0600 - Atmospheric Electricity Detector Part 1

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EPE Online, June 2000 - www.epemag.com - 421

It all started way back at thevery beginning of the 1990s whenthe author saw an article in anastronomy magazine suggestingthat it might be possible to detectthe advent of a meteor by meansof a change in the earth’s electricfield.

Whilst he never succeeded indoing this, confessing that it mightbe due to inadequacies in hisequipment or design, detectingand measuring changes in theelectrical state of the earth’satmosphere soon became aninteresting hobby.

SCENE SETTINGThe ionized layers of the

atmosphere extend from about40km to 200km (25 to 125 miles)above the earth. This ionization iscaused by the “Solar Wind”passing the earth and leaves theupper atmosphere positivelycharged.

There is thus an electric field

terminal of a meter, which willindicate at least one picoamp.The negative side of the meteris connected to a variable highvoltage supply, the other side ofwhich is connected to ground.

If the power supply is nowadjusted so that the currentthrough the meter is zero, thenthe voltage from the powersupply must be the same as thevoltage on the plate.

We can also consider theplate as though it were a battery.Remembering school physics, abattery has potential and alsointernal resistance. Fig.2suggests the arrangementwhereby both these figures canbe determined. The batteryvoltage is E volts and its internalresistance is RB (the resistanceof the atmosphere). Connectedacross the battery is a perfectvoltmeter, i.e. it consumes nocurrent. It is also possible toconnect a resistor R across thebattery by means of switch S1.

With the switch in the offposition the voltmeter will showthe voltage of the battery as E

Investigate Nature’s power-house with this intriguingexperimental design.

ATMOSPHERIC ELECTRICITYDETECTOR by KEITH GARWELL - Part 1

between the upper atmosphereand the earth. Using suitableinstruments, this field can bedetected as it results in aminuscule current through theatmosphere.

If a probe, consisting of ametal plate, is supported on aninsulator one meter aboveground the metal plate willacquire by conduction the samepotential as exists at this levelabove ground. This would betrue of any height, of course, butthe meter is a nice standardunit.

A potential of around 100volts is often present. In otherwords there is often a potentialof 200 volts or more betweenyour nose and toes! Of coursenobody gets electrocutedbecause the resistance of the airis so high that only a very tinycurrent is present. This is whythe actual values are so difficultto measure.

Modern operationalamplifiers make it possible tomeasure the tiny currents butthey object strongly to beingsubjected to such high voltages!

PRACTICALMEASUREMENT

There is a way round thisproblem, as is illustrated in Fig1. The metal plate is supportedone meter above the groundand is connected to the positive

Fig.1. Basic principle of at-mospheric electricity

monitoring.

Fig.2. Equivalent circuit ofFig.1 in which atmosphericresistance can be measured.

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volts. If the switch is now closedthe voltage shown by the meterwill fall due to the current flowingthrough RB and R in series. Callthis voltage V.

As the same current flowsthrough both resistors theirresistance will be proportional tothe voltage across them. V isthe voltage across R and thevoltage across RB is E – V. Thisgives:

RB / R = (E – V) / V

Multiplying by R we get:

RB = ((E – V) / V) x R

and perhaps a more convenientform:

RB = ((E / V) – 1) x R

and finally:

RB = (E x R / V) – R

FIELD EFFECTAlthough primarily intended

for the observation ofatmospheric electricity, themeter described here is verysensitive, quite easilyconstructed and doubtlessadaptable to other fields ofinterest, including pollution.

Out of curiosity, the authorset up the arrangement of Fig.1in his lounge. The meter readingwas zero until the TV wasswitched on, it then fluctuatedrapidly and seemed to berelated to the picture make-up!

Whilst Fig.1 allows thedevice to be used to determine

the potential at a given height aswell as a figure for the effectiveresistance of the air at thatheight, Fig.3 shows a method ofobtaining a figure for theresistance of the air by directmeasurement. It has beensuggested that the resistance ofthe air and the amount of airpollution are inversely related.

In Fig.3 the metal plate ofFig.1 is replaced by a pair ofmetal plates 140mm square andseparated by 20mm. Theseparation can be achievedusing two pieces of plastic tube.Make shallow slits 20mm apartin the side of the tubes with asaw and then push two oppositecorners of the plates into theslits (see photo below).

