Lab
Guidelines
Electrical Laboratory Practice
J. Van der Spiegel
University of Pennsylvania
A. Safety
Observing proper safety precautions is important when working in the laboratory to prevent harm to yourself or others. The most common hazard is the electric shock which can be fatal if one is not careful.
Electric shock
Shock is caused by passing an electric current through the human body. The severity depends mainly on the amount of current and is less function of the applied voltage. The threshold of electric shock is about 1 mA which usually gives an unpleasant tingling. For currents above 10 mA, severe muscle pain occurs and the victim can't let go of the conductor due to muscle spasm. Current between 100 mA and 200 mA (60 Hz AC) causes ventricular fibrillation of the heart and is most likely to be lethal.
What is the voltage required for a fatal current to flow? This depends on the skin resistance. Wet skin can have a resistance as low as 150 Ohm and dry skin may have a resistance of 15 kOhm. Arms and legs have a resistance of about 100 Ohm and the trunk 200 Ohm. This implies that 110 V can cause about 160 mA to flow in the body if the skin is wet and thus be fatal. In addition skin resistance falls quickly at the point of contact, so it is important to break the contact as quickly as possible to prevent the current from rising to lethal levels. Here are several safety precautions one should follow all the time:
- 1. Never work alone.
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2. Use
instruments with three wire power cords (see equipment grounding)
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3. Always
shut off power before touching wires or connectors.
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4. Never
use power cords which are damaged.
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5. Always wear
shoes and keep shoes dry. Do not stand on metal or wet floors.
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6. Never
handle instruments when your hands are wet.
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7. Always
make the connection to the point of high potential at the last step.
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8. Set the current
limiter on power supplies to prevent too large currents to flow (of course
enough to supply the circuit under test). This will protect yourself, the
circuit and instruments under test.
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9. Never
contact the terminals of power supplies.
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10. Always
use wires and connectors with insulated shrouds.
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11. Don't
use too long a cable (short cables will reduce noise pick up as well) and never
have cables lay on the ground to prevent tripping over it.
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12. If
another person can't let go from an energized conductor, swtich the power off
immediately. If that is not possible, use non conductive material to separate
the body from the contact. Act swiftly but take care to protect yourself while
helping the other.
B. Equipment grounding
Grounding is very important. Improper grounding can be the source of errors, noise and a lot of trouble. Please consult the section of "Circuit Ground and Grounding Practice". Here we will focus on equipment grounding as a protection against electrical shocks.
Electric instruments and appliances have equipment cases that are electrically insulated from the wires that carry the power. The isolation is provided by the insulation of the wires as shown in the figure a below. However, if the wire insulation gets damaged and makes contact to the case, the case will be at the high voltage supplied by the wires. If the user touches the instrument he or she will feel the high voltage. If he or she stands on a wet floor or touches a pipe or faucet a large voltage is applied over the user and a current will flow through him or her to the ground as shown in figure b. However, if the case is connected to the ground by use of a third (ground) wire, the current will flow from the hot wire directly to the ground and bypass the user as illustrated in figure c.
Equipment with a three-wire cord is thus much safer to use. The ground wire (3rd wire) which is connected to metal case, is also connected to the earth ground (usually a pipe or bar in the ground) through the wall-plug outlet.
C Signal Interference and Shielding
When doing measurements, in particular of low-level signals, you will notice that the signal carries often a lot of noise.This is usually due to interference from electric and magnetic sources which are present in the laboratory or in nearby spaces. This can include 50 Hz from the fluorescent lights, switching power supplies, motors, a radio transmitter, or even faulted grounding (ground-loop interference; see also section on "Circuit Ground and Grounding Practice". Here are a few precautions that can be taken to reduce the effect of interference.
- Capacitive interference is the result of wires at different voltage placed closed to each other. When the voltage changes in one wire, a voltage can be induced in the neighboring one through capacitive coupling. Examples are wires coming from equipment such as function generators, oscillators, and wires from the outlet (60 Hz). The higher the frequency of the interfering source the larger the interference will be. Another source of capacitive interference are fluorescent lamps. The best way to prevent capacitive interference is to use shielded cable or coaxial cable. The shield around the cable is usually connected to the ground and protects the inner signal carrying conductor from the interference.
- Inductive interference is the result of time varying currents in a conductor such as coils. It is not always easy to prevent this interference. If possible one should work far away from such sources or shield the source with a ferromagnetic shield. Also, keeping large current carrying wires and small signal carrying wires fare apart; and keeping current carrying wires perpendicular to one another.
- Radio frequency (RF) interference is the result of radio transmitters and arcing in motors. To reduce the interference one can shield the low signal carrying wire with a well-conducting shield connected to the ground.
- Ground loops are the results of voltage differences which often exists in the ground plane to which the circuits are connected. This can be eliminated by connecting all ground points in the circuit to only one point which is connected to the earth ground.
Another good practice is to use short cables which carry small signals and keep them fixed. If necessary fix them to the table with tape. To reduce the effect of magnetic pick-up you can twist the wires.
D. Tools
Besides the instruments and electronic components you will be using a variety of tools and cables. Each student group should have their own toolset (protoboard, one wire stripper, one long nose players and a small screwdriver kit). This is available from the RCA lab for $25 (to be paid to the business office in room 559M).
A protoboard is a tool for constructing and testing circuits. It has several strips with lots of holes in them which conduct in one direction. The thin long strips on the sides and in the middle conduct along the long direction while the fat strips conduct along the short direction as shown in the figure below. Protoboards have also posts which facilitates the connection to the power supply and ground. You should follow the following convention: use the black pole for the ground or reference, the red one for the positive voltage and the yellow one for the negative voltage or signal line.
Typical Protoboard
E. Sources of Errors
Doing a lab is usually a pleasant and insightful experience. However, often it can be frustrating, if you get unexpected results and don't know how to solve the problem. The best way to prevent such time consuming errors, is to follow good lab practice and prepare yourself before doing the experiments. One of the common sources of errors in the lab are:
1. Improper grounding (make sure you read the section on Grounding Practices on the EE Lab homepage).
2. Sloppy protoboard organization. Use short and straight wires of different color. The layout of the protobaord should follow close the circuit.
3.Lack of understanding of the equipment.
4. Not reading the instructions or schematics carefully.
5. Not doing the pre-lab.
6. Rushing
7. Never connect a power supply to the output of a function generator. This will damage the function generator.
8.When doing current or resistance measurements with the multimeter, do not put a voltage over the current or resistance input terminal of the multimeter.
F. References:
1. "Guide to Electronic Measurements and Laboratory Practice", S. Wolf, Prentice-Hall.
2. "Lab Manual - The Analysis and Design of Linear Systems", J. Getty, Prentice-Hall, 1994.
3. "Basic Engineering Circuit Analysis", J. D. Irwin, Prentice-Hall, 1996.
4. "Electric Shock Hazard", C.F. Dalziel, IEEE Spectrum, Feb. 1972, pp. 41-50.
Used pending permission of J. Van der Spiegel, University of Pennsylvania