w ' 3 {"1 L“ H O m ;> u) "a >3 51> N H (O "NH? Ly ‘IW II ‘1 li‘ " ll‘li 1 \ ‘1 W | W. l3 N ‘ ‘ \ ‘i l ‘ ; L‘ 1 N ‘1 ‘ H I \ :1 i I I r M —‘|_x__x I—AA mcow TRIGGER CIRCUITS Thesis for. flu Dear“. of M. 5. MlCHiGAN STATE COLLEGE Stalin: Masforakis .1949 THESIS This is to certitg that the thesis entitled \ o '.A O , - \w.l_l."“l_; presented ht] has been accepted towards fulfillment of the. requirements for L4; 3'11 degree in. , 1‘59 __/ llztjnr liruysur Hate,_ 1 _- M-795 THIGG”R CIRCUIE BY Stelios Mastorakis A THESIS Submitted to the School of Graduate Studies of Michigan State College of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Electrical Engineering 1949 m. S E H Tu TRIGGER CIRCUITS Definition Circuits that possess two or more stable ope- rating conditions, that is circuits in which one or more currents or voltages change abruptly from one stable value to another stable value at a critical value of some voltage or resistance and change back abruptly to approximately their original values at a different_critical value of the controlling voltage or resistance are called "trigger circuits," Criterium for a trigger circuit The criterium as to whether a circuit element can serve as the basis of a trigger circuit can be determined from the characteristic current-voltage curve of the element. Let us have the circuit element M, in series with a resistance R, and a battery supply Eb fig. (t). The voltage across the element M is given by the re- lation: EuEb - IR This voltage across the element is also a function of the current through the element according to the equation: This equation represent the characteristic curve of 1-1 the element X. Equation n1=Eb - IR is that of a straight line, xy, the resistance line, through a point on the voltage axis corresponding to the sup- ply voltage E having a negative slope in amperes b! per volt equal to the reciprocal of the resistance in series with the element. Equilibrium values of currents are determined by the intersection of the characteristic curve with the resistance line. \ \“ \ A \\ ‘ c \ \ \\\ \ \ D \\ 3 \£ \ B x \ \ \ \ \ \ \ \ \ \ \ \ \ \\ \\ o “ ¥ If the characteristic curve has a portion whose slope is negative, the resistance line may inter- sect the curve in three points 1,2,5, as seen in fig. (2), indicatin that there are three possible 8 equilibrium values of current. An increase of cur- rent at constant Eb and H from that represented by point 2 is accompanied by a decrease of voltage a- cross the element M. More voltage is thus made avail— able to send current through the resistance and the current will rise further in the same manner. Any increase in current through the element reduces the voltage available across the resistance and thus causes a further reduction of current. Point 2, therefore corresponds to unstable equilibrium and practically is not observed experimentally. If the aLplied voltage is raised progressively from zero, the intersection will move along the branch 0A of the characteristic curve. When the intersection is at A, an infinitesimal increase of voltage will cause the current to fall abruptly to the value of E. Further increase of supply voltage causes the intersection to rise toward C. If the battery voltage is then continuously decreased, the intersection will move down the branch BC until point B is reached at which the current will again jump abruptly to the value corres— ponding to point D. A. It is seen that similar abrupt changes ofcur- rent result if the slope of the resistance line is varied by changing the resistance R, or even if the characteristic curve is displaced vertically or hor- izontal y. With trigger elements incorporating va- cuum tubes this displacement can be accomplished by varying the electrode voltages. From the above ana- lysis it follows that a circuit element whose current- voltage characteristic has a portion with negative slope may serve as the basis of a trigger circuit. Practical Trigger circuits ’he best known trigger circuit is that of Eccles J~.) and Jordan shown in basic form in fig. (3). r--~-+P---~_\a~“--°-i is (3) This circuit functions by virtue of the fact that only one tube at a time passes plate current. Let it be assumed that both tubes can conduct simultane— ously; Then an increase of current in either tube ,7" increases the negative grid voltage of the other tube, which reduces the plate current of that tube. This in turn reduces the negative grid voltage of the first tube and causes further increase of plate current of the first tube. The action is cumulative and only one tube conducts at a given time. In verification of the general theory, let us find experimentally the current- voltage characteristic curve and the resistance line of the curcuit by using the battery Eb connected to the points A and B. The curve of external current versus voltage between A and B is found to be of the form shown in fig. (4), when the battery Eb is con- nected to the points A and B through a resistance R as shown in fig. (3), then the corresponding resis- tance line is of the form of MN in fig. (4). $35. (4) If R exceeds in