# How to calculate the resistor value of a xenon flash circuit to be a set frequency

How can I calculate the value of R2 in the following circuit to have the xenon flash (the rate of the charge/discharge of the capacitor) be set at 10Hz? https://www.eleccircuit.com/xenon-strobe-light-110v-by-scr/

Is this a simple RC calculation? Where R2 is the value we're solving for, 0.1uF is the value of the capacitor and 10Hz is the time constant?

• The neon sets the trigger voltage at around 90V. I’d suggest trying different values of R2 and measuring the flash rate. Feed the numbers into a spreadsheet and graph the results. You should be able to interpolate from that to get the desired flash rate. – Kartman Apr 19 at 1:57

Steps:

Determine the breakdown voltage of the neon tube

Calculate when that voltage will be reached at C1 using R1/C1 to calculate the time constant

While it looks like R2/R3 form a divider that sets the time constant, before NE1 fires, it is an open circuit making the current through that leg 0, so there is no voltage drop across either resistor until NE1 fires, and at that point the SCR is triggered and the flash fires.

That will give you the (approximate) time constant. Change R1 to change the flash rate, but be careful about the power being dissipated by this resistor. This will be temperature dependent.

If you need exact timing, replace SCR1 with an SCR Optocoupler, eliminate NE1 and drive the led on the optocoupler with a 555 or a microcontroller generating a 10hz pulse.

A possible optocoupler is the TLP548J(TP1,F) or the IXYS PS2401.

The average voltage for a half-sine wave is as follows:

$$\V_P=\sqrt{2}V_{RMS}, V_{Avg ~\lambda/2}= 0.45 V_{RMS} = \dfrac{V_P}{\pi}= \dfrac{\sqrt{2} V_{RMS}}{\pi} \$$
$$\V_{trig}=90 V = 37.5 \text{% of } 240 V_{rms}, \text{while } T=RC = 64 \text{% of input }(V_{avg})\$$
thus$$\ V_{trig} \text{cycle time }= \dfrac{37.5}{64} RC \$$

But due to the 50 Hz ripple it will be a very noisy 10 Hz with triggers in 4, 5 or 6 cycles R2 from 2 M to 3.3M and C= 0.1uF.

• then with resonant crosstalk to SCR your could get also sorts of harmonic spikes sparks and smoke from R1, so I would not recommend this circuit. Plus no cell phone would work in any distance that the flash can reach from AGC disturbances.