i have a few 2N5064 SCR that are puzzling me. No matter how many hundreds of thousands of ohms i put between their gates and a 3.2V battery source, it always starts to conduct. I even put a 10 V zener between them, and it still turns on. Looking at the datasheet, i am puzzled. Where minimum gate turn on current is, there is just a dash. Same for minimum voltage. What does this mean? Will they activate with pretty much any current?

Thanks

up vote 3 down vote accepted

2N5064 is a sensitive-gate SCR. It triggers with a maximum of 200uA at room temperature (Tj = 25°C).

No minimum is specified in the official data, as you can see.

Edit: In order to keep it "off" guaranteed, you should keep the gate voltage to less than 100mV, which is guaranteed to not trigger up to Tj = 125°C or so, depending on the manufacturer. In practice that may mean that you'll have to put a resistor from cathode to gate to decrease the sensitivity.

If you really want something sensitive, play with the gate on a MOSFET such as a 2N7000. pA are more than enough typically.

  • So this means that it quite literally triggers with anything? – Anthony B Dec 1 '16 at 1:42
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    @Anthony B No. It simply means that it is guaranteed to trigger at 200 uA. It will probably trigger with less current but it is not guaranteed. – Barry Dec 1 '16 at 1:57
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    Sensitive gate thyristors are meant to be triggered by a pulse transformer between G and K. That's why there is no "off" rating, there usually isn't a connection from G to A so there isn't a chance for misfiring. – Janka Dec 1 '16 at 2:06

Since it's guaranteed to trigger from 200 uA, you'd have to limit the current to less than that to have a hope of it not triggering. From a 3.2 V source, we can compute from Ohm's law that R = 3.2 / .0002.

This tells us that if the resistor is 16K ohms (or less), it's guaranteed to trigger. It doesn't tell us just how much less might be needed to dependably prevent triggering though.

There will undoubtedly be some level of current below which it won't trigger. If you want to play around with getting at least some idea of the maximum current that won't trigger, don't play with how many hundreds of kilohms are needed. Work in something like powers of 10. If a hundred kilohms still triggers, try a few meghoms. If that still triggers, try tens of megohoms. If that still triggers, try hundreds of megohms. When you've established some bounds, then you could do something on the order of a binary search to narrow your lower bound.

Then, of course, pad your result by a fair amount anyway--even if (for example) 5 megohms is enough to prevent triggering with the specimen you have at hand, another one might trigger on half that much current...

Why would you even be wanting to place a resistor from the trigger gate to the +Vbatt??

Expecially one that wouldn't be guarenteed to turn the part on!!

If you were intending to use some type of NPN output coupler to drive the gate it would be responsible to hold the trigger off by clamping the gate near the -Vbatt. Then when the NPN went off you would need a pullup that was guaranteed to turn the part on.

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