Thermal dissipation (heat sinks) information required

[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

 

[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.

 

[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]

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

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.

 

[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]

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

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]

inquiry

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.

 

[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.

 

[adnan_m_s]

Ok hero999 thankl you very much for that advice

 

[ericgibbs]

hi adnan,

Link, explains the basics of heatsink usage.

**broken link removed**

 

[adnan_m_s]

Dear Hero999

I need your help with a thing or two if it is possiple please
first i need to adjust this circuit to charge a 7AH cealed type battery could I replace the SL100 tr with a triac to have more current for charging the 12V 7AH battery? I have chosen this circuit because it has indicators and autoshutt off features so any advices on this

 

[Hero999]

No because a triac becomes latched on onces it's triggered.

This circuit is designed for a mobile phone battery with a voltage of around 3 to 6V.

What are you charging the lead acid from?

If it's just your car then just connect it in parallel with a low value resistor to preven huge current from being drawn.

 

[adnan_m_s]

well,
no i will use an 220/14v adaptor i am planning to make an uninterruptable DC UPS that has a cealed back up battery if i used a power transistor instead of SL100 let us say BD137 or 138 of course according to the right type will that give me a more Amperes current ? 800ma for instance ?
how can i replace some parts of the circuit to get what i want? the 39 ohm the transistor the feeding power to be 14 V things like that?

 

[adnan_m_s]

the bottom line is that I need to do the following ,
i need to have this ups draw current fom mains 220V through a bridge rectifier and keep the battery charged and at the same time when main current is off the circuit should immediatley shift to feeding from the 12v 7AH backup battery now i was thinking after resolving the charging issue to make a small circuit with 741 op to switch the battery on to be the main power source once main 220V is off through connecting the inverting input of the op with the main power after a bridge ofcourse, so when main 220 v is off the op circuit will turn on + battery polar to on state and off course the - will always be on anyway as a common but i am not an engineer i am just an enthusiastic guy who is trying to do anything he can so any ideas or diagrams that you can help me with??