The plates are supportedone meter above ground andthe lower plate is connected toground. The high voltage supplycan then be set to any voltageup to 300V and the meter willagain indicate the voltageacross its resistance and weend up with arithmetic similar tothat used before. In this casethe resistance (Rp) between theplates will be equal to:

((high voltage setting – meterreading) / meter reading) x

meter resistance

TEST RESULTSThe author’s results so far

obtained by using the Fig.1system are as follows:

Taken at a height of 0��5meters from 1 Nov ’99 to 19 Dec’99, from 41 readings theaverage voltage was 83��3V andthe average resistance 10��1 teraohms (one tera ohm = onemillion million ohms). The plateis one tenth of a square meter,so the figure for one squaremeter will be 1��01 tera ohms.

Taken at a height of one

meter from 4 Jan ’00 to 21 Feb’00, from 22 readings the averagevoltage was 69��2V and theaverage resistance 8��14 teraohms, or 0��81 tera ohms persquare meter.

All readings were taken undera blue sky with little breeze duringthe hours of daylight.

Using the two plates of Fig.3a reading under calm but overcastconditions at a height of onemeter gave a reading of 2��4V with100V applied, giving an Rp valueof 4��06 tera ohms. Corrected to aone meter cube gives 4 teraohms. The correction on the twoplates is 0��98.

It is not clear what therelationship is between the twopossible resistance values, ifindeed there is one! There arelots of other questions to beanswered, for example the valuegiven for E at a height of 0��5mwas greater than the one obtainedfor 1m.

This is contrary toexpectations, it would seemreasonable to expect a more orless linear increase in E withheight, and ideally this could besolved by simultaneousmeasurement at several heights.The difficulty here is that thevalues change quickly so theauthor is waiting for a nice calmday when conditions are stablebefore drawing furtherconclusions.

EXPERIMENTALASSEMBLY

The equipment needed toperform such measurements isfairly simple to construct, therebeing only one or two spots wherespecial arrangements arerequired, mostly concerned withmaintaining the required insulationresistance. It can be split into sixparts:

Constructional Project

Fig.3. Principle for measuringthe resistance of the air.

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o) High resistance meterinterface

o) Variable high voltagesupply

o) System meteringo) System interconnectionso) Probes (plates)o) Construction

Note that the 300V variablevoltage supply also incorporatesan isolated �14V supply for themeter. This is necessarybecause, as Fig.1 shows, therecan be a high voltage presentbetween the meter and ground.This high voltage supply isinherently safe because it isdesigned with a very limitedcurrent capability.

METER CIRCUITThe schematic in Fig.4

shows the basic circuit diagramfor the meter interface buffer.(The meters will be discussednext month.) When the input isconnected via socket SK1 to asuitable antenna or probe (e.g.the metal plate mentioned withregard to Fig.1 and Fig.3), thiscircuit allows atmosphericelectricity to be monitored via asuitable meter connected to itsoutput. It is quite simple toconstruct although there are one

or two peculiarities.First note that the use of an

AD795 for opamp IC1 is notessential but if an alternative issought its input bias current mustbe very small because it ispassing through a 100M� resistor(R2). The AD795 bias current isless than 1pA and the offsetvoltage less than 250uV. Theinput resistance of this design is100,000,000,000 ohms, perhapsbetter described as 1011 ohms or100 gigohms (G�).

The circuit is basically avoltage follower hiding behind oneor two minor modifications. Itrequires a power supply ofbetween �12V and �15V.

Resistor R1, in series with theinput to the opamp (pin 3), is aprotection resistor. This meterinterface is intended to be mixedup with static and various otherunpleasant things, at leastunpleasant as far assemiconductors are concerned.As a consequence, R1 is intendedto reduce the possibility ofdamage to IC1. Although R1 is10M�, this value is of noconsequence to the normaloperation because of the effectiveinput resistance of IC1.

Next, one would expect theinput bias resistor (R2) to be

connected to the 0V line.Instead, connecting it to thejunction of resistors R4 and R5effectively multiplies its value by1000. This is how the massiveinput resistance is achieved. R2is already large, 100M�,multiplying it by 1000 gives therequired figure. Thisarrangement is known asbootstrapping, although the termis also applied to other similartechniques. How this works ismost easily seen by goingthrough an example:

Suppose the input at pin 3 is1V, the output at pin 6 will alsobe 1V. The ratio of R4 to R5 isone to a thousand so that thevoltage across R4 will be0��99mV. Let’s call it 1mV as aclose approximation. Under thisthe voltage across R2 will be1mV and the current through itequals 1mV / 100M� (I = V / R),that is 10–3 / 10–8 or 10–11 amps.However, the voltage at pin 3 is1V and the current taken is 10–11

amps. The effective resistancemust therefore be 1 / 10–11,which is 1011 ohms.

The next point to be made isabout resistor R3. This provideswhat is known as the guardconnection. As it is necessary touse an input cable that is


Practical plate construction for use with thetest schematic in Fig.3.

Fig.4. Circuit diagram for the meter interfacebuffer.

