Unijunction transistor LED flasher

Really ?>:-} ...Jim Thompson

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The first control system that I ever designed, when I was still an undergrad at Tulane, used UJTs. It was the throttle control system for a series of LASH ships.

I used cosine-modified-ramp SCR phase-control to drive a DC gearmotor. That drove a lead screw into a hydraulic servo that operated the 5-stage steam valve into the main turbine. Final output was 32,000 SHP. I simulated it all, including ship dynamics, on an HP9100, and then on a PDP-8.

The system hung up on our first sea trial in the Gulf. There were owners, Coast Guard, shipyard people, tech reps, crew, about 150 souls on board. I rated a bunk in a cabin, but most people slept on cots in the hold. We were dead in the water, 200 miles offshore, and I was initially blamed. Helicopters were flying in Baby Ruth bars for the owner's rep. Lots of tense, noisy meetings ensued. It was my theory that the shaft follower on the leadscrew, designed by DeLaval, had a toggle/lockup mechanism. It did.

Fun stuff, for a kid.

Using a resistor in that position, the positive doped silicon tempco of the UJT interbase resistance can be used to compensate for the negative coefficient of the emitter diode drop.

A value of approximately 1E4/(n * V+) provides temperature compensation, where n is the intrinsic standoff ratio (0.65 typical,

Best regards, Spehro Pefhany

On a sunny day (Sat, 26 Oct 2013 15:02:44 -0700) it happened John Larkin wrote in :

Yes, it should be there. I used the UJT as time base generator both for 50 Hz and 15625 Hz in a vidicon camera design. You could sync the UJT by slightly pulling that resistor down, then it would trigger early. When the oscillator was running slower by default, it would sync perfectly. UJT frequency is very stable, largely not supply depending.

I used UJTs as thyristor trigger via a pulse transformer for large 30 A thyristors,

I just bought these for fun, old times.

On a sunny day (Sun, 27 Oct 2013 08:57:16 +0800) it happened pedro wrote in :

You modulate at the receiver end, LCD shutter glasses.

On a sunny day (Sun, 27 Oct 2013 03:22:53 -0500) it happened John Fields wrote in :

Know WHAT?

Yeah, I remember that sync thing now, from the mists of time. A UJT did a lot of stuff, for a part with only one PN junction.

They would even oscillate if you connected B1-B2 backwards.

I'm happy to report that your circuit works just great with a 2N4871 that's been at the bottom of my parts bin for decades. You should've been there. It was like 1977 all over again!

Thank you again for your time machine.

On a sunny day (Tue, 29 Oct 2013 03:25:58 +0000 (UTC)) it happened Don Kuenz wrote in :

You are welcome, that is nice. Actually the cap is 47 uF, (25 was the volts), so the timing is in the ballpark.