SELF-TRIGGERING SPARK GAP "CROWBAR"

Self-Triggering Spark Gap "Crowbar"*VICTOR WOUKt, MEMBER, IRE

Summary-A self-triggering spark gap has been developed (+)for use as a simple "crowbar" at the output of a high-voltuge dc power supply to protect the load in case of load (a) LOAD

The system consists of three spheres arranged in a xT+ Currenttriggered spark gap system, with the center sphere main- Detectortained at a fixed potential with respect to the other two Filter ( Resistorspheres (one at ground, one at high voltage) under steady- Capacitor Thyratron or Chokestate conditions. When the load fails, capacitive coupling \(.)o Chokdrives the triggering sphere to a voltage that causes break-down between the high-voltage sphere and the triggering (b) LOADsphere, with subsequent firing of the entire protective gap HV-system.

Advantages over thyratrons for this "crowbar" applica-tion include the following:

1) the voltage rating is unlimited,2) the current rating is of the order of tens of thou- Fig. 1-Electronic crowbar connections for positive and

sands of amperes, negative high-voltage polarity.3) reliability is greater than that of a thyratron, due to

lack of filament supply requirements, auxiliarytriggering circuits, etc., fire, when the current is excessive in the load. When

4) less costly than the thyratron circuit. the thyratron fires, the load is effectively shorted bythe thyratron, and the energy stored in the power

INTRODUCTION supply capacitor is "dumped" into the thyratron, thusswitching, or commutating, the very large current, from

The use of the "electronic crowbar" [1] has been the load.increasing as higher power vacuum tubes for micro- Although the thyratron circuit is currently usedwave and other purposes have been developed. The widely in such applications, it has two disadvantages:"electronic crowbar" is designed to divert short- 1) Voltage limitation. Thyratrons are currentlycircuit currents from the tube being operated, or under available at comparatively low cost only at voltagestest, to prevent damage to the tube when an internal below 20,000 volts. At higher voltages they are ex-flash-over, arc-back or other vacuum failure occurs. tremely expensive, for high-voltage thyratrons areBecause of the heavy "follow-up" currents available normally simultaneously designed for continuous highfrom filter capacitors of the power supply, a vacuum power rating, thus requiring gas reservoirs, enormousfailure can cause severe damage to the load before filament heating power, etc.the circuit interrupting equipment can be put into 2) Reliability. Although reliability is of paramountoperation. importance, and the load under test may fail only

once in thousands of hours, the thyratron itself mayTHYRATRON "CROWBAR" be inoperative at the time it is required to protect,

due to an open filament circuit, faulty triggering sys-Fig. 1 illustrates the usual connection of a thyra- tem, other causes.

tron-type "electronic crowbar." The thyratron is con-nected across the output that is to be protected, in SPARK GAP PROTECTIONparallel with the load.

The thyratron is fired by a voltage developed When the device to be protected operates at volt-across an impedance that is in series with the load. ages well above the available thyratron ratings, as isThe impedance senses the output current, and sends a the case for high-power klystrons operating at voltagesvoltage impulse to the grid, causing the thyratron to of 100 kv and above, spark gaps must be employed.

The triggered spark gap [2] is illustrated schemati-*Received by the PGIE, August 5, 1960. Presented at the cally in Fiig. 2. It is seen that the spark gap is also

Ninth Annual Industrial Electronics Symp., Cleveland, in parallel with the load, and a signal must be derivedOhio; September 21-~22, 1960.

tElectronic Energy Conversion Corp., New York, N.Y. from the load to "trigger" the gap. This triggering iSFormerly with Sorenlson and Co., Norwalk, Conn. usually done by means of a high voltage impressed

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IRE TRANSACTIONS ON INDUSTRIAL ELECTRONICS

(+)(dor R

I LOAD C

Current CapacitorDetector _ _

Triggered Pulse Amplifier a - Gap Breakdown Voltage El - E2=0.6VGap Transformer Pulse Generator E2-E3=O.6V

El- E3=l.lVFig. 2-Externally triggered "triggered spark gap" system

and triggering circuitry. Fig. 3-Basic circuit of self-triggered spark gap system.

suddenly on the central sphere, which causes a break- and polarity will be as shown. The distribution ofdown to one of the main spheres, the breakdown then voltages at the various points (a), (b), (c), and (d),aretransferring from the triggering gap to the main gap, shown in Fig. 4.discharging the load. Included in the circuit are 2) Let the distances between El and E2 be suchusually a pulse transformer, thyratron, thyratron B+, that the breakdown voltage is approximately 0.6 volt,and bias voltages, etc. with the same relationship between spheres E2 and

By means of multiple gaps, and a "bias" on the E3. Under these conditions, at steady state no spheretriggering gap, this system can be made to operate gap will break down, since the maximum sphere gapover a very wide range of supply voltages [2]. voltage is vor 0.5 volt.

