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home | newest check | boards | help index | log | ps | userlogin | send sysop | slog | status forward | bcm news | users | version | remove cookieG8MNY > TECH 19.04.24 16:31l 96 Lines 4239 Bytes #0 (0) @ WW BID : 5769_GB7CIP Read: GUEST Subj: Unijunction Transistor Circuit Path: JH4XSY<N3HYM<GB7YEW<LU9DCE<LU4ECL<ED1ZAC<GB7CIP Sent: 240419/0729Z @:GB7CIP.#32.GBR.EURO #:5769 [Caterham Surrey GBR] $:5769_GB 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 トトトツトトトトトトトトトトトトトソ 56K ウ / Tag ウ 100R _n_ 2M N ウ b2/. .\e VARIABLE UJT b2 ウ 2N ウ . ウ ウ______ テトトトル 2646 \___/b1 ウ \->ウ 1u === e テトトトトツトトト> Pulse O/P Base View C1 ウ b1 ウ ウ 470R 0V トトトトチトトトトトトトトトトトトトトチトトト 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 レトト250Rトトツトトトトトトトトトトトトトトトトトトトトソ _ウ_ 10W ウ |o| 4 Jules, 50 Watt /_\ === 8uF 800v | | Strobe tube Fuse oトトトエ1N ウ "Nitrogel" | |テトトトトトトトトトトトトトトトトトトトトトトソ 1A | ウ4007 ウ Pulse Cap |o| ウ L-o \トo ウ -340Vテトトトトトトトトトトトトトトトトトトトトル ウ ウ ウ ウ N-o \トトトトト)トトトトトツトト)トトトツトトトトトトトトトトトトトトトトトトトトトトトトトトトトソ ウ On/Off ウ ウ ウ 68K 1W 68K ウ ウ ウ ウ テトトトツトトトトトトトト100Rトトソ 1W 400V ウ _ウ_ ウ ウ ウ 2M Pot ウ ウ u1 ウ 1N /_\ +ウ ウ ウ Variable N ウ テトトトツトエテトソ ウ 4007 ウ === ウ ウ ウ UJT ウ SCR_ウ_ ウ ウ ウ ウ 100uウ ウ ウ タト56Kトトトソ_ テトル 2N _\_/_ ウ )|( 5kV ウ 350Vウ ウ _ウ_ ウ \->ウ 2646 /ウ ウ )|( Pulse 250R ウ ウ /_\' 1u ウ テトツトトトトトトツル ウ 100K )|( 25:1 10W ウ ウ ウ18V === ウ 470R ウ ウ )|( Transformer ウ ウ ウ ウ ウ 470R ウ ウ ウ ウ ウ タトトトトトチトトチトトトチトトトトトトトトトトトチトトトトトトチトトトトトトチトトチトトトチトトトトチトトトル -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 = ォCVs - ォCVd , Where Vs = start volts, & Vd = discharged Volts. 2 2 But as Vs is much larger than Vd , the Vd component can be ignored. ォ 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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