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Circuit Analysis
In the following circuit diodes D1 through D8 act as voltage limiters and
thereby limit the power that can be applied to the driver. A high
wattage 5 ohm series resistor is used as an amplifier load if diode
clamping occurs. In this case fuse F1 is chosen to match the resistors
maximum current based on wattage.
Additionaly, a 1 ampere fast blow fuse (F2) or light bulb is added inline
with the driver side of the circuit. In theory fuse F2 will protect
your amplifier should the circuit module be plugged in backwards.
This will add a little bit of series resistance
Under normal conditions diode clamping does not occur with the load voltage
V=Vin*Z/(Rs+Z). In this case the currents in F1 and F2 is low.
If clamping does occur, excess voltage and current will be passed through
Rs and the diodes.
F1 Rseries F2
3A Fast 5 ohm, 20W 1A Fast Blow
<--o/\/\o-----/\/\---+--------------------------o/\/o--->
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| D1 D2 D3 D4
+-|>|-+-|>|-+-|>|-+-|>|-+
AMP | | | | | DRIVER
+-|<|-+-|<|-+-|<|-+-|<|-+
D5 D6 D7 D8 |
|
<--------------------------------------------+---------->
Notes:
D1 through D8 are silicon diodes with ~0.7V forward voltage drop. Choose the
number of diodes, current etc. depending on desired wattage to the load,
your amplifier wattage, power resistor wattage etc.
This picture below shows the circuit constructed into a WT2 box. Power for
acoustic measurements can be modest, so our lab uses a 25W NAD receiver. This
made F1 unneccessary to protect the 5 ohm 20W power resistor (two 10 ohms in
parallel) and it allowed us to use lower current 1N4001 diodes. A slightly
higher load drive capability was also desired so five diode pairs have been used
for 3.5V peak (max) or 2.45Vrms before distortion occurs. Finally F2 was
replaced with a 12W automotive light bulb that has a DC resistance of about
1 ohm. The effective series resistance in this case is 5+1=6 ohms, which
is not what is indicated on the box label.
If a lower peak voltage is desired, simply put a jumper or switch across some
of the diodes. If space is tight (like this box), aligator clips are a
simple solution
Click Image To Enlarge
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