Multiple LM150 in 1 heat sink

[skmdmasud]

I have a project where i want to run multiple LM150 voltage regulator as constant current output and want to fix the regulators in a single heat sink. I made a prototype with 2 LM regulators and and found out that the output of the regulator is tied with the tab (back metal plate) of the regulator and the current becomes the same in both the regulator.

How can I have different output from the regulators while attaching them to the same heat sink.

Thanks.

 

[ericgibbs]

I have a project where i want to run multiple LM150 voltage regulator as constant current output and want to fix the regulators in a single heat sink. I made a prototype with 2 LM regulators and and found out that the output of the regulator is tied with the tab (back metal plate) of the regulator and the current becomes the same in both the regulator.

How can I have different output from the regulators while attaching them to the same heat sink.

Thanks.

hi,
The TAB [metal plate] on the LM150 is connected to the Vout pin, you need insulating washers and bushes on the LM150 when fixing them to a heat sink.

Without insulation , all the Vouts are linked together

 

[vsaar]

You need a mica isolator between the regulator and heat sink, and possibly a plastic bush to keep the screw from directly touching the regulator.

 

[Reloadron]

To add to Eric's post if you do a Google of TO3 Mounting Kits or TO220 Mounting Kits you will get results like this. Everything from plastic bushings, nylon screws to mica sheets. Just choose a mounting kit based on your needs of isolation. These kits are very common from a host of manufacturers.

Ron

 

[skmdmasud]

Isolating them in that way makes the whole thing a bit complicated, and i dont know if those plastic or mica sheets are available in my area. Lets say i put them on same amp output (identical configuration) on all my regulators then will there be any problem if i put them all in same heat sink?

 

[Reloadron]

You don't mention the case design? TO3 or TO220 not that it matters. You also do not mention the current loads? If you have TO220 case then possibly you could use clip on heat sinks. No, you can't just short the outputs together which is what you would be doing sans insulating heat sink kits. They are very common globally, even in Dhaka, Bangladesh I would immagine. They are not complicated at all. That is how it is done.

Ron

 

[skmdmasud]

You don't mention the case design? TO3 or TO220 not that it matters. You also do not mention the current loads? If you have TO220 case then possibly you could use clip on heat sinks. No, you can't just short the outputs together which is what you would be doing sans insulating heat sink kits. They are very common globally, even in Dhaka, Bangladesh I would immagine. They are not complicated at all. That is how it is done.

Ron

Hi, Its packaging is "TO220 power package (T)". ok if the insulating kit is used as a common hobby related item then it should be available. I have a tube of silicon heat sink paste ...Can i use a thick layer of silicon heat sink paste and mount LM in a PCB and fix the PCB in 90 with the heat sink, so that the pcb takes all stress?

 

[vsaar]

Silicon paste alone will not work as an insulator, because it would smear away when tightening the screw. However, it's okay to put thin layer of paste between the mica to improve cooling.

 

[MrAl]

Hi,

As others have said, you usually insulate the package from the heat sink using special shoulder washers made of nylon with metal screws and nuts like stainless steel. You also either need a mica pad for TO220 packages or an Si pad between the package and heat sink.

If you really can insulate them from the heat sink for whatever reason then the only other choice is to use separate heat sinks or cut a large heat sink into sections with a good hack saw and careful cutting. The heat sinks then can not touch either.

Alternately you might use a different kind of regulator that does allow the pass transistor collectors to be tied together so you can put them all on the same heat sink without isolation.

 

[Reloadron]

You never, ever, absolutely never, ever want to apply a thick layer of thermal compound. It defeats the whole purpose of using it. I suggest you give this link a read.

The whole idea is thermal energy (heat) transfer from the component to a heat sink. We want to get rid of harmful heat.

Attached is an example of what I mentioned earlier when I mentioned clip on heat sinks. Heat sinks are generally made from alloys that are very good conductors of heat. Aluminum is popular as a trade off between cost and thermal transfer abilities.

You can even make your own, note how simple the little clip on heat sink is. You just have to be very sure you can dissipate the heat you generate. You need to do the math involved. Failing to have a good heat sink for your application will result in burned up useless parts. Remember, on your TO220 case the tab is the same as the output pin. The output pins can not be shorted.

Ron

 

[Reloadron]

You never, ever, absolutely never, ever want to apply a thick layer of thermal compound. It defeats the whole purpose of using it. I suggest you give this link a read.