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screened to connect the meterinterface, the screen isconnected to R3. There is thusno voltage difference betweenscreen and the conductor, sothat leakage is minimized.

Lastly, capacitor C1stabilizes the opamp to preventthe possibility of high frequencyoscillation and also serves toprevent interference gainingaccess via either the input oroutput connection.

Note that the connectionlabeled GA is not used.

DUAL VOLTAGEPOWER SUPPLY

Harking back to Fig.1 for amoment, the meter will beoperating at some considerablepotential above ground because,whilst the open circuit voltage onthe plate E is being measured,

Constructional Projectboth the input and the 0V line ofFig.4 will be at the samepotential. This means that thedual power supply for the meterinterface must be isolated fromground and this supply isgenerated in a common unitalong with the high voltagesupply.

In fact there are limits on thepower supply. A lower limit of�12.5V is set by the opamp typein the meter interface circuit (IC1in Fig.4) which must provide anoutput from –10V to +10V.Usually there is a requirementfor an allowance (known asheadroom) of about 2��5Vbetween the supply and themaximum output voltage.

An upper limit is set by theopamp manufacturer and is veryoften �15V with an absolutemaximum of �18V.

Consequently, the valuechosen for the prototype is�14V, which makes allowancefor both requirements. Thissupply is generated using aseparate circuit to be describedin a moment.

A continuously variable highvoltage supply from 0V to atleast 300V of either polaritymust also be generated. Thissupply must be inherently safe,or at least the user must be!This is achieved by arrangingthat the high voltage supply hasa high internal resistance so thatthe output current is very limited.This supply must also beisolated from ground so that itcan be switched to eitherpolarity.

There is also the question ofwhat primary power source is tobe used and a 12V battery waschosen. This makes theequipment portable and alsosafe from mains failure. As inthe author’s set-up the battery issimultaneously charged from themains, it was preferable that a

voltage of between 11��5V and13��8V should be allowed for theprimary source. This is allachieved as shown in Fig.5.

POWER SUPPLYGENERATOR

In this circuit IC2 isconfigured as an oscillatorwhose output is coupled to twotransformer and rectifier circuits.The first, based aroundtransformer T1, generates�14V. The second, basedaround T2, is a variablegenerator which can be set forany voltage between 0V and300V, approximately.

Op.amp IC2 is in fact anaudio power amplifier. However,it will also operate as a poweroscillator and this is how it isused here. Capacitor C2determines the frequency, in thiscase about 50kHz. Diode D1,resistor R8 and capacitor C3 arebootstrap components for pin 7so that the IC can achieve avoltage output close to thesupply rails.

Output pin 5 drives theprimaries of the twotransformers T1 and T2, viacapacitors C4 and C5respectively. The output of T1 isrectified by diodes D2 and D3,smoothed by capacitors C6 andC7, and regulated at �14V bythe Zener diodes D7 and D8.

The output of T2 is appliedto a voltage doubling circuit,comprising D4, D5, C8 and C9.The drive to its primary iscontrolled by TR1 and D6, theaffect of which depends on thebase current produced by theDC voltage applied to R11 fromthe high voltage adjustmentcontrol input.

Note that capacitors C8 andC9 must be rated at 250 voltsworking and therefore all the

The author’s prototypesingle-plate atmospheric

detection platform.

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100nF capacitors have beenspecified at the same rating.

METER INTERFACECONSTRUCTION

The meter interface circuitcan be built using stripboard anda layout is suggested in Fig.6. Itis prudent to use a socket forIC1 so that it can easily bereplaced if it does have amishap. Always ensure that IC1is correctly orientated.

The most important part ofthe construction to be noted isthat pin 3 of IC1 must be bentout sideways so that when theIC is fitted pin 3 is well awayfrom the board. The tworesistors R1 and R2 aresoldered directly to pin 3 andsupported on their wires andshould not touch anything!

During the setting upoperation the input must beentirely screened. This is mosteasily done by mutilating astandard cable mounting coaxialplug. To do this unscrew the cap

and remove the centralconductor and the cable clamp.Then bung up the cap with ascrap of aluminum foil crushedinto a ball. This should be a firmfit in the cap. Replace the capon the body of the plug and thejob is done.

Connector SK2 is a 7-pinDIN socket and SK1 is a TVaerial socket. The latter isspecial in that both center

connector and the screenconnector must be insulatedfrom the aluminum case. Thereare some connectors which arein a plastic molding and one ofthese will be fine. If this type isnot available then the morenormal socket with the metalouter can be used but it willhave to be insulated from thecase by mounting it on a pieceof insulating board.


Fig.5. Dual voltage power supply generation circuit diagram plus variable 300V supply.

