How does this strobe light circuit work?

I took apart the top of my strobe light and couldn’t exactly understand parts of the circuit. Could someone explain how it works?

When the strobe is plugged in, it immediately starts flashing and doesn’t stop until unplugged. The three wires are attached to a knob on the outside, that increases the flash speed as it’s turned clockwise.

• we can't tell you because we don't have the schematic to that. And since only you have access to the board and look at the traces, we have no way of getting that. – Marcus Müller Nov 16 '19 at 21:35
• Not even just a general idea of what the components are doing? I just wanna figure out what the knob specifically is doing and why it needs three wires? – RiFF RAFF Nov 16 '19 at 21:40
• aside from the resistors we can't even tell you what these components are, so no. Draw a schematic, label it with the resistor, capacitor values, and with the diode type and the type of the three-pin component (it's probably a transistor). – Marcus Müller Nov 16 '19 at 21:41
• Are you wondering why this cheap strobe has no On/Off switch? or why one would benefit from using 3 wires on a pot? – Tony Stewart Sunnyskyguy EE75 Nov 16 '19 at 22:21
• Not sure if the knob is the knob the user turns or the orange potentiometer. If it the former then the knob as three connections and is a potentiometer. The outer two pins are your power and ground while the middle is your output voltage that varies from your input voltage to ground. This is "usually" how it works. – Joey Nov 17 '19 at 0:17

Pic shows 2 diodes, 2 electrolytic caps, one thyristor, and another diode which is probably a diac, plus assorted passives. Schematic should probably not be entirely unrelated to something like this:

Diodes at the input and two electrolytic caps form a voltage doubler which generates a DC voltage equal to double peak AC voltage.

1µF cap is slowly charged through the 1Meg pot. When it has enough voltage to trigger the diacs (there are two on this circuit for some reason) they fire and the thyristor conducts, discharging the 1µF cap into the transformer primary. This creates a high voltage spike on the secondary which triggers the xenon tube.

It's a simple circuit, but:

Capacitors charged to high voltages are dangerous and this is essentially a ghetto defibrillator. Do not mess with this circuit, do not bring anything conductive near it (including fingers) unless you know what you're doing...

This circuit is probably similar to what you have. It is from my high school electronics class, 45 years ago. The first project that I ever built. Too dangerous to be built in schools today. THIS CIRCUIT CAN KILL YOU!

C1/D1/D2/C2 are a voltage doubler. C2 quickly charges to about 300VDC. C4 also quickly charges to about 160V.

C3 slowly charges until the threshold of the diac is reached. It then discharges through the SCR gate. This causes the SCR to conduct, which causes C4 to discharge through the trigger coil, causing a high voltage pulse on the trigger coil secondary, which ionizes the Xenon gas.

When the Xenon gas is ionized, it creates a path for C2 to discharge. The C2 energy is what causes the flash. A higher value C2 will cause a brighter flash, but you won't be able to fire it as often. After C2 is discharged, the cycle repeats.

The trigger coil used to be sold by Radio Shack. It has a very high turns ratio, 1:100, probably more.

Why are all 3 leads connected to the potentiometer? It is an old habit of EEs. With only the wiper and one end connected, a intermittent wiper would cause infinite resistance. With the 3rd lead connected, an open wiper will default to be the same as fully CCW, or fully CW.

Note that since Circuit Lab doesn't have models for the SCR or Diac, equivalent circuits were used. I didn't model the Xenon tube, so the plots are not valid after the SCR fires. C2 would normally discharge through the Xenon tube and the cycle would start over again.

To keep the plot readable, the trigger transformer only has a ratio of 1:5, in reality it is much, much higher.

Since I was not allowed to have 2 editable schematics in an answer, the first was changed to a picture.

simulate this circuit – Schematic created using CircuitLab

A variable resistor with TWO leads has one connection to the end of the resistor track and the other to the "wiper". The resistance between the leads varies as the wiper is moved along the track.

A variable resistor with 3 leads has a wire to each end of the resistor and one top the wiper. As before you COULD use this as a variable resistance between the wiper lead and either of the ends - as one resistance increases the other decreases.
However, the more usual use - which has a hint in the name "potentiometer" is to apply a voltage across the whole track and to "tap off" part of the voltage with the variable wiper. A "potentiometer" is a device which supplies variabkle "potential" = voltage.

In the two example circuits provided by others the resistor is used in effectively the 2 wire mode (joining one end to the wiper does not affect the result) but many circuits make use of the voltage variable aspect. If you load the 'pot' wiper lightly (eg the resistive load is a high value compared to track resistance value - eg 1K pot track resistance and 100k load resistor), then the tapped off voltage is not much affected by the load. If the load resistance is small (similar to track resistance or less than say 10 times larger) then the load will affect the tapped voltage and the calculations, while not complex, are affected by the load.

In this example the 3 wire arrangement may supply an "effective voltage" established by the pot position to establish a reference or trigger voltage or a charging current for a capacitor.

A clear well focused well lit photo of BOTH SIDES of the PCB would help.