The whole idea is thermal energy (heat) transfer from the component to a heat sink. We want to get rid of harmful heat.

Attached is an example of what I mentioned earlier when I mentioned clip on heat sinks. Heat sinks are generally made from alloys that are very good conductors of heat. Aluminum is popular as a trade off between cost and thermal transfer abilities.

You can even make your own, note how simple the little clip on heat sink is. You just have to be very sure you can dissipate the heat you generate. You need to do the math involved. Failing to have a good heat sink for your application will result in burned up useless parts. Remember, on your TO220 case the tab is the same as the output pin. The output pins can not be shorted.

Ron

<EDIT> I see MrAl beat me to it. </EDIT>

Then I screw up and double post.

 

[skmdmasud]

Then I screw up and double post.

Thanks for posting twice , it made the topic very clear .
Mica is available it is most probable used in iron (cloth de-wrinkled lol). the difficult part will be finding plastic screw.
I always thought heatsink paste were good conductor of heat...isnt that its purpose?

 

[Reloadron]

OK, when metal is "finished" it has what is called a "micro finish". How well is it machined to a finish. How really flat is it? The idea here is that two surfaces must mate. The problem is nooks and imperfections in the mating surfaces. Tiny little cracks and blemishes will exist. If the surfaces were perfect the heat would transfer very well. Nothing is perfect. The idea behind thermal compound is to fill those tiny microscopic cracks and flaws, that is all it should do. You don't need very much. Too much defeats the purpose.

Ron

 

[MrAl]

Hi,

Yes the thermal resistance of heat sink compound increases with the thickness of the layer. A layer two times as thick has twice the thermal resistance as one layer has.

You dont need plastic screws. You can use steel or stainless steel. On the other side of the heat sink you use a nylon shoulder washer which keeps the screw centered in the hole at all times so never comes into contact with the side of the hole. This is really the best way because of the physical endurance of the metal screw vs nylon. Note the screw doesnt have to be isolated from the device package tab, only isolated from the heat sink metal so one shoulder washer per screw is enough. The holes are drilled out to accommodate the shoulder washers not the screws so they will have a significantly larger diameter than the screws. You can also add insulation to the screw threads too if you like, leaving enough room for the shoulder washer and nut on the end.

 

[bountyhunter]

Alternately you might use a different kind of regulator that does allow the pass transistor collectors to be tied together so you can put them all on the same heat sink without isolation.

Most common TO-220 regs come in an "isolated" tab TO-220 package which does not require any insulation. I just bought one today that was a 7805 device in the isolated tab T0-220.

 

[bountyhunter]

Hi,

Yes the thermal resistance of heat sink compound increases with the thickness of the layer. A layer two times as thick has twice the thermal resistance as one layer has.

You should always use a straight edge of some kind to scrape the excess grease off the face of the device. You only want enough grease in place to fill the microscopic air voids in the surface of the metal, not enough to move the transistor tab away from the heatsink surface.

 

[skmdmasud]

Hi,

Yes the thermal resistance of heat sink compound increases with the thickness of the layer. A layer two times as thick has twice the thermal resistance as one layer has.

You dont need plastic screws. You can use steel or stainless steel. On the other side of the heat sink you use a nylon shoulder washer which keeps the screw centered in the hole at all times so never comes into contact with the side of the hole. This is really the best way because of the physical endurance of the metal screw vs nylon. Note the screw doesnt have to be isolated from the device package tab, only isolated from the heat sink metal so one shoulder washer per screw is enough. The holes are drilled out to accommodate the shoulder washers not the screws so they will have a significantly larger diameter than the screws. You can also add insulation to the screw threads too if you like, leaving enough room for the shoulder washer and nut on the end.

Plastic Shoulder washer is a great idea dont know it i can find it. I was also thinking about applying pressure with a metal rod in the top plastic body of the regulator something like pressure on top side "o - metal rod" --> o|| <-- back metal side.

Thanks.

 

[skmdmasud]

Coincidentally I was changing a battery in UPS and found out they have used shoulder washer and thermal pad in their heat sink. I took a picture for people like me who dont know how it looks like. Sorry the image quality is not that good
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the screw in the image with white ring like washer is i guess a shoulder washer and the voltage regulator in the left is placed over a grey heat sink pad.

 

[skmdmasud]

I finally made it. Thanks to all the members of electro tech online for their great support and innovative idea.

**broken link removed**
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[Reloadron]

Nice going and glad it worked out.

Ron