2B Circuit Building

INTRODUCTION TO CIRCUIT BUILDING

   In learning to deal with circuit construction, the novice student may need the following guidelines. Please read and study the advice carefully since the equipment you will be using throughout the 2B labs is delicate and quite expensive; abuse of this equipment will not be tolerated and connecting a circuit incorrectly may damage power supplies or measuring instruments. Please check with your instructor to see if your circuit is correct before turning on the power; this way, if you "fry" an instrument, it will be your instructor's fault, not yours!

Eleven things to remember:

1. Assemble your circuits with the power disconnected and turned off.

2. Double check all connections before turning on the power.

3. If you have any doubts, have your instructor check your circuit before turning on the power.

4. How to turn the power on and off: Make sure all voltage and current setting dials on the power supply are turned to zero. Turn the power switch on. Slowly turn up the power to the desired level. Turning off the power is the reverse of the above; turn down the power to zero then turn off the power. This method introduces less current shock to the circuitry.

5. Do not use both hands at the same time when checking equipment with the power on. Put one hand behind your back so you don't complete a circuit that includes your heart. A current through your heart of 0.1 A for a few seconds will induce fibrillation.

6. Before you change a setting on any device, try to predict the outcome first by physical reasoning. This is most important if you want to understand what is going on.

7. For non-auto scaling meters, like the VOM and hand-held DMM's, start with the dial setting with the largest numerical value. Do this for voltmeters and ammeters alike. This method will help you avoid damaging the meter by sending too much current through it on a low setting. Start with the largest setting and move down gradually to smaller settings until you have the setting that provides the most significant figures (for digital readouts) or where the scale increments are furthest apart (on "analog" or meter readouts). Also, never measure a resistance value while a current is already through that resistor!

8. Never pull any plug out of its jack by pulling on the wire. Pull on the plug housing instead, or you may pull the wire right out of the plug.

9. Avoid mishandling the equipment by dropping it or putting it on the table carelessly. Electronic equipment can be very delicate.

10. If something is broken, don't put it back on the cart. Have your instructor check the suspect equipment and fill out a repair tag describing the problem. You will not be punished for reporting broken equipment.

11. Put away your equipment as neatly as you found it. Coil all power cords. Put away connecting wires by type and color. Make sure all equipment is turned off. See that your lab table area is cleared and clean. Garbage goes in the garbage cans, not around your table.

Circuit building tips:
1. Color code your wiring as best as possible. Use red wire for high potentials and black wire for ground or low potentials.

2. Set up the actual circuit first and then insert the necessary measuring equipment. In our class, the actual circuit alone is usually simple; things start getting complicated when measuring equipment is applied.

3. Constantly check connections to make sure they are not loose (do this safely!). Half of the problems students have with circuit construction are due to bad connections.

4.   Layout your circuit in the simplest possible manner with the fewest possible wires. This makes it much easier to troubleshoot your circuit if you encounter problems.

Types of equipment and measuring instruments you will use:

1. VOM: Stands for volt-ohm meter although it can usually measure current as well. This is the classic analog meter.

2. DMM: Digital Multi-Meter. In this class, a small, hand held, battery powered instrument that does essentially what a VOM does except it gives you a digital readout. Be careful with these meters as they can give spurious readouts and are more delicate than they appear.

3. HP-DMM: The Hewlett-Packard digital multi-meter is powered by a cord and provides highly accurate and precise values. It has auto-scaling and control of the significant figure in the display. Nonetheless it still does essentially the same job as a VOM.

4. DC power supply (also called a "TPS" for transistorized power supply): The Heath Kit DC power supply is a delicate beast. Be careful not to demand too much current. This should be an item to check when you are troubleshooting a circuit.

5. High Voltage power supply: This Heath Kit power supply should be treated with respect as its high voltage could harm you.

6. HP function generator: This instrument provides alternating voltage (AC) at variable frequencies in three different signal forms: sinusoid, square, and triangle.

7. Dual trace oscilloscope: This instrument is the basic measuring tool for many labs. It is by far the most complicated instrument to master in this lab, yet its basic function is simpler than even the VOM's.

Types of circuit elements you will encounter:

1. Resistor: A device that "resists" the flow of charge through it.
2. Capacitor: A device that temporaily stores electrical energy.

3. Battery: A portable source of electrical power, usually of low voltage.

4. Inductor: This is used in AC applications and exploits the properties of changing magnetic flux; it "resists" a changing current.

5. Diode: Ideally, this device will allow charge to pass through it in one direction only.

Common concepts (see glossary also):

1. Voltage: A difference in electric potential between (or across) two points. The two points may be in space or actually located on a specific body or circuit.

2. Current: The flow of charge through a material body, or a vacuum, with respect to time.

3. Resistance: The opposition a body presents to the flow of charge though it.

4. Capacitance: The ratio of the amount of charge a body holds to the electric potential it consequently possesses (C = Q/V).

5. Inductance: The property of a circuit component (or even a circuit) that resists the change of magnetic flux through it.

6. Ohms' Law: A basic although not fundamental law commonly applied in circuit analysis. It is a linear relation between voltage, current and resistance. Bear in mind that although Ohms' Law is a linear relation, this does not mean all circuits elements are linear, indeed most circuit elements are non-linear and do not obey Ohms' Law.