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Transistors in series Answered

if I were to put transistors in series (and hook up there bases together through proper biasing resistors) would it increase the voltage capacbility of the circuit? I need a way of switching on and off about 1000 volts really fast. for example, would 2 750 volt transistors in series be able to handle 1500 volts or not? just a HV pic from my old, dead NST

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westfw

9 years ago

No, not as you describe, because you'd have to have the base voltage of the "top" transistor up above it's collector voltage in order to get any base current flowing. I don't know if there's a configuration offhand that does what you want...

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guyfrom7upwestfw

Reply 9 years ago

hmmm... wouldn't it still work but there would be like a voltage drop of 2 volts? OR would I have to apply a bit more voltage to the top transistor, or would it be a lot more... what about a npn and a pnp back to back?

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westfwguyfrom7up

Reply 9 years ago

I'm not sure; it's been a long time since I've analyzed transistor circuits in detail.
But... If you have a total voltage V, and the transistors are OFF, then you want to have V/2 across each transistor to get the voltage multiplying effect you want. That means the collector of the top transistor (assuming NPN) is at V/2, and you need it's base to be at V/2+0.7 or so to get current flowing across the B-E junction.
(but when they're ON, the voltages drop a lot, so you only need it to be at Vcebottom+0.7 to STAY on.)
I think this sort of thing is why SCRs, Triacs, IGBTs, and assorted exotic vacuum tubes get used in most real-world HV switching circuits.

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guyfrom7upwestfw

Reply 9 years ago

huh, forgot about scrs, might be perfect in my application.

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tech-kingguyfrom7up

Reply 9 years ago

high voltage scrs are expensive, and have large copper heatsinks.

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westfwtech-king

Reply 9 years ago

Perhaps, but they're cheaper and no bigger than transistors of similar ratings. And they actually exist. (Digikey has ONE (1) transistor in stock with a Vce rating of 1000V or more. Two pages of SCRs...)

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killerjackalope

9 years ago

It might work if you made a voltage divider in the circuit and turned them on at the same time well, something like this... Now assuming the voltage is half over each one then it should work, then you just turn them on with whatever base voltage you fancy... Now if it was one KV and they were both 750V rated it might work, that would allow for differences in resistance, since small ones can be an issue with simple divides, maybe add a small resistor of the same value to each collector leg, making the tolerances higher... Please note I could be completely wrong here but it makes sense to me...

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tech-king

9 years ago

i dont think so. why do you need to switch 1kv? and why cant you get a 1kv mosfet from fairchild?

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guyfrom7uptech-king

Reply 9 years ago

I suppose I could, it's just that they are like 5 bucks.... When I could you 2 low voltage transistors for like a dollar

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NachoMahma

9 years ago

. Interesting question. Wish I knew the answer. If no one comes up with an answer, I'll ask my Dad. He's an EE, but, since his background is transmission and distribution, he may not know.