Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

heat sink

Status
Not open for further replies.

unlv007

New Member
I have a three terminalvoltage regulator IC -LM 317 and i have been told to "heatsink" it.
How should i do this? How do you heatsink an IC or a transistor?
Regards
 
To add to what blueroomelectronics has said. The amount of power depends on the size of the heatsink.
 
To get more technical, you first calculate the power dissipated in the 317. This equals the voltage in minus the voltage out times the output current (Vin-Vout) x Iout = Power dissipated.

Then you buy a heatsink that can keep the temperature of the device below its maximum rating. This is typically 125C but for reliability, you usually want to keep it below 100C.

Heatsinks are rated in the thermal resistance to air in degrees C per watt. The 317 case also has a thermal resistance from junction to case, listed in the data sheet, which varies depending upon the case type. This must be added to the heatsink thermal resistance. The temperature rise is then watts dissipated times the total thermal resistance Trise = Watts x (TRsink + TRcase). Trise is added to the ambient temperature to get the junction temperature.
Note: This is for a heat sink that is in open air.

For low power you may not need a heatsink. In that case you use the 317 thermal resistance of the case to air, which is also in the data sheet.

To minimize the mounting thermal resistance it is good practice to use thermal grease between the case and heat sink (Radio Shack among others sell such grease).

Heatsinking is also discussed in the LM117/317 data sheet from National Semiconductor. (It's amazing the information that's in a data sheet. Unfortunately, few people seem to read them.)
 
her "What are you doing up there?"
him "err just downloading some datasheets dear, i'll only be five minutes"



her "i wish he'd just look a nude women like normal blokes............"



:)



i have found though, that if the power requirement isn't too high, if you over specify the regulator, you don't need a h/s at all. which saves money, time and board space. that's usually the 78/79 type regulators though
 
A good rule of thumb is "if the part is warm/hot to the touch, then a heatsink would be a good idea".

An alligator clip can be used as a heatsink for component leads whe soldering.

Don't try to save a few pennies by omitting the thermal grease.

Earlier this week I stopped at a local hardware store looking at aluminum tubing, and realised that some of the 'u' shaped aluminum stock, when cut to the right size, would be about right for TO-220 heatsinks. I can't think what the stock is called, but it's square or rectangle, with just 3 sides.
 
I noticed on some datasheets that dedicating some PCB area as a heat sink might be a solution.
 
stevez said:
I noticed on some datasheets that dedicating some PCB area as a heat sink might be a solution.

I've never tried that, but it should work if there's enough copper...
 
well FWIW you can go cheap and just poprivet a fat washer on the back side. Just remember, the heatsink is not electricly isolated from most parts and may have a substantial voltage.
 
walshlg said:
well FWIW you can go cheap and just poprivet a fat washer on the back side. Just remember, the heatsink is not electricly isolated from most parts and may have a substantial voltage.

Good Idea Walshlg, not very elegant, but simple and cheap!

I'm sure most of us have loads of scrap, in our junk boxes, that would work for heat sinks. Ya just gotta think outside the ol'e junk box (I know, bad pun...:rolleyes:)
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top