Best way to implement thermal protection?

mightydub · 2nd October 2009, 10:08 PM

Please share your opinions and thoughts on the best way to implement thermal protection for an amplifier - techniques I have seen are:

1) cut AC power
2) turn off the current source loading the input LTP (for example Adcom)
3) turn on a fan
4) trigger a limiter on the input, reducing drive

1 and 2 are drastic but effective. 3 helps the symptom but doesn't address the underlying cause. 4 has the advantage of not stopping the party but could be ignored leading to long term damage.

I'm experimenting with shunting current away from the bases of the OPS drivers, similar to the SOA protection circuits discussed by Doug Self, but triggered by over temp rather than SOA constaints. It occurred to me that with ThermalTrak transistors it is really easy to monitor junction temp in the output stage.

What are your thoughts on the pros / cons of these approaches?

MJL21193 · 2nd October 2009, 10:12 PM

5) use the right size heatsink(s)

richie00boy · 2nd October 2009, 10:14 PM

Monitor heatsink temperature and either mute the input or disconnect the output.

It's only needed though in cases of abuse, normal use it should not overheat, if it does the heatsinks are too small.

Upupa Epops · 2nd October 2009, 10:23 PM

Give detecting element ( e.g. Airpax ) directly ( or near ) power device...

mightydub · 2nd October 2009, 10:55 PM

Quote:

Originally Posted by richie00boy

Monitor heatsink temperature and either mute the input or disconnect the output.

It's only needed though in cases of abuse, normal use it should not overheat, if it does the heatsinks are too small.

The point is to be able to construct an amplifier that wouldn't need huge heatsinks sized for worst case, and conservative thermal limits. Monitoring heatsink temp for bias and overtemp tells you the integral of the power for the last several minutes, not what's happening right now.

Instantaneous junction temp can be far above the heatsink temp. You could have a much smaller heatsink designed for "average" power, and even let it run fairly hot, knowing that by monitoring the junction temp you can keep the output stage safe from harm by turning the power down.

CBRworm · 2nd October 2009, 11:02 PM

I myself try to size HS appropriately

Then thermal fan control, followed by power down. I have never had an amp overheat to the point of the AC kill tripping, but the protection is there.

2nd October 2009, 10:08 PM	#1
mightydub diyAudio Member Join Date: Dec 2006 Location: Where the sky loves the sea	Best way to implement thermal protection? Please share your opinions and thoughts on the best way to implement thermal protection for an amplifier - techniques I have seen are: 1) cut AC power 2) turn off the current source loading the input LTP (for example Adcom) 3) turn on a fan 4) trigger a limiter on the input, reducing drive 1 and 2 are drastic but effective. 3 helps the symptom but doesn't address the underlying cause. 4 has the advantage of not stopping the party but could be ignored leading to long term damage. I'm experimenting with shunting current away from the bases of the OPS drivers, similar to the SOA protection circuits discussed by Doug Self, but triggered by over temp rather than SOA constaints. It occurred to me that with ThermalTrak transistors it is really easy to monitor junction temp in the output stage. What are your thoughts on the pros / cons of these approaches?

2nd October 2009, 10:14 PM	#3
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	Monitor heatsink temperature and either mute the input or disconnect the output. It's only needed though in cases of abuse, normal use it should not overheat, if it does the heatsinks are too small. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

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2nd October 2009, 10:12 PM	#2
MJL21193 Account disabled at member's request Join Date: Mar 2007	5) use the right size heatsink(s)

2nd October 2009, 10:23 PM	#4
Upupa Epops diyAudio Member Join Date: Aug 2003 Location: Prague,Czech Republic	Give detecting element ( e.g. Airpax ) directly ( or near ) power device...

2nd October 2009, 11:02 PM	#6
CBRworm diyAudio Member Join Date: Jan 2007 Location: S. Florida	I myself try to size HS appropriately Then thermal fan control, followed by power down. I have never had an amp overheat to the point of the AC kill tripping, but the protection is there.

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