Thermal Protection Circuit

cm961 · 18th January 2003, 05:34 PM

Hi everyone,

I'm trying to understand this amp's thermal protection scheme. I've drawn out the schematic of it and it doesn't make sense to me.

The fused neutral line goes through 2 thermal switches (1 for each channel) (normally closed) in series and then connects to the live live. In paralelle with these two thermal switches is an LED w/ resistor. This is the temp. indicator LED.

So the basic operation would be, under normal temperature the switches are closed and the LED is off. On overheat, the switches open and the LED gets current. The things I don't understand are the following: If the switches are closed all the time, it shorts the live to the neutral and the fuse blows. If the switches are open (overheat condition) the LED comes on, but how does that protect the amp?

I can draw a schematic if I'm being too confusing. Any help greatly appreciate, I'm sure I'll get this amp working soon!

Pete

dhaen · 20th January 2003, 08:20 AM

Pete,

You're right: You are being confusing

Try posting a schematic. Draw it in MS paint if necessary. Then I'm sure you'll get some suggestions.

Cheers,

cm961 · 21st January 2003, 04:19 AM

Thanks John,

I figured it out. It was a really stupid mistake made by me. I was using my continuity tester to trace the circuit, and what I didn't figure was that the transformer coils were in series with the circuit, yet causing basically no voltage drop. So I'll explain the circuit, more clear this time (I hope).

The normally closed thermal switches are in paraelle with the LED indicator + resistor. All this is in series with the transformer. So what happens is that the resistor and LED are normally shorted. If one of the switches opens, the current flows through the resistor and LED. The resistor is 10k, and the impedance of the transformer at DC is very little, so the resistor drops about all the voltage. The LED gets some too and turns on, and since the resistor and LED are dropping all the voltage, the transformer coils get none and thus the power supply is effectively disabled.

I've made a few really amateurish mistakes on this amp, but I'm happy to annouce that I've got both channels functioning now. One has only 4 of 6 transistors hooked up, but I just have to order more and I'll solve that.

Thanks to everyone on diyaudio, you've all been very helpful.

Pete

18th January 2003, 05:34 PM	#1
cm961 diyAudio Member Join Date: Jun 2001 Location: Peterborough, Ontario	Thermal Protection Circuit Hi everyone, I'm trying to understand this amp's thermal protection scheme. I've drawn out the schematic of it and it doesn't make sense to me. The fused neutral line goes through 2 thermal switches (1 for each channel) (normally closed) in series and then connects to the live live. In paralelle with these two thermal switches is an LED w/ resistor. This is the temp. indicator LED. So the basic operation would be, under normal temperature the switches are closed and the LED is off. On overheat, the switches open and the LED gets current. The things I don't understand are the following: If the switches are closed all the time, it shorts the live to the neutral and the fuse blows. If the switches are open (overheat condition) the LED comes on, but how does that protect the amp? I can draw a schematic if I'm being too confusing. Any help greatly appreciate, I'm sure I'll get this amp working soon! Pete

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20th January 2003, 08:20 AM	#2
dhaen diyAudio Moderator Emeritus Join Date: Jun 2002 Location: U.K.	Pete, You're right: You are being confusing Try posting a schematic. Draw it in MS paint if necessary. Then I'm sure you'll get some suggestions. Cheers,

21st January 2003, 04:19 AM	#3
cm961 diyAudio Member Join Date: Jun 2001 Location: Peterborough, Ontario	Thanks John, I figured it out. It was a really stupid mistake made by me. I was using my continuity tester to trace the circuit, and what I didn't figure was that the transformer coils were in series with the circuit, yet causing basically no voltage drop. So I'll explain the circuit, more clear this time (I hope). The normally closed thermal switches are in paraelle with the LED indicator + resistor. All this is in series with the transformer. So what happens is that the resistor and LED are normally shorted. If one of the switches opens, the current flows through the resistor and LED. The resistor is 10k, and the impedance of the transformer at DC is very little, so the resistor drops about all the voltage. The LED gets some too and turns on, and since the resistor and LED are dropping all the voltage, the transformer coils get none and thus the power supply is effectively disabled. I've made a few really amateurish mistakes on this amp, but I'm happy to annouce that I've got both channels functioning now. One has only 4 of 6 transistors hooked up, but I just have to order more and I'll solve that. Thanks to everyone on diyaudio, you've all been very helpful. Pete

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