Now you can protect your HiFi amplifier from damage due to excess heat. This circuit will cut off the power to the amplifier board at the moment it senses a high temperature. It automatically connects power when the temperature returns to normal. The circuit uses an NTC Thermister as heat sensor and it also has audible warning and reset provisions.
The circuit uses the popular timer IC NE 555 as a temperature controlled switch. Its trigger pin 2 is connected to a potential divider comprising VR and the NTC Thermister. Thus the voltage level at pin 2 depends on the resistance of Thermister and the setting of VR. NTC (Negative Temperature Coefficient) Thermister has high resistance in normal temperature and the resistance decreases when the temperature increases. The Threshold pin 6 of IC is used to reset the IC if needed. When the pin 6 gets higher voltage than pin 2 IC will reset and output becomes low.
Power to the amplifier board is given through the Normally Connected (NC) contacts of the relay and the switch S2. So that power to the amplifier board will be available through the NC contacts of the relay and S2 when the relay is in the de-energized state. Resistance of the Thermister is set by VR so as to keep the trigger pin 2 of IC high at normal temperature. When the temperature inside the amplifier cabinet increases, resistance of Thermister decreases so it becomes conducting. This makes the trigger pin 2 of IC low and its output becomes high. T1 conducts, so that the relay energizes and buzzer beeps. This will break the power supply to the amplifier board. The relay will de-energize automatically when the temperature returns to normal
Keep Thermister near the Heat sink of the amplifier IC. Adjust VR for the particular temperature level at which relay energize.