K3000F1 - Sidac Thyristor

[oc disorder]

Have this primitive lethal 240 volt strobe for repair.
From what I can tell the the Sidac Thyristor K3000F1 needs replacing
but apart from Farnell in Australia who have a minimum order of 500 I cannot seem to find a source for 1 !
Mouser list it as obsolete.
K3000F1 Littelfuse Thyristors - Diacs, Sidacs, SCRs & Triacs
Well I am prepared to buy 10.
I managed to completely ruin my multimeter with it as the capacitors were still charged...!
Anyway the owner has asked about it and although I am feeling like trashing it it has a good case (earthed) and reflector so perhaps I should redesign it
or at least make it work with more readily available components.
The K3000F1 sidac breakes down at aprox 300 volts ..is there any other way to
construct an equivalent using zeners ?
Its approx like the attached but for 240 and with more capacitance.
Tube and trigger trans have been subsituted.
No brand name on it either.
Doubt if it would be approved these days but I suppose when its all sealed up its not so lethal.
Any suggestions welcome.

[Jazz P Bass]

300 Volt sidac

You can certainly try this guy.
Information Unlimited

[R.G.]

Yep. It is primitive, and lethal.

I found the same schemo here.

Before giving up, I'd order some of Mouser's K2500G Sidacs for $0.83 each and try them. They just might work.

I also think you could get two 150V Sidacs in series to work **if** you used a high resistance, perhaps 10M or so, in parallel with each one to equalize the voltages across them; you may also need a small capacitor, under 100pF maybe, to equalize the dynamic voltage changes.
Edit: Having looked at the circuit, I think the K2500G will work fine. The biggest difference will be that it will trigger a little earlier (i.e. faster for a given pot setting).

What that circuit does is to rectify the AC line to DC, about 320Vdc for 240 in, same for a doubled 120Vac in. The rectified DC charges C3 through R2 and R3 until the Sidac breaks over. That dumps C3 through the trigger transformer, which ionizes the flash tube. That then dumps the charge in C1/C2 into the flash tube. The value of R1 is presumably big enough to prevent the AC line pulses from keeping the flash tube on and thereby melting it down on the first flash. 8-) When C1 and C2 are empty, so is C3, and the flashtube and SIDAC quit conducting.

Then C1 and C2 charge up again, and C3 starts charging through R2 and R3.

The energy per flash is what is stored in C1 and C2, that being (1/2)*11uF*320V*320V, or about 0.56J. Power is frequency times joules per flash.

The charge time on C3 is the time to charge 680nF to 300V from 320V, so call it five time constants. The time constant in question is C3 and R2+R3, which varies from 220K to 5M, so the RC is 0.149S to S, giving flash intervals of about 0.75S minimum to 17 seconds max. That seems long, so perhaps the breakover isn't that many time constants. I'm guessing as I haven't dug through the circuit in detail. But it's in the ballpark.

You could safe-en it up by putting an isolation transformer ahead of it, even though 320Vdc from a transfomer isn't all that much safer. It's low power, so a small isolator, perhaps even a pair of 24Vac devices back to back would work.

The 300V on the Sidac isn't critical.

[Steve Conner]

Plenty of other 300V Sidacs are available anyway. ST make them, but they sell them under the name "Trisil". They can't call them Sidacs because it's a trademark.

No amount of zener diodes will do. If you want to know why, read this:
http://www.st.com/stonline/products/...e/anp/5628.pdf

In a pinch you might be able to get by with an ordinary thyristor. If you leave the gate terminal disconnected, they'll often self-trigger when the anode-to-cathode voltage exceeds the rated maximum. However, reliability could be degraded, and you may not be able to find one rated as low as 300V these days.

It would be easy to modify the trigger circuit to work with a thyristor and diac, anyway.