Question on heatsinking

[imix500]

I've been using this little kit from MPJA- a kit from electronic rainbow named LCD-T1, it uses two lm34 sensors. Once the meter is callibrated it's great. The sensors response is pretty fast, but varies depending on how well coupled they are to what you are measuring.

[lineup]

Originally posted by AuroraB


LM35 - slow?

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.

Thanks AuroraB.
This gives me the option to use LM35.

It is a nice and useful little chip.
As it outputs 10 mV for each degree C = linear scale.
Together with some Op-amps, one reference and your multimeter
it makes a not too difficult circuit to measure temperature.
With good precision, I think.

It should not cost very much.

LM35 - Precision Centigrade Temperature Sensor
• Calibrated directly in ° Celsius (Centigrade)
• Linear + 10.0 mV/°C scale factor
• 0.5°C accuracy guaranteeable (at +25°C)
• Rated for full -55° to +150°C range
• Suitable for remote applications
• Low cost due to wafer-level trimming
• Operates from 4 to 30 volts
• Less than 60 µA current drain
• Low self-heating, 0.08°C in still air
• Nonlinearity only ±¼°C typical
• Low impedance output, 0.1 Ohm for 1 mA load
http://www.national.com/ds.cgi/LM/LM35.pdf
.

http://www.national.com/pf/LM/LM35.html

lineup

[PeteM]

For my DOZ I used the "bells & Whistle" circuit here

http://www.cpemma.co.uk/thermal.html

Using small good quality fans with ball bearings makes for very low noise.

Once the circuit is adjusted and supply voltage to the fans is tweaked the set up is reliable. Only during hot summer use could you hear faint fan noise My thermistor monitored air flow temperature.

[AndrewT]

Hi,
the Rth s-a of a sink is roughly inversely proportional to the square root of the length.

If the sink you know is 3inches tall and has Rth s-a =0.6C/W then an 8inch tall section will have an Rth s-a ~=0.37C/W

Rth s-a (8")=Rth s-a (3") / Sqrt(8/3).

Remember to apply the correction factor for delta T (the difference between the contact point/face temperature and the ambient temperature)
Remember also, that the sink only matches the manufacturer's data if the whole contact face (or the manufacturer's stated method) is at contact face temperature. A large sink that is too thin will not transfer adequate heat to the far extremities compared a to smaller sink. This results in the fins at the larger distances running at a lower temperature than the manufacturer's test method and the effective Rth s-a can be much higher than the the data sheet and WILL be at least some margin higher than the datasheet.

The result of these two factors is that the semiconductor runs at a temperature significantly higher than expected if these corrections are omitted from your model.

[geminni]

How about adding fan? On a website http://www.aavidthermalloy.com
there is wery friendly search engine There i can input LFM for forced convection. I put LFM for sunon 80x80mm fan that has 40CFM, in LFM it is about 584LFM. With this fan and 300mm long heatsink i get 0.3C/W. I wonder how to calculate with two fans? Will it be double LFM, 1168? If it is like that then i get 0,21 c/w just like i need for amplifier