Prototype high voltage flash tube control circuit.
Fig. 1

Unijunction Transistor SCR Photo Flash Control Circuit

by Lewis Loflin

Warning this page deal with high voltage circuits. This could cause injury or worse. I present this for information purposes only.

Date 7/31/2021. To see full size image right-click and open image in new tab.

This is a continuation of my unijunction transistors page. See Understanding Unijunction Transistors Theory Operation.

In Fig. 1 is my prototype unijunction transistor silicon-controlled rectifier (SCR) firing circuit.

Here I trigger the high voltage trigger transformer for a xenon flash tube.

See Understanding Xenon Flash Tube Circuits.

Unijunction transistor SCR driver circuit.
Fig. 2

Fig. 2 is the complete schematic to a unijunction transistor being used to fire a 600-volt SCR. In this case I used a BT137-600E triac that does operate as an SCR.

The pulse-flash rate is controlled by the 4.7uF capacitor and the 1 mega-ohm potentiometer.

A 0.022uF capacitor is charged to 300-volts through a 100K resistor.

When SCR1 is turned on by a voltage spike from B1 from a 2N2646 unijunction transistor the 0.022 capacitor discharges through the primary of trigger transformer L1.

A pulse of several thousand volts on the secondary ionizes the xenon gas in U1. This creates a conductive path for the 24uF capacitor to rapidly discharge.

This creates as bright white flash with a duration in microseconds. U1 exhibits what is called "negative resistance".

Merriam-Webster defines negative resistance as "a resistance phenomenon (as exhibited by an electric arc or vacuum tube) in which the voltage drop across the circuit decreases as the current increases.."

This is a property of fluorescent lights, NE-2 neon bulbs, etc. This allows a rapid discharge of the 24uF capacitor.

When the capacitor voltage drops below a certain level the tube turns off. The charge capacitor will begin to recharge through a 1.8K resistor.

Unijunction transistor output waveforms.
Fig. 3

Fig. 3 is the oscilloscope display of spike pulses from B1 to fire an SCR gate.

High voltage flash tube circuit with SCR.
Fig. 4

Because this circuit uses high voltage it is time to discuss a high-voltage power source.

Fig. 4 illustrates the use of a 12-volt inverter circuit. This uses a power transformer, 2 transistors, and few resistors to create a self-oscillating high voltage square wave. This is then rectified and filtered.

Another option is a voltage doubler circuit that will create a 340-volt output from a 120 VAC line. I don't recommend this unless you know exactly what you are doing.

See the following:

YouTube video Transistor HV Generator.

Flash tube high voltage generator prototyping circuits.
Fig. 5

Fig. 5 is the actual inverter power source circuit with a 12-volt battery. This powers this flash tube circuit. banner.

Related video to above:

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