The meter interface board mounted in its case. A pieceof card is placed between the board and bottom of

case to prevent any “short circuits”.

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An aluminum case needs tobe used, measuring about76mm x 50mm x 25mm. TheDIN socket should be fitted atone end and the TV coaxialsocket at the other.

POWER SUPPLYCONSTRUCION

The secondary winding onthe high voltage transformer T2is 150 turns of 36 gauge(0��2mm) enameled wire. Forthose who haven’t played thisgame before, use a shuttle. Thiscan be made from a piece ofcard or plastic that will passthrough the center of the toroidand which has a slot at eitherend so that the wire can bewrapped round the shuttlelengthwise.

It is as well to first put 10turns on the toroid the hard way,take it off and measure it so thatthe required length for 150 turnscan be worked out. Allow plentyfor the ends. The wire is muchcheaper than patience!

The primary for transformerT2 has only six turns. For T1,the primary has eight turns andthe secondary 16 turns. As theother windings have only a few


Meter interface unit complete withcrocodile clipped connection link, plus

the constructedpower supply board.

Close up details of the constructed power supplyboard.

COMPONENTSResistors

R1 10MR2 100M high ohmic cermet filmR3 4k7R4, R6 1k (2 off)R5 1MR7 5k6 1% metal filmR8 1k 1% metal filmR9 10k 1% metal filmR10 5k6 1% metal filmR11 22k 1% metal filmR12 4k7 1% metal filmR13 100kR14 22kR15 3M

All 0.25W 5% carbon film or betterunlessotherwise indicated

CapacitorsC1 100n polyester, 12.5mm spacingC2 2n2 polystyreneC3 to C10 100n polyester, 250V (8 off)C11 220n polyester, 250V

MiscellaneousB1 12V battery (see text)B2 AA cell (2 off)SK1 coax socket, insulating (chassis mounting) (see text)SK2 7-pin DIN socket (chassis mounting)S1 d.p.d.t. miniature toggle switchS2 s.p.s.t. miniature toggle switchS3 d.p.d.t. toggle switch, 240V AC ratedS4 s.p.d.t. toggle switch, 240V AC ratedPL1 coax plug (cable mounting)PL2 7-pin DIN plug (cable mounting)ME1, ME2 panel meter, 0.1mA full scale deflection (2 off)T1, T2 ferrite toroid B64290K618X830 (25mm diameter) (2 off)

Stripboard, 0.1-inch, 39 strips x 39holes; stripboard, 0.1-inch, 12 strips x13 holes; metal case 75mm x 50mm x25mm; metal case to suit powersupplycontrol unit (see text); knob for VR3;8-pin DIL socket; crocodile clip; 36gauge (0.2mm) enameled wire (seetext); nylon nut and bolt to suit (seetext)(2 off each); aluminum plate 316mm x316mm x 2mm (or thicker); supporthardware (see text); 6-way cable,lengthto suit; connecting wire, solder, etc.

See also theSHOP TALK Page!

$30Approx. CostGuidance Only

(Excluding batteries, meters,cases, and hardware)

PotentiometersVR1 10k miniature preset, squareVR2 50k miniature preset, roundVR3 10k carbon rotary

SemiconductorsD1 to D3, D6 1N4148 signal diode (4 off)D4, D5 1N4007 rectifier diode (2 off)D7, D8 14V 400mW Zener diode (2 off)TR1 BC548 npn transistorIC1 AD795 opampIC2 TBA820M power opamp

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turns, so the wire can bethreaded through. Plasticcovered connection wire will do,e.g. 1/0��6.

Assembly details for thepower supply board are shownin Fig.7. There is nothing specialabout the layout so it can bevaried to suit differentcomponent sizes. Be carefulwith the cuts in the tracks sincethese must be done on thereverse side. It is easiest topoke something through fromthe front to help mark a cutposition.

The two transformersshould be fastened to the board,ideally using a nylon nut and

Fig.6. Meter interface strip-board component layout, de-tails of breaks required in un-

derside copper tracksand wiring to off-board

components.

bolt, passing the bolt through apiece of plastic, through thecenter of the toroid and througha hole in the matrix board.Tighten the nuts just sufficientlyto hold the toroids in place.

NEXT MONTHIn the final part next month

we conclude the constructionand describe the metering andmonitoring probes.

Fig.7. Power Supply strip-board, topside component

layout and underside coppertrack break details. Note thatthe two toroid transformers

are bolted to the board usingnylon nuts and bolts and two

strips of plastic

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0600 - Atmospheric Electricity Detector Part 1

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Transcript of 0600 - Atmospheric Electricity Detector Part 1