Tab Electronics Guide to Understanding Electricity ... - Sciences Club

278 Chapter Eight
Figure 8-17
Speaker protection
circuit.
ac
In
D1
R1
LED1
C3
300 F
+
ZD1
4.7
k
390
k
390
k
11
k
D9
DPDT
Contacts
+
D3 C4 C5
R2
100 k Q2 Q3
D8
+
Relay
Q4
F1
R4
10 k
Right
in F2
Left
in
R5
10 k
C1
300 F
C2
+
+
D4
D5
D6
D7 Q1
Q5
R3
56
k
+
C6
220
F
Right
in
Left
in
thumps; and (3) it provides a visual “status” indication of the amplifier’s
operation utilizing a typical LED indicator.
Operational power for the Fig. 8-17 protection circuit is normally
obtained from the secondary of the power transformer used in the
power supply for the audio power amplifier (the current drain of this
circuit is very low, so it shouldn’t represent any significant load to any
large power transformer used to provide high currents to an audio
power amplifier). The connection terminal extending from the negative
side of C3 should be connected to the center-tap of the power transformer—the
transformer center-tap will always be the circuit common
point (see Fig. 8-16). The connection point extending from the anode of
D1, labeled “ac in” in Fig. 8-17, is connected to either “hot” side of the
transformer secondary (e.g., either 25-volt secondary wire coming out of
the 50-volt center-tapped transformer of Fig. 8-16).
Referring to Fig. 8-17, D1 and C3 make up a simple half-wave rectifier
circuit, providing a rectified and filtered DC voltage from the AC operational
power obtained from the secondary of the power transformer.
For example, if you connected this circuit to the 50-volt transformer secondary
of Fig. 8-16 (as described previously), the DC voltage at the cathode
of D1 should be approximately 35 volts (25 volts 1.414 35.35 volts