Handbook for Sound Engineers

Heatsinks and Relays 373

For a triac, VD is about 1.5 V; for SCRs, about 0.7 V.
For transistors it could be from 0.7 V to more than
100 V.
The following is an example of how to determine the
minimum surface area required for a flat aluminum
heatsink to keep the case temperature of 75°C (167°F)
for a triac while delivering a load current of 15 A, at
25°C (77°F) ambient and a voltage drop across the triac
of 1.5 V

Using Eq. 13-7

Using Eq. 13-6

It is important that the case temperature, Tcase, does

not exceed the maximum allowed for a given load

current, IL (see typical derating curves in Fig. 13-70).

Eq. 13-6 gives the surface area needed for a verti-
cally mounted heatsink. With free air convection, a
vertically mounted heatsink, Fig. 13-8, has a thermal
resistance approximately 30% lower than with hori-
zontal mounting.

In restricted areas, forced-convection cooling may be

necessary to reduce the effective thermal resistance of

the heatsink. When forced air cooling is used to cool the

component, the cubic feet per minute (cfm or ft^3 /min)

required is determined by

(13-8)

where,

1 W is 3.4 Btu,

temperature rise is in °C,

Q is the heat dissipated in watts,

'T is the heatsink mounting temperature minus the

ambient temperature,

K is the coupling efficiency (0.2 for wide spaced fins,

0.6 for close spaced fins).

13.2 Relays

A relay is an electrically operated switch connected to

or actuated by a remote circuit. The relay causes a

second circuit or group of circuits to operate. The relay

may control many different types of circuits connected

to it. These circuits may consist of motors, bells, lights,

audio circuits, power supplies, and so on, or the relay

Figure 13-7. A typical derating curve for solid state devices.

'TTcase–= Tambient

75 qC–= 25 qC

= 50 qC

WV= DIL

=1.5u 15

=22.5 W

A 133 W

'T

= ------ - in^2

133

22.5

50

= u----------

59.85 in

2

=

Model A

Model B

TC case temperature–C°

25

20

15

10

5

0

I 0 10 20 30 40 50 60 70 80 90 100

Rms on-state current–AL

Figure 13-8. Thermal resistance for a vertically mounted

(^1) e 16 inch aluminum plate of various dimensions.
40
30
25
20
15
10
7
6
5
4
3
2.5
(^2) 1 1.5 2 2.5 3 4 5 6 7 8 9 10
Side dimension of square plate—inches
$T/W = Q
Thickness:^1 / 16 inch
finish: bare
position: vertical
TC
TA: 50oC
Q
Thermal resistance—
oC/W
cmf Btu he
60
=----------------u0.02 temperature rise
1.76Q
'TK
---------------=