The carbon-composition resistor

Chapter The carbon-composition resistor

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • It’s always a good idea to short out all filter capacitors, using a screwdriver with aninsulated handle, before working on a high-voltage dc power supply. I recall an instancewhen I was repairing the supply for a radio power amplifier. The capacitors were holdingabout 2 kV. My supervisor, not very well acquainted with electronics, was looking overmy shoulder. I said, “Gonna be a little pop, now,” and took a Phillips screwdriver, makingsure I had hold of the insulated handle only, and shorted the filter capacitor to the chas-sis. Bang! I gave my supervisor a brief explanation while he took some deep breaths.Even if a supply has bleeder resistors, they take awhile to get rid of the residualcharge. For safety, always do what I did, whether your supervisor is around or not.Impedance matchingA more subtle, more sophisticated use for resistors is in the coupling in a chain of am-plifiers, or in the input and output circuits of amplifiers. In order to produce the greatestpossible amplification, the impedances must agree between the output of a given am-plifier and the input of the next. The same is true between a source of signal and the in-put of an amplifier. Also, this applies between the output of the last amplifier in a chain,and the load, whether that load is a speaker, a headset, a FAX machine, or whatever.Impedance is the alternating-current (ac) cousin of resistance in direct-current(dc) circuits. This is discussed in the next section of this book.The carbon-composition resistorProbably the cheapest method of making a resistor is to mix up finely powdered carbon(a fair electrical conductor) with some nonconductive substance, press the resultingclay-like stuff into a cylindrical shape, and insert wire leads in the ends (Fig. 6-5). Theresistance of the final product will depend on the ratio of carbon to the nonconductingmaterial, and also on the physical distance between the wire leads. The nonconductivematerial is usually phenolic, similar to plastic. This results in a carbon-composition resistor.Carbon-composition resistors can be made to have quite low resistances, all theway up to extremely high resistances. This kind of resistor has the advantage of beingpretty much nonreactive. That means that it introduces almost pure resistance intothe circuit, and not much capacitance or inductance. This makes the carbon-composi-tion resistor useful in radio receivers and transmitters.102 Resistors6-4A bleeder resistor is connected across the filter capacitor in apower supply.