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G8MNY  > TECH     28.11.23 08:13z 97 Lines 4240 Bytes #23 (0) @ WW
BID : 61228_GB7CIP
Read: GUEST
Subj: Unijunction Transistor Circuit
Path: SP7YDD<SR8BBS<SR1BSZ<EA2RCF<LU9DCE<GB7YEW<VE3CGR<PE1RRR<KA1VSC<GB7CIP
Sent: 231128/0802Z @:GB7CIP.#32.GBR.EURO #:61228 [Caterham Surrey GBR]
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 & 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.

+12V 컴컫컴컴컴컴컴컴커
       56K                                     / Tag
                    100R                     _n_
       2M       N                         b2/. .\e
    VARIABLE   UJT b2  2N                     . 
        ______   쳐컴 2646                 \___/b1
              \->
    1u ===      e 쳐컴컫컴> Pulse O/P     Base View
    C1            b1  
                     470R
 0V 컴컴좔컴컴컴컴컴컴컨컴

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 & UJT.


PRACTICAL MAINS LIVE STROBE CIRCUIT

                   +340V
          旼250R컴쩡컴컴컴컴컴컴컴컴컴커
         __  10W                     |o|  4 Jules, 50 Watt
         /_\      ===  8uF 800v        | |  Strobe tube
 Fuse o컴캑1N        "Nitrogel"       | |쳐컴컴컴컴컴컴컴컴컴컴커
   1A |   4007      Pulse Cap        |o|                       
L-o \훟      -340V쳐컴컴컴컴컴컴컴컴컴켸                        
                                                               
N-o \컴컴)컴컴컫컴)컴컫컴컴컴컴컴컴컴컴컴컴컴컴컴컴            
On/Off             68K 1W                       68K           
                    쳐컴쩡컴컴컴100R컴        1W    400V    
         __           2M Pot                        u1     
     1N  /_\   +      Variable     N            쳐컴쩡늘커   
     4007     ===                UJT       SCR__         
           100u        읕56K컴커_   쳐 2N    _\_/_      )|(  5kV
           350V    __           \-> 2646     /        )|(  Pulse
        250R        /_\'      1u     쳐쩡컴컴컫  100K    )|(  25:1
         10W         18V       ===         470R         )|( Transformer
                                   470R                 
          읕컴컴좔컨컴컨컴컴컴컴컴컨컴컴컴좔컴컴컨컴좔컴좔컴컨컴켸
                   -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 & 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 & 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
Strobe Energy = 첖Vs - 첖Vd , Where Vs = start volts, & Vd = discharged Volts.

          2                        2
But as  Vs  is much larger than  Vd  , the Vd component can be ignored.

              x 8 x 10(^-6) x 680 x 680  = 1.85 Watts/Pulse (Jules)

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 & 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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