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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 26th Dec 2018, 6:50 pm   #1
Murphy Maniac
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Default Choosing a Thyristor

I wonder does anyone know which thyristor type would be a suitable replacement for a CRS1/40 400 p.i.v. 1A thyristor. It requires a 3V trigger and about 50mA holding current. This appears to be an obsolete type and the only ones which I have found are at silly prices.
Thanks.
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Old 26th Dec 2018, 9:54 pm   #2
G4_Pete
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Default Re: Choosing a Thyristor

Hi,
one way is to use the thyristor search and filters on RS or Farnell and just filter on the specs or close to them and all the nearest type numbers pop up.
Pete
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Old 26th Dec 2018, 10:27 pm   #3
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Default Re: Choosing a Thyristor

This is close, you won't need a heatsink at 1A;

https://uk.farnell.com/littelfuse/2n...220/dp/9556753
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Old 27th Dec 2018, 11:33 am   #4
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Default Re: Choosing a Thyristor

Thanks for that, I can get on with the job now, no excuses!
Best Wishes,
Geoff.
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Old 27th Dec 2018, 6:26 pm   #5
Murphy Maniac
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Default Re: Choosing a Thyristor

I have been looking at, and comparing data sheets for the thyristor suggested (2N6397G) and I notice that the maximum trigger voltage is 1.5V.
When using an SCR1/40 a pulse of greater than 3V is required to trigger it. The pulse generator in the circuit I want to build is designed to produce such a pulse (circuit included) ie. greater than 3V. Will this damage the 2N6397G ?
If so, can anyone suggest a way out of this difficulty which does not include forking out a fortune for an obsolete SCR1/40 ?
The transistor in the trigger circuit is a unijunction TIS43.
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Old 27th Dec 2018, 6:42 pm   #6
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Default Re: Choosing a Thyristor

I think damage is unlikely, you could insert a resistor or 2 x silicon diodes in series with the to gate to drop voltage, or connect 2 x silicon diodes in series from gate to ground
to act as a 1.2V zener, cathode of one diode to ground of course.
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Old 27th Dec 2018, 6:46 pm   #7
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Default Re: Choosing a Thyristor

I'd regard it as good practice to include a low value resistor - 10Ω or so - in series with the thyristor gate.

With the circuit as it is, there is virtually nothing to limit the gate current when the TIS43 avalanches. The timing capacitor (one of C3-C6), the bottom half of the unijnction's channel, the 0.1μF coupling capacitor, and the thyristor gate-cathode are effectively all in series, forming a pretty low-resistance discharge path. But adding a small resistor will I limit the current to under an amp, which everything will be happy with, without interfering with how it's supposed to work.
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Old 27th Dec 2018, 7:30 pm   #8
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Default Re: Choosing a Thyristor

Intrigued as to what the rest of the circuit is doing now! I built something like this many years ago to fade disco lights in and out by varying the pulse frequency around the mains frequency.
Extract from the gate limits attached. (Farnell/Littlefuse)
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Old 27th Dec 2018, 8:17 pm   #9
Murphy Maniac
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Default Re: Choosing a Thyristor

That's brilliant, thank you so much for the advice. The thyristor is used to trigger a gas discharge tube (Ferranti NSP2) acting as a stroboscope with a range of 10Hz to 250Hz.
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Old 28th Dec 2018, 1:44 pm   #10
Argus25
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Default Re: Choosing a Thyristor

Quote:
Originally Posted by kalee20 View Post
I'd regard it as good practice to include a low value resistor - 10Ω or so - in series with the thyristor gate.
Good idea. I was going to suggest a 47R. One trick is to simply put in a variable resistor in series with the gate and increase the value until the scr stops triggering, then use about 1/5 to 1/4 that value. This way the SCR gate is well protected. The resistor also prevents false triggering on high frequency interference too. One other way to help limit the current is to put a small resistor in series with the UJT's B2 connection in the order of 10 to 47R.
Also I don't recognize that UJT, but a good one for the job is a 2N3980.

Last edited by Argus25; 28th Dec 2018 at 1:53 pm.
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