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Old 8th October 2002, 10:57 AM   #1
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Default Amp Protection Circuit - Here!

Many folks here have been looking for an Amplifier Protection Schematic. Many links have been provided with many pointing to Rod's project. Here is a circuit from Elektor published in the 1980s. It is relatively cheap and the components are readily available. The circuit is very reliable.

The functions provided by the circuits are as follows:
Power on Delay, Output DC protection for speakers, Delayed speaker connection (all of these accomplished thru' 2 [or 3] relays) and Thermal protection (a fan and indicator LED can be switched on at a predetermined temperature of the heatsinks).

Not shown in the schematic are 15-0-15 Volt 15VA transformer and a 12 volt regulator (LM7812).

This is the schematic that has all functions mentioned above except the thermal part:
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File Type: jpg amp protection schematic1.jpg (42.9 KB, 3051 views)
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Old 8th October 2002, 11:07 AM   #2
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This is the PCB layout for the above protection circuit:
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File Type: jpg amp protection pcb1.jpg (13.3 KB, 2345 views)
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Old 8th October 2002, 11:09 AM   #3
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The component overlay is here:
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File Type: jpg protection pcb overlay1.jpg (34.8 KB, 2293 views)
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Old 8th October 2002, 11:14 AM   #4
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The temperature sensor circuit is optional. It only switches on a Fan and indicator LED through a relay but does not disable the speakers or the input to the amp. These can be configured by the user if desired.

Schematic of the Thermal Sensor:
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File Type: jpg thermal prot. circuit1.jpg (21.6 KB, 2341 views)
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Old 8th October 2002, 11:16 AM   #5
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PCB artwork for the Thermal Sensor:
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File Type: jpg thermal prot.pcb1.jpg (3.6 KB, 2085 views)
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Old 8th October 2002, 11:17 AM   #6
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Component Overlay for the Thermal Sensor PCB:
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File Type: jpg thermal prot.pcb overlay1.jpg (4.1 KB, 1692 views)
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Old 27th November 2006, 07:58 PM   #7
edl is offline edl  Hungary
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Lot thanks!
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Old 27th November 2006, 08:23 PM   #8
lineup is offline lineup  Sweden
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Can you give some basic
instructions.

how and where to attach the inputs from an amplifier

I guess for example that T8 and T9, in temp sensor,
are to be put on main heatsink of power amp?
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Old 28th November 2006, 01:16 PM   #9
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The circuit needs a 12-0-12 or 15-0-15volt 300-500mA transformer. You could tap the power after the rectifier diodes and run an LM7812 to get the 12 volt DC supply for the relays.

Amp outputs are wired from NC contacts of the output relays to R1 and R4 of Figure 1. Ground connections have to be completed.

Lineup, you are right about the temp. sensing transistors. You can do a variety of things with Re3 contacts - mute the input, switch on fans, trigger a temperature variable fan circuit; or if set to sense very high temperatures of output devices/heatsink, cut off power to the output stages of the amp etc.,
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Old 28th November 2006, 01:27 PM   #10
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Nice find. Do you have the original article? If you scan the artworks in and save as PNG (lossless compression) instead of JPG (losy compression) you would achieve usable artworks
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