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Old 25th July 2006, 06:20 AM   #1
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Default Temperature sensors in an amp

I'm looking in to the project of adding a temperature sensor to an audio amp as a means of thermal shutdown (if the temp gets too hot, a relay will open to shut the circuit down).

Now, there are the temp sensor ICs that sense the temperature themselves, and then there are the temp sensor ICs where you attach a simple diode connected transistor to them (and they measure temp through the Vbe junction).

==> Is there an advantage to one of the above mentioned sensor topologies?

==> Where should I place the sensor in a power amp to measure the hottest point (I would think right on an output transistor, or perhaps on the heatsink for the output transistors)?

==> How should I fasten the sensor to the hot point? The sensor has to stay in place, and I also need to insure a good thermal interface (should I use thermal grease?).

Thank you!
-RT
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Old 25th July 2006, 06:39 AM   #2
jleaman is offline jleaman  Belgium
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Well it is a good idea to have this type of protection BUT you should never push the amp transistors to get hot and thermally break down. Meaning past there specification. What you should do is use the proper heat sinks.

Class a amplifiers need to be at a certain temperature tho. The rumor / Theory is that they sound better when warmer. Tis true with a few class a amps that i have too. What kind of amp are you building more info other than you want to protect it is needed.

Jase
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Old 25th July 2006, 07:33 AM   #3
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thermistors with a lug mounting to fix on the transistor mounting screw may be a good option, or may be solder a lug on the sense transistor.
Just a suggestion

Gajanan Phadte
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Old 25th July 2006, 07:45 AM   #4
rpapps is offline rpapps  Antarctica
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How about one of these.
Cheap (AU$5), self resetting, bolt on to heatsink and control relay.
Cheers
Rob
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Old 25th July 2006, 09:22 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
make a decision.
Latching or non latching?

If you adopt latching temp cut-off then the amp shuts down and you should arrange for either auto reset @ switchon or manual reset.

If you adopt non latching then the amp (or part of it) shuts down until the temp sensor/controller says it's cool enough to switch back on. The disadvantage is that the amp temperature cycles causing fatigue in the semiconductor devices that are suffering the worst heating effects. Repeated cycling WILL cause premature failure.
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regards Andrew T.
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Old 26th July 2006, 06:20 AM   #6
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Aha! Thank you all for the feedback, these are questions that I have to consider more carefully.

I was planning on a class AB amp using ONsemi's ThermalTrak transistors. I wanted thermal protection because I have never used these transistors before, and thus I'm not *exactly* sure how well they will set their own bias currents in the output stage. If they go in to thermal runaway, I'm hoping to save them before they loose their smoke.

I would think to use a latching system: if the unit does go in to thermal overload, I want it to stop alltogether because there is probably a problem somewhere that should be addressed.
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Old 30th July 2006, 04:59 AM   #7
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Some transistors don't have a "tab" to connect to (TO-247 types like MJL4281) so the heatsink is a good place to connect the thermistor.

Make the sense circuit sensitive, so when/if the heatsink is hotter than normal, the amp shuts off.
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