ballast resistor (10K–1M) to get an appealing result. Likewise, if there’s no effective oscillation, then merely swap the connection leads of the auxiliary coil or the secondary coil (not both at the same time).

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Below is a photo of my pocket neon glow lamp tester in action. At the time, I used a cheap mobile phone power bank to power up the entire tester circuit.

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You should go further…

The pocket neon lamp tester circuit presented here is enormously flexible. By choosing different components, you can easily modify the entire design for other purposes. For example, you can build a high-voltage DC generator/pulser for your next neon ring counter project or a pocket high-voltage conductivity tester for occasional examination of your “heritage” high-voltage diodes/tubes/display panels.


The following casual plots show the output of the pocket neon glow lamp tester when the oscilloscope was cross-probed across the output connector X1 (with a neon glow lamp inserted in the test socket). Look closely, as you may be inspired!

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Keywords : Pocket, Neon, Glow, Lamp, Tester
Writer : delon  |
13 Jul 2019 Sat   
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