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When to use a heat sink


FireFly
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I was looking at various datasheets for bridge rectifiers for the "0-30 VDC STABILIZED POWER SUPPLY" project. I'm trying to understand when a heat sink is required. For example, the datasheet for the NTE5329, 6A bridge rectifier:

http://www.nteinc.com/specs/5300to5399/pdf/nte5329_31.pdf

...states "Recommended mounting position is to bolt down on heatsink with silicone thermal compound for maximum heat transfer with #6 screw."

But how big a heat sink does it need? Is there any way to tell from these spec sheets (bridges, transistors, etc.) if my usage is low enough to not require a sink? Some slightly lower rated bridges don't mention a heat sink or have a screw hole.

Also, I think the photo for this power supply project seems to show a little heat sink in the upper right corner. Is that for Q2 = 2N2219 NPN transistor? If I didn't have that photo, how can I tell from the datasheet? Or do I just have to order 6 of everything for the first few months until I get more experience?! :-\

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If you are using a 6A rated bridge rectifier in place of D1 through D4, which are rated at 2A, then you are never going to need a heat sink for this rectifier bridge. It will never work that hard in this circuit.

The Heat Sink in the picture is for the 2N3055. The data sheet for this transistor gives some information regarding heat transfer. Most heat sinks are also rated for their heat transfer.
As a general rule, if you are using the upper limits of the current rating of the power supply, it is going to get the power transistor hot. If you are only using less than 1 amp, you can probably get by without it.

MP

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Thank you, MP. Sounds like 3/4 experience and 1/4 mathematical.

The heat sink(?) I was referring to in the picture is the black "collar" towards the upper right of the board right beside where the 3 2N3055 connection wires attach to the board. Isn't that the Q2 = 2N2219 and a little, round, black heat sink or is it something else?

That's what made me wonder in the first place about knowing when to use a sink especially if only working from schematics.

As far as heat sink ratings, I guess I should pay more attention to what was attached to old heat sinks that I salvage to give me a hint of what the sink is capable of.

Thanks again for your info.

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You are correct that there is also a small heat sink on the board in the picture. The text shows that there is much on-board short circuit protection and does not mention heat, so I do not see the reason for it. Perhaps the author was playing it safe.

Would those who have built this project please comment about the heat generated by the parts?

MP

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  • 2 weeks later...

Hi Firefly, a heatsink is used to carry heat away from a component conducting the heat to the surrounding air, since the heatsink has a large surface. Heat is caused by the power that the component is dissipating (power = the current through it X the voltage across it).
The 2N3055 can have up to 3A through it. If the output voltage is set low (say 1V) then the voltage across it is the difference between the rectified input voltage of about 32V and the output voltage which could be 1V, therefore in this case the voltage across it is 31V. The power dissipated is 3 X 31 = 93W. So it has a big heatsink.
The 2N2219 would pass up to 0.15A and in this case have 30V across it so it would dissipate 0.15 X 30 = 4.5W. So it has a little heatsink.
The rectifier diodes also pass up to 3A but have only 1V across them when conducting. They each conduct 1/2 the time so they dissipate only 3 X 1 X 1/2 = 1.5W. In a bridge 2 diodes conduct simultaneously so it dissipates 3W. For its amount of surface, that would be quite warm but not too hot.
When building or designing you must consider power since even resistors must have their power calculated.

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