New Page 6

VINTAGE RADIO WORLD - RESISTORS

resistors

what resistors are and how they work

Resistors in common use in valve radio receivers

Pictured left, top: half watt resistor. Middle: 2 watt resistor. Bottom: a typical wire-wound resistor

Resistors used in valve radio receivers are commonly made from carbon. These types are low-wattage ones, which means they can only handle small currents. They limit current by converting the electricity into heat, which is dispersed by convection (heat rises) into the surrounding air. In many elderly radios, they are in the form of a carbon stick - physically quite large but still only capable of modest power dissipation. Modern equivalents may be much smaller in size but capable of dissipating higher power levels.

Higher wattage resistors are made as ceramic tubes with resistive wire wrapped around them. They act almost like electric fire elements, though not as hot, of course! These are used as 'ballast' resistors in AC/DC sets, dropping the mains voltage to a suitable supply level for a chain of valve heaters. They also are used for HT smoothing, forming time-constant circuits with electrolytic capacitors.

Ballast resistors commonly go open-circuit. Care must be taken to fit correct values and wattages when replacing these.

How they work

If we connect a resistor across a voltage source, then a current will flow, the strength of which will then depend upon the potential difference as provided by the source of voltage and the amount of resistance in the circuit. The end result is a voltage drop for a given flow of current which is dependent upon the ohmic value (resistance) of the resistor. Without a flow of current through a resistor, there can be no voltage drop across it.

Potential difference is measured in voltage (V) and the relationship between current and voltage is a simple one. For a circuit with a given resistance, doubling the potential difference (the voltage) will double the current flow.

EXAMPLE OF RELATIONSHIP BETWEEN CURRENT AND VOLTAGE

V =	1	2	3	4	5
I =	.1	.2	.3	.4	.5
V/I = ohms	10 ohms	10 ohms	10 ohms	10 ohms	10 ohms

The OMEGA shown above is the symbol used to represent resistance.

I have shown it as an image because the symbol when used as standard text may not be recognised by web software, rendering the text effectively unreadable.

The symbol (I) is used to represent current. Current is measured in amperes, (usually shortened to amps or simply A). We can see that the ratio V over I remains constant. This is because the voltage divided by the current is equal to the resistance as measured in ohms (W). In our example above, the circuit has a resistance of 10 ohms.

Ohms Law

This states that the current through a conductor is directly proportional to the potential difference across it. However, Ohms law only holds good under stable conditions (in other words, no temperature changes) and only applies to ohmic conductors: that is, conductors that obey Ohms law! These are the commonplace conductors made from carbon and from metal alloys.

How to use Ohm’s Law

The useful thing about Ohms law is that it allows you to calculate the value of any one of resistance, current or voltage if you know the other two. You can obtain the value of resistance in a given circuit by dividing the voltage by the current. If you divide the voltage by the resistance, then you obtain the current: if you multiply the current by the resistance you obtain voltage. Although the working values are in volts, ohms and amperes, an ampere is a rather large unit for electronics and you may find it useful when doing simple calculations to substitute milliamps (mA) for A and kilohms (k) for R. You must substitute both, however.

One milliamp is one thousandth of an Amp. Ten mA are therefore one hundredth of an Amp and one hundred mA are one tenth of an Amp.

Power

Power is the term used to describe either the rate of production of electrical energy (for example, an electric generator) or the rate of dissipation of electrical energy (for example, a light bulb). The symbol for power is P. We use the term watt as the unit of power. It is named after James Watt.

Although small torch type bulbs will state current consumption, large light bulbs that use mains electricity give a power rating in watts. The higher this rating, the greater the light output (and the greater dissipation of power).

To find power value, simply multiply V X I. Example. A torch bulb is rated at 6V, 0.5A. Multiply six by a half and the answer is three: the bulb is a 3 watt bulb.

VINTAGE RADIO WORLD