Disadvantages of this circuit are complexity, par- 2ticularly of the triggering circuit, and added time de- 3) Now consider what happens when the outputlays between inception of load failure and gap firing voltage is suddenly shorted to ground (simulating, fordue to the auxiliary circuits. example, a vacuum failure in a high-power tube).

Point (d) goes to ground but, since the capacitor Cl

SELF-TRIGGERING SPARK GAP cannot discharge, because it has no path throughwhich to discharge, the potential across the capacitor

The circuit developed for this application [3] in- remains constant, and therefore point (b) tends tocludes all the advantages of the thyratron circuit, plus maintain the same initial difference of potential be-the advantages of a triggered spark gap circuit, but is tween (b) and (d), and drops rapidly towards a poten-considerably simpler than, and operates more rapidly tial to ground of as shown in Fig. 4.than either of the previous two, because of lack of de- 2laying in the triggering circuit. One minor disadvan- 4) The difference in potential between sphere Eltage is that it has a limited operating range of approx- and E2 now increases, because the potential of pointimately 2 to 1 in voltage, for a given mechanical con- (a) has not yet changed, due to the impedance of thefiguration. resistor R3. (The time scale of Fig. 4 is micro-

The self-triggered spark gap is illustrated in Fig. 3. seconds, whereas "R3 C2" is usually hundreds of mi-The power supply is shown, along with:

TV-Load Failsa load capacitor "C2" for filtering purposes, T2-Gop El-E2 Firesthe load being protected, T3-Gap E2-E3 Firesthe spark gap consisting of three spheres El, E2 v (a) (a)

andE3,() (4 -two divider resistors Rl and R2, V/the trigger-voltage developing resistor R3, and 2 (b) (the "voltage carrying" capacitor Cl. (c)-(I\Ib__

The operation is as follows: | Zlg ix 4 5 Scsec1) Assume an output voltage V, and Ri = R2. R3 _v I I I

is small, to avoid introducing excessive load regula- '2 lI Ition; therefore, at steady state, with capacitor Cl

V Fig. 4-Time variations of voltage to ground from variouscharged, the voltage across it will be equal to 2iX portions of self-triggered spark gap system.

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SELF-TRIGGERING SPARK GAP "CROWBAR"

croseconds.) Therefore, the voltage across gap El- The asymptote to which voltage of point (b) willE2 increases rapidly, and, since it can withstand an -V

' ~~~~~~~~nowtry to move after the load fails is - so that theover-voltage for only a fraction of a microsecond, the 4spark gap breaks down after a delay of approximately difference of potential between El and E2 tries to1/2 microsecond, after VE2_E1 exceeds 0.6 volt. 3

5) When spark gap 1-2 breaks down, the potential reach 4V, and spark gap E1-E2 will fire.of sphere E2 rapidly builds up toward that of point (a), However, sphere E2 will now tend to return tothe initial output voltage, V. voltage - which is the breakdown voltage of gap E3,

The buildup of voltage of the spark gap of spheres 2 'E2 and E3 is shown in Fig. 4, where it can be seen and this is the limiting factor which does not permit athat the voltage between sphere 2 and sphere 3 now given mechanical configuration to operate over aincreases, until the voltage exceeds the breakdown wider voltage range.voltage, 0.6 volt, of the gap, and this gap also breaks Thus, although the supply voltage could be droppeddown. theoretically to as low as Vand still have gap El-

There are now two spark gaps in series, or, with tcertain sphere configuration, the spark will switch be- E2 fire, gap E2-E3 would not fire subsequently.tween spheres 1 and 2 to spheres 1 and 3, thus short- Further investigation of this circuit reveals that ifing the output directly through the sphere gap. the ratio of Rl to R2 is not unity, but R2 is less than

This entire commutating action takes place in Rl so that the voltage across capacitor Cl is greaterabout 3 microseconds maximum, so that the total Vt .

amount of energy dissipated in the load during thet initially, no particular advantage is gained in

shorting conditions cannot be excessive, and the load the operating range. This is true, because, althoughis protected. point (b) will now be driven more negative than it might

have previously, the spark gap voltage must be set toa higher percentage of V, and therefore the range over

PRACTICAL DESIGN LIMITATIONS which the system will be self-triggered is reduced tothe same value of approximately 2 to 1.

If the sphere gaps could be relied upon to have ac- Because of the construction inaccuracies dis-curately predetermined breakdown of 0.5 volt, then an cussed above, spacings that correspond to 0.6 volt be-actual supply voltage range of 2 to 1 could be achieved tween the "minor gaps" and 1.1 volt minimum overwith a given mechanical setting. Thus, if E1-E2 the "major gap" are recommended. Therefore, inbroke down at 0.5 volt, E2-E3 at 0.5 volt, and El-E3 practice, the range of firing for a given mechanicalat 1.0 volt, then the system would operate satisfactor- configuration of the sphere gaps is approximatelyily at V volts, but would fire automatically at voltages 0.65 to 1.above V.