magnitude the value of the re- ciprocal of the slope of the curve at point 0, then abrupt changes of current thrvugh R and of voltage Wt. 15-? between A and B can be made to occur by varying Eb or shifting the characteristic by changing the operating voltages of the tubes. As R is increased, MN becomes more nearly horizontal and in the limiting case, when R is infinite, becomes the voltage axis. The external current is then zero, but changes in electrode voltages can cause an abrupt transfer of current from one tube to the other and a reversal of voltage between A and B. The need of more than one voltage supply is avoided by the use of the circuit of fig. (5), in which the coupling between tubes is made by means of voltage di- viders. j R‘ ‘tlL-vcvw— ' B 4 G *- ---t R‘ 3-b t. A.“ ' tie-(5) Let the total current flowing in the left-hand lead resistance be i,, equal to the sum of i3, of plate current and i; of current in the potential di- vider. The voltage at the plate of the left—hand tube is: Eup‘ "' 3'in Volts The entire difference of potential across the ‘ 10 't '- ‘~ I ,1 1 first potential divider, AhCRCh, is therefore: of which, [ten-M») + 5...] (“:9“) volts ‘ n ‘ O W ‘ I appear across the lower section 01 the diVider, he, consequently the grid of the right-hand tube is held at a potential of [( Fwy-LR” + EM'lg Rc ) - EM? volts R.+a1 This grid voltage is sufficiently negative to en- sure that the plate current of the right-hand tube will be very small if not actually Zero. In other words, the right-hand tube is biased near the cutoff level, in some cases below the cutoff level. Consequently, the right-hand plate potential has risen nearly to the level of the plate-supply voltage. The potential of the right-hand plate is therefore almost equal to ENp Hence the voltage across the second potential di- vider is: 5”,, EM, volts If the grid current of the left—hand tube could be neglected, this grid would be held at a voltage of: (E... 4- EM,)( flies.) volts above the potential of the M point or Although this highly positive value is reduced nearly to zero by the actual grid current superimposed upon the network, the grid potential remains slightli positive, being high enough to permit the is current to flow through the tube with the low internal voltage dI’Op Of: Eu?“ LtRb Hence, the entire system of highly unequal currents and unequal voltages is now self-consistent and self- perpetuating, there being no unbalanced voltages tend- ing to produce or assist any further change, rcuit l~—'° Design pf the Trigger C The design of a trigger circuit such as the one shown in fig. (6), is a relatively easy matter. All that is required is to make sure that with one tube con- ducting the grid of the other is below the cutoff value, and that the tube which is nonconducting maKes the grid of the opposite tube zero or even positive. mo zoo . volfs £13.“) Cnoosing Rb 25,000 ohms. Ep 200 volts. Ek 45v. ‘- and using a 6J5 triode: r _ soc _ m use Ip (total) - 1275:7300 - 0.0\/8 al‘lgo The load line intersects the zero grid voltage cha- racteristic at a.current value of 5.4 ma. fig. (5). This leaves for the voltage at the plate of the left hand tube: \ 200 - (0.00b4) (25,000) 8 200 - 155 = “5 v. The cutoff grid voltage for a 6J5 operating at 200v. plate voltage is -le. according to the characteristic curve. In order to be sure that the circuit will not trigger prematurely, we shall be safer by placing the grid of the nonconducting tube VT; at a level consid- erably more negative with respect to the cathode than this value; let us decide for instance at ~25v. With the cathode established at a level of 45v., point N’must therefore be at a level of 45 +(-25) = so v. Let us assume that VT. is conducting and V”; is non- Pb conducting. Point A will then be at a level 0 45~'65s“°* while point B will be at a level about 200v. if the re- sistances RC and R; of the voltage divider are of high value compared to the 25,000 ohm load resistor; I As p01nt N must be at a potential of 20 v. and A at 110 v. . . . . I . , it means that the reelstances RC and RC must be in tne ratio; "0-30 , q ‘7fi7" ‘ 7F ‘ ”f ~ I In this case He can be made 430,000 and RC 100,000 ohms . Let us check now whether the resistors RC and RC, which will make tube VT; nonconducting (by making its grid 25v. negative with respect to the cathode) will also make the grid of tube VT zero with respect to the O cathode as we had assumed. With tube VT; nonconducting, point B will be just a few volts below the 245v. level owing to the small current taken by the voltage divider. With the resistors 1 I r¢ , . ,A 1 o and RC equal to 400,000 and 100,000 onms respectively :0 C I 0 w po nt M would be at a level of approximately 55v. or 10v. positive with respect to the cathode level. When MI is connected to the grid of tube VE, grid current will be- gin to flow and will prevent the grid from becoming mom than a fraction of a volt positive with respect to the cathode. If this check had revealed that the grid failed to become zero or pOSitive, another start with different load resistor, (cathode resistor) would have to be made. . " ‘L Triggering ketnods A common method of triggering the circuit is to insert a negative pulse in the grid of let us say VT. through a small coupling capacitor in series, fig. (7) Rc R: I 4 I Re :: Rt. 3 ' M m— u —-.