Handbook for Sound Engineers

Resistors, Capacitors, and Inductors 251

noticeable when trying to adjust a bias voltage with an

old oxidized pot.

Sometimes the pot can be cleaned by spraying it with

a contact cleaner or silicone and then vigorously

rotating it. Usually, however, it is best to replace it

because anything else is only temporary.

Any dc voltage present on the pot is also a source of

noise. Such voltage is often produced by leaky coupling

capacitors at the input connector or output circuit of the

wiper, allowing dc voltage to appear at the wiper

contact. If there is a resistance between the resistor and

the wiper, the dc current flowing through the wiper

contact to the output stage will create a voltage drop.

Because the wiper is moving, the contact resistance

constantly changes creating what looks like a varying ac

voltage. Using Fig. 10-13, the value at VLoad, whether ac

or dc, can be calculated with Eqs. 10-10 and 10-11. If

the wiper resistance is 0—i.e., a perfect pot—the output

voltage VLoad is

(10-10)

where,

If a pot wiper has a high resistance, Rw, the output

voltage VLoad is

(10-11)

where,

.

10.2 Capacitors

Capacitors are used for both dc and ac applications. In

dc circuits they are used to store and release energy such

as filtering power supplies and for providing on

demand, a single high voltage pulse of current.

In ac circuits capacitors are used to block dc,

allowing only ac to pass, bypassing ac frequencies, or

discriminating between higher and lower ac frequen-

cies. In a circuit with a pure capacitor, the current will

lead the voltage by 90°.

The value of a capacitor is normally written on the

capacitor and the sound engineer is only required to

determine their effect in the circuit.

Where capacitors are connected in series with each

other, the total capacitance is

(10-12)

and is always less than the value of the smallest

capacitor.

When connected in parallel, the total capacitance is

(10-13)

and is always larger than the largest capacitor.

When a dc voltage is applied across a group of

capacitors connected in series, the voltage drop across

the combination is equal to the applied voltage. The

drop across each individual capacitor is inversely

proportional to its capacitance, and assuming each

capacitor has an infinitely large effective shunt resis-

tance, can be calculated by the equation

(10-14)

where,

VC is the voltage across the individual capacitor in the

series (C 1 , C 2 ,˜˜˜ Cn) in volts,

Figure 10-12. Construction of a wirewound resistor.

Wiper

Wiper

End view

A. Fixed card wirewound resistor.

B. Tapered card wirewound resistor.

VLoad V 1

Ry

R 1 +Ry

------------------

©¹

= §·

Ry

R 2 RLoad

R 2 +RLoad

=--------------------------

VLoad Vw

RLoad

Rw+RLoad

---------------------------

©¹

= §·

Vw V 1

R 2 Rw+RLoad

R 2 ++Rw RLoad

---------------------------------------

©¹

= §·

Figure 10-13. Effects of wiper noise on potentiometer

output.

Vin

C 1

C 2

RLeakage

R 2

R 1

RW

RLoad

V 1

VW VLoad

CT^1

1

C 1

----- -

1

C 2

----- - }^1

Cn

++----- -

=-----------------------------------------

CT=C 1 ++C 2 } Cn

VC VA

CX

CT

------

©¹

= §·