Electronic gas lighter

Outside look

This gas lighter (sold by Blokker in Europe) is powered by 1x AA battery, and generates 4KV (approximately) pulses at a 20 Hz-rate when pressing the button.
I thought it might be interesting to take a look inside and see how the high voltage pulses are generated.

Inside look

1. High voltage output wires to the tip of the gas lighter

2. High voltage capacitor (rated 2 KV)

3. High voltage transformer

4. Switch to connect the AA battery to the circuit

5. Blocking oscillator coil

6. Transistor of the blocking oscillator.

Circuit diagram

Q1, R1 and the primary of T1 form a self-oscillating blocking oscillator running at a frequency of about 6 kHz. The positive pulses at the base of Q1 have a width of about 10us and a peak voltage of about 12V. T1 has a secondary winding with about 100x the amount of turns of the primary, resulting in high voltage pulses of 10us with a frequency of 6 kHz, which is a duty cycle of about 16%.

D1 rectifies the voltage at the secondary winding, so only the positive pulses pass. C1 is grounded via the primary winding of T2 and is slowly charged with the 16% duty pulsed current. The interesting part of the circuit is D3, which is a reverse-biased regular 1N4148 diode, that will break down at minimum 100V reverse voltage. So when C1 reaches about 100V, D3 will break down and will provide the gate of the thyristor THY1 (SCR/Thyristor) of enough current to start conducting. The SCR used here is a sensitive gate type, meaning it will trigger (start conducting) at gate currents of 40 to 200uA.

When THY1 conducts, it will connect C1, that is charged to 100V, across the primary of T2. This means that C1 will discharge into the low resistance primary of T2, causing a huge but very short current spike in the primary of T2. The turns-ratio of T2 is about 50x. At the secondary winding of T2, we find a voltage peak of 4 to 5KV. C2 is added to form an LC-resonance circuit with the inductance of the secondary winding of T2. So the output voltage is not a sharp peak, but rather stretched out due to the self-resonance of the LC circuit. Due to C2, the energy at the output is available for a longer time-span, generating brighter sparks.

To finish the story : when C1 is completely discharged and the current is lower than the hold current of THY1, THY1 will stop conducting, so C1 is not 'paralleled' with the primary winding of T2 anymore and can start charging again. It takes about 50ms to charge C1 to 140V (in my lighter, the break-down voltage of D3 was 140V), so we have a repetition rate of about 20 Hz between the high voltage pulses at the secondary of T2.

The flash of a camera works similarly, by generating a high voltage with a fly-back transformer from a single 1,5V battery. A capacitor is charged to this high voltage, and its energy released into the trigger-coil of the xenon flash tube when the flash is activated. This generates a short high voltage pulse that ignites the xenon flash tube.

Tasers also work similarly to generate high voltage pulses.
Fascinating how you can generate any voltage from a 1,5V battery using rather simple electronics. The energy that can be delivered in these short high voltage pulses is limited and not life-threatening. The energy at the output is the energy stored in C1, which is released into T2 and is equal to 1/2 * C * V2 = 0.5 * 100nF * 140V = about 0.001 Joule.
10 Joules is considered hazardous.
50 Joules is considered lethal, independent of the time in which this energy is released.

See also : High voltage pulse generator (>30KV)