Due to the small amount of instability in any sparkgap system, it is best not to design the spark gap CONCLUSIONspacings too close to the maximum operating voltage;hence the recommendation of 0.6 volt in Fig. 3, and in A self-triggering spark gap has been developedprevious discussions.previous discussions. ~~~which operates successfully for the same "crowbar"

However, assuming that the spark gap could be re- whchoperatesccesfly th e same crwar' . . ~~~~~~~~~purposeas described by thyratrons [1]. It includeslied upon to operate at V volts and below, with the in- all the advantages of the thyratron and the triggereddividual gaps operating at 0.5 volt, then this system spark gap circuit, and has the added advantage ofwould still trigger if the supply voltage were to drop

V very fast operation due to its "self-triggering" char-to 2. acteristics. Tests similar to those made by Parker

To recognize this, referring to Fig. 4, all voltages and Hoover [1] show vividly how small size fuses are

are to be divided in half. Thus, point (a) will be at completely intact after being shorted across a highV energy content power supply protected by the triggered

as will point (d) initially. Point (b) will now be at spark gap. In addition, several volunteers havetouched the outputs of power supplies at voltages as

W.Uhen steady state is reached, the voltage across high as 60 kv with only minor physical discomfort.V ~~~~~~~Thisis not to be construed as meaning that the spark

capacitor Cl will also be -4. gap can be depended upon for human safety.

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IRE TRANSACTIONS ON INDUSTRIAL ELECTRONICS

REFERENCES [2] J. Pearson, "Pulsing System for Stanford UniversityLinear Accelerator," W. W. Hansen Labs. of Physics,Stanford University, Palo Alto, Calif. Tech. Rept. No.

[1] W. N. Parker and M. V. Hoover, "Gas tubes protect high 173.power transmitters," Electronics, vol. 29, pp. 144-147; [3] V. Wouk, Sorenson and Co., Inc., "Protective spark gap,"1956. U.S. Patent No. 2, 840, 766; June 24, 1958.

Problems Encountered and Solved in Starting Up ofComputer Control Systems *

ROBERT P. ADAMSt

Summary-Techniques for improving the accuracy and safety which are to be improved, and to adjust the manipulatedof computer-controlled systems are presented. The scope of variables to meet the output objective in spite ofthe paper is limited to those techniques which are being variation and the disturbance variables. A study ofused successfully in operating computer-controlled processes. the process is made to describe mathematically the

INTRODUCTION output variables as a function of the disturbancevariables and the manipulated variables.

Two of the most important questions to be consid- Typically, in addition to the variables named, thereare intermediate variables which can be measured.ered when planning a computer control system are:

1) Is it possible to get the required accuracy from the These can be thought of as redundant measurements

process instrumentation? 2) Since the computer con- since they depend on the manipulated and disturbancetrols an entire process, how can fail-safe features be variables.incorporated to protect the process if the computer or The redundant intermediate variables can often bethe process instrumentation fails? Several techniques used to increase the accuracy of measurements. As-which partially answer the above questions have been sume that for a particular process the manipulatedincorporated in operating a computer-controlled chemi- variables include flows and temperatures, and thatcal process. among the intermediate variables there are means of

measuring the concentrations of various compoundsACCURACY through the use of analytical instruments. If the

The typical process is characterized by variables variables can be tied together by a mathematicaldescription and if the intermediate variables can bethat can be set by the computer, such as flows and maue oeacrtl hntemnpltdvn

temperatures (manipulated variables); outside uncon-esrdmr cuaeyta h aiuae ai

temperatures (mnpltdvables, it is possible to improve the accuracy of thetrolled disturbances such as ambient temperature, raw manipulated variable measurement by mathematicalmaterial quality changes, or aging catalysts (disturbance adjustment. Consider an example of this technique.variables); and the action of the manipulated variables Assume for the moment that a concentration isand disturbance variables combined with the process being measured with a chromatograph (Fig. 1). Theitself which result in the desired product, the product chromatograph is reproducible to about 1/4 of onequality, or the dollar operating return. These results v .are known as output variables. The objectives of pecntoranig-huprodlhuhtea-computer control are to pick those output variables curacy is only about two per cent at any time. Undercomputer control it is possible to take advantage of*Received by the PGIE, July 25, 1960. Presented at the tesottr erdcblt omk nisrmn

Joint Automatic Techniques Conf., Cleveland, Ohio; whose accuracy is within 1/4 to 1/2 of a per cent. InApril 18-19, 1960. Fg ti hw httecmue pn av

tPaper written while with Thompson Ramo Wooldridge Prod- Fi.1itsshw tathecmuropnvleAucts Co., Beverly Hills, Calif. Presently with Packard which allows a sample of the process stream to flowBell Computer Corp., Los Angeles, Calif. through the sampler. The computer starts the timer,

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