-*l'l't~—— fig. (1) The charge on the plate of this capacitor cannot be altered instantly due to the resistance in series with this capacitor. Hence, by applying a negative pulse at ELK the drop in the potential of point 3 pro- duces a corresponding drop in potential at g; no al- teration in the potential difference between the plates of the capacitor being possible in this first instant. If VT, was already not conducting, that is the grid 3' was blocked below cutoff, the tube does not respond, the negative pulse being disregarded. However, if tube VT.has been carrying the heavy current, the sudden re- duction of the positive potential of its grid causes a l2 sudden and violent drop in current accompanied by a simultaneous rise in current of tube VTQ. If the in- stantaneous current values meet and pass, the circuit triggers, flipping over to the Opposite terminus con- dition. The steeper the pulse the better because a slo- ping wave front permits a portion of the available voltage to be expended in the capacitor C., so a de- creased portion is available for use at a: The cap- acitor g.cannot be made larger to compensate a sleping wave front because its twin 9; ties the grid E to the inactive potential source 2, and the desired rapid rise in potential of brmust be accompanied by an al- teration in the charge Of.9£ which for that reason must be kept as low as possible at that phase of the triggering action; a large value of capacitance de- creases the sensitivity by limiting the suddenness of the rise of current in the plate resistor which is important for the triggering action. Actually, con- densers of value 25 or Sogfi can be used. The circuit having switched the main current over to VTz, then disregards any additional negative pulses imposed at point a while awaiting the occurrence of the next negative pulse applied at point b. In some applications the points a and b are joined together, fig. (8), pulses from a single external chan- nel being applied simultaneously to each coupling cap- acitor, C,and C2. R‘ R‘ I ‘F D a. 1‘ a; n —-NV\IWVWVV~—1I fi'h F‘s-U) In this case, the use of the commutating capa- citors G:and G; is necessary to prevent the circuit from stalling when equal negative pulses are applied simultaneously to coupling capacitors C.and 0;. Suppose that tube Vfl, is conducting, applica- tion of a negative pulse at the common input ter- minal causes a sharp drop of plate current through tube A. The resultant sudden rise of plate voltage transferred through 9L to the grid of tube B over- powers the negative pulse, of external origin, that is applied through Czto the grid of tube VT;. The first pulse flips the main current over from A to B, the next pilse flips it back. The size of 01 and p; is not critical. Usually they are twice as large as C,and 02° Trigger Circuit using pentodes A modification of the basic curcuit is obtained by using pentodes as shown below fig. (9). _ 1 Kg J-c‘ a. 85 C:_L L can?!“ I l; I 1 v1. w, I i l Can‘t-0‘ «Hose 1; 9‘ ‘ ’ o w Kg «Host. 3517;“ T ‘ 7 hwy»? \ $1 7 c IL. fis- (Q) Here, the suppressor grids of pentodes serve the same function as the triode control grids of the cir- cuit described above. The screen grids are used in the normal manner and the control grids are used for triggering the circuit. One very desirable characteris- tic of this circuit is that the circuit may be triggered by a very small negative voltage impressed upon the control grid of the conducting tube, but it is insen- sitive to positive voltages applied to the control grid of either tube. The functions of the control and sup- pressor grids may also be interchanged but the result- gering voltage D 0 ing circuit is then sensitive to tri: T C); of either polarity. . é single pgntode, under certain conditions, may also be used as a trigger tube. If the suppressor voltage of a pentode is varied with screen voltage, the change in suppressor voltage being prOportional to the change in screen voltage and in the same direction, the curve of screen current versus screen voltage is of the form shown in fig. (W). L‘2 fl, . (to) A negative voltage impressed upon the suppressor grid causes electrons that have passed through the screen grid to turn back to the screen so high screen current results. A positive increment of sup- pressor voltage (decrease of negative voltage) allows more electrons to go to the plate and thus decreases the screen current, which means that the suppressor- screen transconductance is negative. Under proper operating conditions the screen current decreases with a positive increment of suppressor voltage even when the screen voltage is given an equal increment. An increase Auzin the screen voltage is accom- panied by an eyual change Ace, of suppressor voltage. AR; would by itself change 1c, by the amount At‘g/Qz and Au, acting alone would change 'ch by the amount A“: 93?. ° the net change of l