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Ignatron triggering Circuit: Troublesome NPN Transistor

aero_jason

Aug 11, 2012
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Hey Everybody!

I'm working on my Senior Project, and a triggering circuit we recently re-did is not working. We are attempting to reproduce some work a student did last year. A piece of his circuit is the attached image (with my terrible handwriting on top). I am really confused by the way he used the two transistors, I've never seen this before. Can anyone offer some insight into the logic behind this? If there is any, that is! It certainly isn't working for us.

Thanks!!

--Jason
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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OK, you're going to have to tell us more about the student and the source of the signal.

Is the student majoring in electronics?

And what are the characteristics of the signal?

I could understand the circuit a little more if the input signal were open or grounded, or perhaps if the source of the signal was only able to sink current.

Do you really have to replicate this part? Or do you just want it to work?

Lots more information needed.
 

aero_jason

Aug 11, 2012
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Is the student majoring in electronics?

He just graduated, I believe, with a Bachelors in Electrical Engineering, and this was his senior project.

And what are the characteristics of the signal?

It's output from an Arduino Uno R3, which is a "single board microcontroller." So, it's supposed to be +5V when it's on and 0V when it's off. It's "Digital voltage" (according to my teammates) which means it's not designed to draw any current. However, it doesn't ground out, so when we turn the circuit on it's at 0.65 volts and it sits there.


I could understand the circuit a little more if the input signal were open or grounded, or perhaps if the source of the signal was only able to sink current.

Do you really have to replicate this part? Or do you just want it to work?


The full diagram is attached. We just want it to work. I just don't undertand transistor functionality very well at all, and the "triggering signal" (the 5V that is connected to the emitter of the NPN on the left of the first image) isn't getting through to the rest of the circuit. I'm pretty much stumped right now, but I'm going to dust off my circuits textbooks this weekend to see if I can't get a better grasp on why this isn't working.


Does that help at all, or am I still providing too little info?

--Jason
 

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I'm really at a loss as to what that's supposed to achievve

The arduino should be able to drive the 74HCT input without any problems, so I can only assume this is an inverter because of some issues with the PWM output (otherwise you'd just change the code to remove the need for the circuit.

I would try removing Q2 and connect the base of Q1 to the Arduino output via the 4k7 resistor.

I also wonder why 74HCT and not 74HC. It doesn't seem that TTL compatible inputs are needed...
 

CocaCola

Apr 7, 2012
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I'm not fully sure what the end goal is, but if the goal was just to invert the signal why was in not simply done in software?
 

john monks

Mar 9, 2012
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This is a typical analog circuit for minting a high speed output transition for a digital circuit.
This method is faster that what one transistor will give you. Sometime a logic level must go through it's transition quickly for the next device to operate properly.
 

gorgon

Jun 6, 2011
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This is a typical analog circuit for minting a high speed output transition for a digital circuit.
This method is faster that what one transistor will give you. Sometime a logic level must go through it's transition quickly for the next device to operate properly.


I wonder what 'high' speed means here? To get real high speed you should get rid of the pullup resistor, and add a transistor to pull up too.

How about a proper buffer (inverter) from some high speed logic family?

TOK ;)
 

aero_jason

Aug 11, 2012
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So, more updates.

We found our problems:
1) We bought the wrong transistors, they were rated for 4V and we fried them.
2) Our Arduino code "turned on" the port, but before that happened the code never declared the port as an "output," so instead of grounding out the port it was "reading" it like an input, so it hovered at like .6V instead of 0V. Than means even when we tested it with the correct transistors, it didn't work

The weirdest thing is the code worked a few months ago, and we got the ignitrons to fire. How that was achieved is a mystery to me at this point.

I don't know why he designed it like this instead of changing the "fire" signal to "low" instead of "high."
 

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My teammate found it!

For those interested:

http://www.play-hookey.com/digital/experiments/ttl_inverter.html

This is a digital inverter, which uses a transistor-transistor logic circuit

Heh, almost exactly what I said. This is a circuit to invert a signal that can sink but not source current.

Since your uC almost certainly has symmetrical CMOS outputs, you can dispense with the transistor as I said.

Your ex-student seems to have taken his design from somewhere that used TTL and he either didn't know, or didn't realise that parts of the design were there to overcome limitations in a logic family that he wasn't actually using.
 
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