Message 29376 in TECH.WW
From: G8MNY
Date: Fri, 04 Sep 26 07:19:00 Z
Newsgroups: TECH.WW
Subject: Unijunction Transistor Circuit
Message-ID: <4138_GB7CIP>
Path: N2MH4|DK0WUE|PI8ZTM|CX2SA|ED1ZAC|GB7CIP
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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Correction Apr 18)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Unijunction transistors are strange animals, not often found they have 2 bases
that are on the same bar of silicon with an emitter in the middle. They exhibit
a -ve impedance characteristic emitter to base 1 and as such easily form simple
oscillators. But as they are slow devices they are not useful for radio work.
TIMEBASE
Here is an example of a Unijunction transistor used for a simple timebase from
5 sec period to 20 Hz, for higher speeds change C1 for 0.1uF or 10nF.
HOW IT WORKS
When C1 charges up to the trigger voltage of the UJT, it fires & discharges the
C1 producing a voltage spike on the Base 1. The 100R is for protection of the
load and UJT.
PRACTICAL MAINS LIVE STROBE CIRCUIT
+340V
/_\ === 8uF 800v | | Strobe tube
-340V
This strobe circuit uses the UJT pulse circuit to trigger a small SCR that
discharges a 400V u1 cap into an pulse ignition transformer (small highly
insulated coil). The AC pulse of the 0.1uF and Transformer, the brief gate
trigger pulse & the low current through the 68K insure the SCR can't remain on
between pulses. The 100K limits the trigger circuit voltage to 200V.
This stepped up high AC voltage spike applied to the outer trigger electrode
starts up a discharge in the 50W strobe tube that discharges the special pulse
capacitor that was charged up to 680V DC.
The whole circuit is mains live and housed in a well earthed metal box with
plastic shafted pot & main switch frame mounted off the case.
The large coiled strobe tube is at the prime focus of a reflector (car
headlight sized reflector). Smaller power tubes can be used just change the 8uF
for a smaller value to suit the tube flash & continuous power ratings.
2 2
2 2
But as Vs is much larger than Vd , the Vd component can be ignored.
But @ 20 Pulses per second that is 37 Watts! Hence the 50W rating of the tube.
The common photographic flash gun tubes, are well over run at 100x brighter
than strobe tubes and last only a few thousand flashes as they suffer immense
thermal shock at each flash.
Strobe tube flashes for several tens of million of flashes!
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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