adnan_m_s

Hi every body,

I am an electronic enthusiastic, but I need to know some information on how to select the proper heat sink , I have told that there are equations for this but can any one tell me in simple terms , let us say I have to dissipate 7Watts from a pass transistor of a voltage regulator circuit what is the proper value of the heat sink to sue I mean (C/W)
let say I have a piece of 2w heat dissipation without sink and 20w to heat sink what should I do?
Adnan_M_S

Gayan Soyza

If I need a heat sink I won't go deeper measurements in the Heat sink.

All I do is I fix a small one & see.After a certain period working the circuit if cannot touch or its getting too much heat I'm replacing with another big one.

It depends upon the space enclosure also.Some times I wont place heatsinks too but still the circuit is working perfectly.

For a 7W Power dissipation a small one will be ok.But for cheaper components even a bigger one is not enough.

Also in many regulators there is a feature called Thermal Shutdown Feature.(It automatically shuts down if it exceeds there temperatures or overload situations).But I haven't experienced this.All I got burned before it shuts down

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stevez

The Radio Amateur's Handbook published by the ARRL (American Radio Relay League) describe's a reasonable method for determining the proper size of heat sink. I can't tell if it would fit your definition of "simple" or "proper" but it would help to show you some of the issues relating to heat sinks.

A few things to think about:

A. The difference between the hot transistor (or other component) and the air that will ultimately take away the heat is significant. If you wanted to keep your component case at 25 C it won't happen without refrigeration if the surroundings are at or near 25 C. The hotter your component and the cooler the environment the smaller the heat sink.

B. Heat flows by conduction from the component to the heat sink and is quite dependent on the way the component is fastened to the heat sink.

C. The heat sink must give up heat to the air and less heat is transferred in still air as compared to air that is moving relatively fast.

D. Related to 'A' - if other components are giving up heat they add to the demands of your heat sink by increasing the temperature of the surroundings.

As Gayan suggests, experimentation may prove to be the simpler way. For critical applications the "proper" method is likely good design (calculation) with experiments for verification.

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adnan_m_s

guys ,
what if i used a relatively big heat sink for those transostors, and forget all about the rest do you think that will do for 7 W dissipation

Nigel Goodwin

That's pretty well my technique - use a good size heatsink and see how hot it gets! - if it gets too hot, then increase the heatsink size!. There are formulas for calculating it, but generally you don't know the spec of the heatsink - unless you're buiyng them new of course.

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Gaston

don't under estimate the power of a fan either. i have a pentium three cpu heatsink with a small fan attached about two inches by two inches and an inch deep and it keeps my lm3886 cool to the touch. i had a smaller heatsink with no fan and it would burn the piss out of my finger

j.p.bill

That's pretty much the technique - put on the biggest sink in the box. If it's still too hot, rig a fan to push air over it.

eng1

How big is a 'big heatsink'?

I would suggest to calculate the thermal resistance of the heatsink you need and avoid to use or buy an oversized one. With typical values - as you didn't give details about the tansistor - I think that 10°C/W should be ok.

Rs = (Tjmax-Ta)/PD - Rjc - Rcs = (150-25)/7 - 6 - 0.4 = 11 °C/W

adnan_m_s

well, i am now using now the aluminum heat sinks taken from a pc case power supply, what do you think? it is about 70mm*40mm*(2.5-3mm) with additional 6fins (about 7mm*(2.5-3mm) what do you think , is it enough?

eng1

I think it is enough!

adnan_m_s

hi everybody

I have assembeled a regulator circuit using LM338K IC and it is an adjustable regulator IC as you may well know now the tricky part is the heat sink, unfortunatley I can't get a ready one , but I am thinking of an aluminum 110mmX120mmX10mm piece do you think it is enough to dissipate the heat and keep the element under 125 centigrade
here are the figures
1- I will be using a 19 v rectified input to get a 12 Volts regulated out Put
2- I am planning on getting 5A 12 V from it
a question worth asking is the heat PD required = (input in volts -out put in volts) Multiply the amperes? so is it about 19-12 = 7*5 = 35 watts(PD)?

Hero999

Why not use an old CPU heatsink from an old PC?

Both cheap and readilly available.

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adnan_m_s

I am afraid it won't be enough because I think power dissipation would be something like 35 watts and I do not want to use any fans, I also want to keep the element even lower than the 125 degrees say 90 or 80 if I could , because I think this would be more stable don't you think?

Hero999

A typical CPU dissipates more than 100W so I think a CPU heat sink will dissipate 35 Watts without needing a fan.

The LM338K is thermally protected so nothing bad is going to happen even if it does overheat.

You could probably get hold of an old PC for free so you've got nothing to loose in trying it.

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adnan_m_s

Ok hero999 thankl you very much for that advice