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Hastur 31st January 2007 01:58 AM

Question on heatsinking
Do you think that a 8" section of the extrution from the r-theta group order a while ago would be sufficent for a single channel A30?

twitchie 31st January 2007 03:22 AM

I've been asking myself that same question, and I think the R-Theta group buy profile is 9012, so C/W/3" section is listed as 0.6:

I'm not sure about the math, but I think this means a 6" section will give you a (ideal conditions) C/W of 0.3 - 100W idle load will raise the heatsink temp to 30C over room temp. Double again to 12" and you should get 0.15 and 15C for a 100W load, so figure between 15 and 30C - closer to 30 under average heat transfer (that's 30C over room temperature) for an 8" section. Some will chime in with a link to the heatsink calculator/simulator and you can plot your heat points etc for a real simulation.

sawreyrw 31st January 2007 03:36 AM


Your assumption is incorrect. The thermal resistance is not inveresly related to length. The thermal resistance of a 6 inch section will be greater than 1/2 the resistance of a 3 inch section. My guess is it will be around .4. There is a link to the R-Theta calculator on the page you mentioned.

Hastur 1st February 2007 06:31 PM

So I can assume 8" section would be ~.3 C/W and since a lot of people have had success with the big conrad heatsinks listed at .21, but taking into account the 1.4 multiplier since they rate temp different, that makes it effectively .3 I should be fine.

Cool, well not cool, but not boiling either.

lineup 1st February 2007 06:48 PM


let's sink the temp :clown:

Question 1:

Anybody know of a temperature monitor project?
Good if it is a project for monitor power amp heatsinks.
- 1 sensor at heatsink, close to Transistors
- some indicators: LEDs
or maybe
- measure millivolt per degree with multimeter at testpoints
- digital display (advanced monitor)

Question 2:

What would be a suitable sensor?
- LM35 is a dedicated IC, gives 1mV raise / each degree C
but I have been told it is very slow .. like 5 minutes to reach value
- BD139, TO-126 transistor could be used, ~2.2 mV / each degree
in change of VBE, volt base emitter
.. but would need some Op-Amp and some calibration to get a nice scale


lineup 1st February 2007 06:59 PM


I asked a similar question 1 year ago.
I got this nice answer:

Originally posted by Arius

I'd treat the BD139 as a silicon PN junction diode.
In that case, excite it with 10uA.
You should get about 2.3mV/degC sensivity with that.


The ThermalTraks are supposed to replace this temp dependent bias generator circuit (their internal diodes, that is).

What Aruis tells me is:
1. I should use only like 10uA current flow in the BD139
if using it as temperature sensor
2. I will get ~ -2.3 mV / degree C in temp raise

Original Topic:
On Semi ThermalTrak - Power Transistors

Temperature Control Department
Lineup Audio Lab

Another good topic on heatsink and temp control, temp tracking
where also Nelson Pass contributed some good advice:
MOSFET amp problem - Heatsink and Temp tracking



DSP_Geek 2nd February 2007 06:21 AM

Would that group buy still be open, by any chance?

Allen Newby 2nd February 2007 12:02 PM

I use a water cooling system with a flow controller to monitor the coolant flow and temperature. This shuts the amp down if things get too hot, or if the flow fails for any reason.

I made use of a temperature sensor sold in the UK by Maplin. This may be OK for monitoring a conventional system providing it doesn't get too hot. It is certainly easier than messing about with Vbes, bandgap references, instrumentation grade opamps:

Temperature Module

This module will display up to 69.8 Celcius and can be rigged to alarm. You can connect an external temperature probe, and there is a display bezel for it too:.

Piercarlo 2nd February 2007 12:16 PM

Re: Question on heatsinking

Originally posted by Hastur
Do you think that a 8" section of the extrution from the r-theta group order a while ago would be sufficent for a single channel A30?
dissipation ability is related to:

1) heathsink surface

2) Air speed flowing on this surface (and hence is dependent on mounting: vertical mounting allow more air flow to run on heathsink surface than horizontal).


PS - "Bulkity" of heathsink is more related to its thermal capacitance rather than its surface. More fins act more efficiently of more weight.

AuroraB 2nd February 2007 12:38 PM


Originally posted by lineup

- LM35 is a dedicated IC, gives 1mV raise / each degree C
but I have been told it is very slow .. like 5 minutes to reach value

Not so.. the chip reacts in seconds- it's the heated mass and your thermal coupling that sets the response.
I use it with good results for measuring internal temp's in sounding rockets!
Any DPM set to 0-1V gives you direct readout of temp. If you want alarms or switch off, use a comparator or a small dedicated DPM.

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