An Experimenter's Experimenter's Electrometer Sniffing out subtle electrostatic phenomena By Richard Hull The field of Electrostatics concerns itself with charges, potentials, and forces, and is often considered to have first been studied in the late 18th and early 19th centuries. In today's modem world, electrodynamics is the dominant study in electrical engineering. Electrostatics is making somewhat of a come back, and the forces and charges involved can be used for a number of novel purposes in our day-to-day lives. Xerography is a prime example. The experimenter can arm him or herself with an old classic electroscope and investigate a number of rather large electrostatic effects. In order to gain real insight and do more subtle and difficult experiments, an electrometer is often needed. The electronic electrometer is nothing more than an ordinary voltmeter. The salient difference between your digital VOM and the electrometer is one of input impedance. This relates to the amount of load that the instrument places on the circuit under test. Most good electrometers have input impedance's of 200 teraohms. This is tens of thousands of times greater than the average electronic VOM(I - 10 megohms). It will immediately amaze the experimenterjust how electrical the world really is, once armed with an electrometer. Modern electronic electrometers are very expensive. $5,000 will buy a fairly nice instrument, while $10,000 would be needed for a superlative one. Keithley Instruments and Victoreen are major suppliers of these specialized instruments. This article will supply the experimenter with schematics and a broad overview for assembling a first class electrometer with a very modest outlay of cash. Actually, this is a unity gain impedance translator or electrometer IX amplifier, in the s trictest sense, as the experimenter will have to supply the readout system, (usually a VOM or oscilloscope.) It will be assumed that the experimenter has a modicum of experience in assembling simple electronic circuits. DISCUSSION OF THE CIRCUIT REQUIREMENTS
As the frequency response of this junior electrometer is limited by the low pass filter circuitry to around 10Hz, no special PC boards are needed. Point to point or wirewrap type connections are all that is required. I have assembled many of these circuits on the simple pad per hole, 2"x3" circuit boards found in local Radio Shacks. The key point is shielding and insulation. The front end of the system utilizes a special precision CMOS FET Integrated Circuit by National Semiconductor. (LMC 6081) This special IC can be purchased from DigiKey. The only other IC is an LM-324N and can be found at most Radio Shack stores. I recommend first quality machined pin IC sockets (DigiKey). Radio Shack also has aluminum chassis cases which will be needed for shielding. The input connector must be a Teflon insulated, chassis mount, female, UHF connector type PL-259. The output connector is unimportant and can be a RCA type phonojack. I chose a BNC chassis jack for my output connector. The system is powered by two common 9 volt transistor radio batteries. Parts layout is only critical in that the part of the PC board containing the input IC (LMC 6081) be located immediately at the input connector.'The critical input pin to this IC (pin #3) is carefully lifted and not inserted in its socket at all. The
two components from this IC lead to the input connector center terminal are connected floating in mid-air!! All batteries and wiring should avoid this area and be on the opposite side of the board and shielded container box. Beyond this, the builder is totally at liberty to do as he or she pleases. I have a proposed lay out diagram included. GETTING STARTED
Begin by soldering the sockets to the PC board. Next make all connections to all the various tie points. When the board is wired, add the battery clip leads. The box should be a fully enclosed all metal item. Place the input connector at one end and the output connector at the other end of the box. For convenience in some models, I put the input connector on the top of the box at one end. Holes must be drilled for standoffs or long machine screws to support the board near the input connector. The batteries are best placed in the opposite side of the box near the output connector. Make sure at each step of the drilling process that all components will fit in the box and not short out or touch one another. The power switch should also be positioned near the output end of the box. If you still haven't picked up on it, the input end of the box is special and PRIVATE! Only the input IC and the ultrashort input connection, and air, are allowed. You are on your own from this point! Good luck. I expect the builder to rely heavily on the supplied visuals to complete the circuit. TESTING AND CALIBRATING THE INSTRUMENT
