Ok so this is going to seem like a common sense question, and yes I have consulted the DataSheet. I am using a L7805 Voltage Regulator to drop 12V 10Amp power coming from my ATV 12V plug outlet down to 5V with a maximum draw of 200mA for an Arduino Nano based project. I know the L7805 can achieve this, from the research I have done and my circuit board is complete aside from soldering the L7805 in. My problem is that I don't know how big a heat sink I need for the L7805. It itself will stick out of my project box so it'll have airflow on it, but how big of a heatsink do I really need to have because I am seeing different sizes and styles. I really don't want to ruin anything and have to get a new regulator.
Well, the L7805 is just an automatically adjustable resistor. So 7 volt drop at 200ma is how many watts? Your heat sink must dissipate at least that many watts. Are you sure you want to do this?
Hang on, I gotta do some math to find out the watts. And I don't know any other way to get what I am doing. This project is portable, so I have a 2 way switch wired in. One side of the switch can supply my Nano with power from a 9volt battery, and the other side of the switch is getting the 5volts that will be made via the regulator which is connected to a barrel jack port. I modified an 12V outlet cord with the other end of the barrel jack. So while in the Atv I can plug it in and when I get out I can switch over to battery.
5V @ 200mA = 1W
7V @ 200mA = 1.4W
That's according to the online converter I use for all my projects. Would the the wattage coming out of my ATV's outlet need to be factored in?
What is the resistance of the outlet?
The reasonable replacement for the L05 is a buck converter. Almost no heat.
You need to find "Thermal Resistance Junction to Ambient" in the datasheet. Different devices in the TO-220 package has different values (probably caused by different test conditions). Assuming 50 °C/W and 1,4 W dissipation the junction will be 70 °C over ambient temperature. If the 200 mA is only maximum and is not reached for long periods of time (minutes) you should be OK with no or only small heatsink.
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Hi,
One of these T0-220 Heatsinks should do the trick.
Mount it vertically is ideal, but if you have to mount it on its back, don't crowd components around it.
Tom... 
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Since you read the data sheet you know about the needed bypass capacitors, both bulk and high frequency. Be sure you read the manufacturer's data sheet for the 7805 they are not all the same. Bulk you can always go higher but not lower. I always use two caps each at the needed capacitance that compensates for the -+ part of the tolerance and covers me if one fails open. Using 100µF capacitor with a ±20% tolerance could legitimately vary from 80μF to 120μF so for my design I would rate it at 80μF.
You should also consider why you'd want to give away more in HEAT than your circuit consumes. If you're already laid out for a 78L05, Recom makes a drop-in converter at $3.25. These are around 80% efficient.
https://www.digikey.com/en/products/detail/recom-power/R-78E5-0-0-5/2834904
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That got to me. This thread talks about a "L7805" - which is a somewhat odd but apparently common reference number for the standard TO-220 or similar 1 A regulator.
A 78L05 is a totally different animal. 
Sorry for that. Yes, the 78L05 is a TO-92. I just somehow mistyped it. Using 2.8W to supply a circuit that needs under 1W just seems horrid. Still, people love these linears for ease of use.
I am not sure about resistance... I'll look at the buck
Yes, I have a capacitor on the output, that was the first thing I checked.
Because that is what I have, a L7805 Voltage Regulator. It was best thing I had on hand.
How about the input? and you need both bulk and high frequency, normally 1 cap will not do it.
This is the drop in converter I use.
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I can easily add another one on the input side but it'll be crowded as I would be using a second 100uF 25v Cap.
If the 7805 is installed remove it and replace it with the drop in converter. If you don't know how to remove it , cut the pins and
solder the drop in converter pins to the cut pins on the pads.
DON'T USE THE 7805.
You are a bit harsh on the poor regulator. If the project works well with it and can afford the power "wasted" why change it? The switcher may add unacceptable noise or increase idle power consumption or bring some other unforseen complication.
I tried the same thing and soon smelled smoke. I put a buck regulator as a pre regulator to get from 12V to 8V and the the 7805 can convert 8v to 5v without getting warm. The 7805 is a linear regulator & dissipates the drop as heat, whereas the buck converter has a transformer and doesn’t dissipate the drop. Velleman WMA402 or 404 IIRC.
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