A New Electronic Timer. P.L. Nonn and J.C. Sprott. February 1968 PLP 179. Plasma Studies. University of Wisconsin

Transcription

1 A New Elecronic Timer by P.L. Nonn and J.C. Spro February 968 PLP 79 Plasma Sudies Universiy of Wisconsin

2 n pulsed plasma experimens i is ofen pecessary o have rigger pulses a various adjusable imes. n he course of developing a selfconained magneron conrol uni for use on he ocupole he imer circui described herein was designed and consruced. Fig. shows a schemaic diagram of he imer. The inpu rigger signal (eiher exernal or inernal) closes SCR causing capacior C l o discharge hrough he zener diodes Z l and Z2 producing a 3 0 vol square pulse which is used o charge capacior C2 hrough he variable resisor R l. R2 is adjused so ha afer a ime = R C2 l he volage across C2 becomes sufficien o fire he unijuncion ransisor which discharges C2 ino R 3 and fires SCR 2 which discharges C and C l 3 ino he oupu resisor R. 4 The par of he circui enclosed by doed lines in Fig. is consruced on a prined circui board and can be reproduced from a phoographic negaive. Fig. 2 shows he layou of he PC board. Because of he large variaion in holding curren for SCR's of he same ype i is necessary o choose suiable SCR's from a large bach. f he holding curren is oo low he imer will no rese. For proper emperaure compensaion a mylar capacior should be used for C2. The imer covers a range of 2 orders of magniude. For greaer range addiional choices of C are needed bu he m nimum delay due o l sage delay in he wo SCR's is 4 sec. The ranges can be made inegral muliples of one anoher by hand choosing capaciors or by insering a rimmer capacior in each range excep in he range wih he longes delay.

3 2 The bigges advanage of his imer circui over oher circuis currenly in use is ha over a considerable range ( down o N 2% of he full scale) he delay ime can be read direcly off he en urn vernier dial. A fourchannel version of he imer has been consruced and is shown picorially in Fig. 3. This uni covers he inerval 0 sec 0 msec in wo ranges. The oal cos of pars if bough singly from Allied is $ 40/channel. Tess made on a represenaive channel of his imer revealed he specificaions lised below: npu mpedance Minimum level Minimum pulse widh 35 K +00 V 2 sec Oupu mpedance Volage Rise ime Decay ime Power Volages ( unerminaed) V < 0. sec 0 sec dling curren Charging curren Recycle ime Jier (Half widh of gaussian) Lineariy < 0. rna 3 ma.25 sec.004 % < 0.5 %

4 3 The imer is remarkably insensiive o emperaure and line volage changes. Using a simple zener regulaed supply he ime inerval changed by only 004% when he line volage changed from 90 o 20 vols. A % change resuled when he imer module was cooled o liquid freon emperaures. As an added feaure an inerchangable PC board can be consruced which provides a repeiive oupu pulse wih coninuously variable rep rae. The circui for he repeiive pulser is shown in Fig. 4J and is similar o he imer circui excep ha he inpu SCR circuiry is omied. C 3 was reduced o..0 f in order o reduce he rese ime o 25 msec. f shorer ime inervals are desired C 3 can be reduced furher (a he expense of oupu pulse widh) or R 5 can be made smaller. consan of R l The ime inerval is given by he ime and C 2 and has all he desirable feaures of direc readou J sabiliy and lineariy. Acknowledgmen We express appreciaion o Tom Lovell for assisance in consrucing he imer paricularly for layingou he PC board and for preparing he drawings and phoographs for hi s paper.

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