With the shielded box still apart. get a 1.5 volt battery and connect it to the input. Turn the unit on. Measure with a VOM the voltage on the battery. Write it down. Now go to the output with your VOM and adjust the gain potentiometer until the reading exactly equals the input voltage of the battery. `This sets the instrument for unity gain. You are calibrated! Next, we have to check out the finished system. Assemble the box and case for final use. I prefer to use a banana plug with a stiff 24 gauge rod soldered inside the plug to make a vertical whip antenna if the input connector is on top of the instrument. If not, just use a short flexible wire soldered to a banana plug and connect it to a stainless steel salad bowl which is setting on a clean drinking glass. It is crucial that no path of any resistance exist to ground from the isolated and insulated metal object. Ground the chassis of the instrument (a good solid earth ground is needed.) Connect the output to an Oscope or VOM. Set the scope's sweep for a long period sweep of 10 seconds across the screen or more. Set your meter for the 10 volt range if you have no scope. Turn on the instrument. Now move about a bit and notice the varying voltage you are impressing on the bowl or antenna! The impedance is so high (teraohms) that the instrument can resolve voltages that would be overloaded and swamped back to zero by other meters of lower impedance. If you have encountered any problems in getting the system to work, you have made a simple mistake somewhere. Check your batteries. Are they good? Are they hooked up right? Are the ICs in their sockets correctly? Recheck all wiring. You can blow the input IC very easily by touching the input lead while your body is electrostatically charged. I hope you purchased a spare! PROTECTING YOUR INVESTMENT
It seems bizarre that we use static sensitive components in an instrument and then use them in static experiments. The instrument gets its ultrahigh impedance from the use of feedback and the delicate micro thin layer of insulation separating the FET gate lead from the main substrate in the device.'This is a layer which is easily punctured by static. Needless to say, such puncture means the destruction of the device and IC. The wise application of the
electrometer device is the best precautionary measure one can take. Never leave anything hooked to the input connector when the instrument is not in immediate use! The size of the isolated capacity (item hung up as an antenna and hooked to the input) relates to the amount of charge collected, and thus the voltage collected. This instrument will respond to voltages in the range of +/- 7 volts or so. Large collectors around high voltage equipment such as Van DeGraf Generators, Wimhurst machines, Tesla Coils, etc., will collect hundreds of volts and thus destroy the input IC. The voltage collected also relates to proximity of the source of charge for any given collector size. If in doubt, use small collectors or a whip antenna hooked to the input. Never walk across the room and just touch the input collector or connector. You may carry thousands of volts on your body! Always ground yourself on the grounded instrument box. Remember, the box must always have a solid connection to a real earth ground to function properly. SUMMARY
Your meter can be used to determine the polarity of various insulators. Plastics are all negative. Polished, clean glass is always positive. Tapping one's toe in time with a piece of music while sitting on a modern carpet can induce +/-10 volt potential change on a can of Spam 5 feet away! The study of atmospheric electricity can be investigated. A people proximity detector could be devised. No matter what your use, you will see in short order, that every object is continuously exchanging charges with other objects in our everyday world. We exist in a sea of charge. For more information on electrostaties, an excellent book by A.D. Moore entitled Electrostatics is available again at a very modest cost and many experiments are suggested to introduce the amateur scientist to the fascinating world of electrostatics. PARTS LIST AND SOURCES FOR THE EXPERIMENTER'S ELECTROMETER • • • • • • • • • • • •
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R1-10 megohm 114 watt film resistor 5% R2, R4, R5 -10K ohm 1/4 watt film resistors 5% R3 - 10K ohm potentiometer linear taper (Pc type) R3A - 3.9K ohm 1/4 watt flm resistor 5% R6, R7 - 100K ohm 114 watt film resistors 5% Cin - 10pf disc capacitor 600 volt C1, C2 .22 ufd 50 volt mylar capacitors 10% ICI - LMC 6081 integrated circuit Digikey #LMC6081 INND IC2 - LM 324 integrated circuit Digikey # LM324ANND 8 pin machine pin socket Digikey #A400-ND 14 pin machine pin socket Digikey #A401-ND Metal box or cabinet to suit. Small enclosures available at any local Radio Shack (must be metal) 2-Battery clips for 9 volt batteries (local Radio Shack) Female UHF connector, chassis mount (PL-259 type) DigiKey # ARF1007-ND Circuit board, pad per hole, small, 1X2 or 2x3 preferred found at local Radio Shacks BNC or RCA type chassis mount output female jack. Local Radio Shack.
Sources: 1. Local Radio Shack stores and outlets.
2. Digikey Electronics, 701 Brooks Avenue South, Thief River Falls, MN 56701-0677 1-800-344-4539 Digikey order line.
Fig 1. Electrometer Amplifier Layout on Bottom of Board 40 K bytes
Fig 2. Basic Electrometer Circuit 25 K bytes
