voltage circuit question

[quadmasta]

I just got a device that runs off of 5VDC at 2000mA and I want to run it off of a 12V source.

I got two LM 7805 voltage regulating transistors that take 5-37VDC and output 5VDC at up to 1A.

Can I run two of those parallel to one another to get 2A output?

Will I need a stiffening capacitor for each transistor?  Will I need a heatsink?

[BobA]

Some voltage regulators do not share load well.   This can cause one to get into trouble while the other is not doing anything.  If you look at the data sheets for the 78XX series you can find circuits for higher current regulators.   They use an external transistor that can handle the current load. That would be your optimal way to regulate a higher current load with a 78xx device.  You can also get TO3 devices that do the same and will handle 3 amps such as an lm323K.

[MonMotha]

You will most decidedly need a heatsink for any linear regulator solution here.  You're talking about dropping (12-5)*2=14W on the regulators (to only deliver 10W to the load).  Even with a reasonably good Theta_JA of 3 degC/W, you'll see a temp rise of 42C (108F) above ambient!

If you do want to use a linear solution, the venerable LM323 is probably one of the few devices you'll be able to suitable heatsink.  Note that on this device, Theta_JC is 2 degC/W, which means that even with a "perfect" heatsink, you'd still get a 28C (82.4) temp rise.

This is enough power that you might want to find yourself a switch mode power supply.  There are some solutions out there for 12V to 5V capable of serveral amps.  You may have luck using devices designed for automotive use.

You can try building one yourself (the topology you probably want is the "buck" topology), but without some serious electronics experience, this is likely to be more frustrating than successful.

[quadmasta]

Looks like I might just pony up the cash for the auto adapter for it.

[quadmasta]

My friend John suggested I look into using a transistor to amplify the output amperage of the 7805 and gave me this link but didn't really have time to explain it.

http://www.williamson-labs.com/480_xtor.htm#emitter-follower

If I'm understanding it correctly, the input and output voltages are the same, it just amplifies current as needed.  I just don't know how to calculate the resistor values or what I'd need for a "signal generator."  Can anybody give me more insight or places to look for information?

[MonMotha]

Basically, you're using an external pass transistor which can handle higher current than the one built into the 7805.  This transistor will still dissipate 14W, and it will need to be heatsinked to the excess.  In fact, you'll probably need more than one transistor just due to the power dissipation, and then you have to make sure they're well matched.  You simply cannot get around the power dissipation issue using a linear solution like this.  Total power dissipation MUST be at least 14W due to the nature of this kind of regulator.  14W is a surprisingly large amount of power - things will get TOASTY.

If you are interested in using an external transistor, there should be an example in either the 7805 datasheet or a related app-note.  The 7805 with its feedback ends up acting as the "signal generator" in the example you posted.  See figure 14 of the fairchild datasheet.  May differ in others.

Seriously, just go buy a switcher.  These have efficiencies topping 90% in some cases.  Even if you don't care about the power efficiency, you won't have to deal with the heat.