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components |
resistors and capacitors in brief | ||
        
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This summary page contains basic information about the use and action of resistor and capacitors in valve (tube) radio circuitry. More specific technical info about resistors or capacitors may be selected from the buttons. Resistors
Perhaps the most common components in any radio are resistors, in a variety of forms. Symbols encountered in valve radio technology are not the same as those used today. Physical appearance has changed, too, with the old large stick types becoming obsolete. Despite their large size, they were capable only of modest power dissipation and modern, smaller resistors can often handle as much, or considerably more, power. Pictured left: almost all variable resistors resistor are in reality potentiometers. A variable resistor is usually a potentiometer with the 'slider' (the centre tag connection) wired to one of the track ends. Typical uses in radio include volume and tone controls.
Resistors in series When
we connect resistors in series by wiring them end to end,
their individual values simply add up, the total resistance being the sum of the
individual values in the circuit. EXAMPLE: 100 ohms in series with 200 ohms in series with 50 ohms = 350 ohms total resistance. Resistors
in parallel When
resistors are joined side-by-side, the combined value of
resistance is always less than the value of the lowest value resistor in the
network. An example: if we have two resistors wired in parallel, one being 200 ohms and the
other being 100 ohms then the
total value of resistance must be less
than 100 ohms.
But,
how much less? We can use this formula to calculate the total value of the
resistance from any network of 2 parallel resistors: Rtot = R1 X R2 over R1 + R2 |
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Preferred values In practice, it is easiest to choose a resistor nearest the value you require, bearing in mind that resistors come as so-called ‘preferred values’. Usually, the nearest preferred value is near enough: few valve circuits need exact values, especially with the higher resistance ranges. So, if you decide you need 185 ohms you will choose the nearest available, in this case 180 ohms. Capacitors Or, to give them their old name, condensers. The second most All rights reserved. © VRW 2008/2010 |
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common passive
component in radio technology, these things come in a very wide range of sizes,
forms and types for many purposes. In valve radio receivers, most of the
physically larger capacitors were either of a tubular form, with a paper
dielectric, or of a flat form with a mica dielectric. Paper types are common in
AF (old LF) circuits for coupling and decoupling purposes at audio frequency -
inter-valve coupling, tone control etc. Mica types do similar functions but at
much higher frequencies: RF (old HF) and also, in conjunction with inductors,
tuning and alignment of tuned circuits.