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 Forum index » DIY Hardware and Software » Circuit Bending
Voltage Regulator Overheating - Heat Sink Needs?
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Cyeos



Joined: Mar 17, 2010
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PostPosted: Fri Aug 11, 2017 8:24 pm    Post subject: Voltage Regulator Overheating - Heat Sink Needs?
Subject description: Troubleshooting big circuit bent project running off LM7806.
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Hello!
Potential newb question.
Having trouble finding existing related topics at the moment, and other electronics forums touch on the subject but are often going over my head.

I have a bent circuit using an LM7806 voltage regulator off of a 12V DC wall wart to get 6V DC. It is a TO-220 package, currently using this little heat sink:
http://www.mouser.com/Search/ProductDetail.aspx?R=230-75ABvirtualkey56720000virtualkey567-230-75AB

Other parts of the circuit want 12V, so that's why I'm not just using a 6V adapter to begin with, which would obviously be easier.

It works, but I measured about 320mA being drawn from the 6V line, at which the regulator gets too hot to touch very quickly. The bulk of the heat must be coming from the 6V difference from supply(?)

If I understood what I read around at all, I am needing to dissipate around 2W.

1) Should this overheating be happening at that level of current draw?

2) If the overheating is expected, I imagine I need a bigger heat sink, but I do not understand the "thermal resistance" system to figure out how much bigger I need to go than the heat sink I am currently using. Can anyone tell me how to calculate?

Thanks in advance!
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PHOBoS



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PostPosted: Sat Aug 12, 2017 3:35 am    Post subject: Re: Voltage Regulator Overheating - Heat Sink Needs?
Subject description: Troubleshooting big circuit bent project running off LM7806.
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2W seems about right and yes they can get pretty hot with that kind of power dissipation. I've never done
any calculations myself and it's not that easy as there are a lot of variables. Usually I just put a heatsink on
it and if it gets too hot to touch I use a bigger one. Embarassed A quick search led me to this page which might be
helpful and shows how many variables there are.

Something else you might be able to do is reduce the current draw for the 6V section. For example if there is
an amplifier in it which is drawing most of the current you might be able to replace that with a 12V one.

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gdavis



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PostPosted: Sat Aug 12, 2017 10:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

I played around with the calculator in PHOBoS' link with the numbers from the heat sink and LM7806 data sheet and the 2W of dissipation (sounds about right). Came up with a heat sink temp of about 40C which would be pretty warm (edit: used wrong value, see two posts down). According to a quick google, too hot would be about 50C to 60C.

The problem is that voltage regulators are meant to do just that - regulate. You've trying to convert 12 volts to 6V which is a 50% drop. That's asking a lot. As you would probably surmise, you can do it as long as you keep the current down and/or heat sink adequately, but it's less than ideal.

You may want to look into a DC-DC converter which is more efficient and may be accurate enough depending on what your powering, A combination of a DC/DC converter and regulator might be something to consider if you really need an accurate 6V.

Like PHOBoS I also wonder what you're powering and if it really needs 6V.

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Last edited by gdavis on Sun Aug 13, 2017 1:56 pm; edited 1 time in total
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PHOBoS



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PostPosted: Sun Aug 13, 2017 4:20 am    Post subject: Reply with quote  Mark this post and the followings unread

hmm 40C actually seems pretty low with 2W dissipation and such a small heatsink. I did try some
calculations myself and it depends a lot on the ambient temperature you use. Without a heatsink it
gets easily above 100C. A DC-DC converter might indeed not be such a bad idea it's definitely more
efficient, although it could cause some unwanted noise.

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gdavis



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PostPosted: Sun Aug 13, 2017 1:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
hmm 40C actually seems pretty low with 2W dissipation and such a small heatsink.

Oh! I was using the 7.5 C/W quoted in the mouser page. I just looked at the data sheet and that value is with forced convection. Natural convection is 57C over ambient at 2W which is 82C with 25C ambient. Using 28.5C/W in the calculator gives the same value.

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PHOBoS



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PostPosted: Sun Aug 13, 2017 2:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

I hadn't even noticed that and used the 7.5C/W too and I got similar results (around 40) untill I read this part:
Quote:
Some people choose 20 °C for the room temperature, some others choose 25 °C or even 27 °C (300 K), but these temperatures are too low for our calculation. One should always consider the maximum possible working temperature of the circuit: in hot summer day you may easily exceed the above values. For home applications, values around 50 °C are usual, for industrial applications is not uncommon to go up to 60 °C or more.

Than, if you don't have a blower and all the cooling is done by natural convection, it's safer to further increase it by 5 to 10 °C, especially if your heat sink is internal. The reason is because without a blower the temperature in the neighbourhood of the heat sink is higher than in the rest of the room.

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gdavis



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PostPosted: Sun Aug 13, 2017 2:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

I used 25C since that most likely matches the situation of sitting there testing out your new circuit with the chassis open and putting your finger on it to see if it's getting hot. But ya, at some point the different ranges need to be taken into consideration before you pack it in a chassis and expect it to work over a variety of conditions.
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