Why my 7812 blow up?

[XantheFIN]

Hi!


I thought this being safe:

" These regulators employ internal
current limiting, thermal shutdown, and safe−area compensation"

https://uk.farnell.com/on-semiconductor/lm7812ct/v-reg-14-5v-to-35v-to-220/dp/1467365


Input is 24V.
Shortened: Attempted use lm7812ct to power small 250mA 12V DC motor. There is toggle switch in +24V line which goes then to regulator.

I put right in the legs 0.2uf and 0.4uf caps as they had enough voltage rating for output and input.
There is 0.1uf cap right at the dc motor as i tought i am being wise by doing that.


Happiness lastes only some 30 seconds in total run time.  Before just trying out any diode here and there i ask directly from pro's.. Where i failed?

[floobydust]

Brushed DC motors have arcing at the brushes which makes transients beyond what the IC will take.
They also act as generators when spinning down, so you need protection diodes!
Your caps may be too small to help with the arcing as well.

You soldered the IC to an aluminum heatsink? 

[XantheFIN]

Brushed DC motors have arcing at the brushes which makes transients beyond what the IC will take.
They also act as generators when spinning down, so you need protection diodes!
Your caps may be too small to help with the arcing as well.

You soldered the IC to an aluminum heatsink? 

Not soldered 
It might look fancy as there is thermal paste and couple glue 'spots' and other side terminal keeps it in place. Its not touching anywhere electric at all.

[floobydust]

Why no mounting screw? The IC is dissipating several watts- which is beyond glue.
Add the usual two protection diodes like 1N4001 (input-output and output-GND) and the next IC should last...

[TimFox]

Applying an external voltage (possibly from motor brush transients or inductive kickback) to the output of a 78xx is dangerous.
The minimum protection (as floobydust said) is a reverse diode at the output (preventing negative voltage) and a reverse diode from output to input (preventing the output voltage becoming more positive than the input voltage).

[XantheFIN]

Why no mounting screw? The IC is dissipating several watts- which is beyond glue.
Add the usual two protection diodes like 1N4001 (input-output and output-GND) and the next IC should last...

I didn't yet mind machining hole for the screw and its not glue which is between there to be exact in details.. as i told its thermal paste is between there full cover the IC and its very thin layer to heatsink so not acting as insulator. Glue is just quick test help holding it up there mostly on sides until i get time machine it better with screw hole or better heatsink. It was more like that thermal glue application which been used on devices.
There is no air gap. I have that usual thermal pad mat or sticker thing but i remembered only after.


I will test tomorrow with the diodes. Thank you. 

[TimFox]

The thermal compound is to fill microscopic gaps between two flat metal pieces.
However, you need force to hold the device against the heat sink:  the usual method is a screw (possibly using insulating washers), but some heat sinks have a leaf spring to press the tab against the heat sink.

[floobydust]

OP, going from 24V to 12V at 0.25A with a linear regulator will make a lot of heat. I would say that heatsink is too small for 3W.
When a 7812 overheats, it's OK it will just shut down.

[bitwelder]

The thermal compound is to fill microscopic gaps between two flat metal pieces.
However, you need force to hold the device against the heat sink:  the usual method is a screw (possibly using insulating washers), but some heat sinks have a leaf spring to press the tab against the heat sink.

So, if not ready yet to make a final hole for the screw, at very least the IC should be pinched to the heatsink with some clip?

[rsjsouza]

The thermal compound is to fill microscopic gaps between two flat metal pieces.
However, you need force to hold the device against the heat sink:  the usual method is a screw (possibly using insulating washers), but some heat sinks have a leaf spring to press the tab against the heat sink.

So, if not ready yet to make a final hole for the screw, at very least the IC should be pinched to the heatsink with some clip?

Yes. The thermal compound alone will not make a good thermal contact, it is part of a system that comprises the device, heatsink and a fitting mechanism (leaf spring or screw). The fitting mechanism is particularly important when you consider the thermal expansion that can potentially increase the distance between the device and the heatsink.

Also, the heatsink is small when compared to the amount of power dissipated by the 7812, unless you use adequate forced cooling (a fan).