Voltage Regulator Question

[KeyboardKowboy]

I've built a voltage regulator using the LM7805CT voltage regulator (Fairchild), used to drive TLC5940 led driver ICs. I tested the voltage regulator circuit, and its working fine, outputting a solid 5V regardless of voltage input (between 5v and 18v). When using a 9V battery, my circuit would work, but when driving a lot of LED's at once, it would start acting flakey. I deduced that I was in need of more current. I replaced the 9V battery with a 12V 2Amp power supply, the voltage regulator properly outputs 5V and the extra current available has fixed any issues I was having.

Here's my question: The LM7805CT states "Output current up to 1A". Does this mean that regardless of input power source current, it will only be outputting 1A of current? Or will it output up to the current being supplied, which in this case is 2A, thus possibly creating an issue? I just want to fully understand that statement... thanks for any help.

Link to the LM7805CT datasheet: http://www.fairchildsemi.com/ds/LM/LM7805.pdf

[adafruit_support_bill]

Generally it means that the device will reliably operate at up to 1A continuous output with appropriate heat-sinking. You may get more out of it for short periods.

[KeyboardKowboy]

Ok, so it doesn't matter on the input necessarily then. But because there's 2A available, and if the circuit draws over 1A, then the voltage regulator will be outputting over 1A, which is above it's spec. Am I understanding that correctly? Thanks for your help arduwino.

[adafruit_support_bill]

That is right. The load determines the current demand.

Being a linear regulator, the actual current handling is partly dependent on the input/output voltage differential. A 7805 will be working pretty hard to regulate 12v all the way down to 5v. You would be better off with a supply voltage in the 7-9v range.

There are switching regulatorsthat will handle the higher current and be more efficient. But it will cost you a bit more.

[Entropy]

Ghetto approach to 12v-to-5v switching regulators - cannibalize a charger for cell phones that have a mini-USB charging port.

[westfw]

Since the 7805 has current limiting and overtemp protection built in to it, it should shut down if the limits are exceeded for "too long."

[KeyboardKowboy]

Excellent. Thanks everyone for the help. I appreciate it!

[zener]

The big idea to remember is that the current you can really get is limited by heat. Heat comes from power dissipated. Power dissipated in a linear regulator = (volts in - volts out) * current. What this all means is that if you don't provide enough heat sink you MIGHT get MUCH LESS than 1 amp.

[KeyboardKowboy]

Good to know, thanks Zener. I did notice (my finger will attest) that the regulator got really hot when using the 12V 2A power supply. I ran it for about 20 minutes, and the circuit ran fine and the regulator didn't switch off. This simple regulator is just something I threw together to test out a possible fix to the original issue with my led driver circuit being under powered. I'll be utilizing a breadboard power supply from adafruit or sparkfun for the finished product...

As a side note, I have a bunch of small heat sinks laying around, couldn't hurt to attach one to the backside of the "built in" heat sink on the regulator, right?

[adafruit_support_bill]

couldn't hurt to attach one to the backside

Anything's better than nothing. I've even used bits of scap aluminum on occasion. And air-flow helps too. If you don't have a fan, at least position it so that natural convection can help with the cooling.

[westfw]

couldn't hurt to attach one to the backside of the "built in" heat sink on the regulator, right?

Right. I remember actually looking at the numbers once, and even a rather modest heatsink results in a pretty big improvement over the TO220 package itself.

[stinkbutt]

One of the better options for heat sinks only costs about $0.04.

[KeyboardKowboy]

One of the better options for heat sinks only costs about $0.04.

Haha, thanks for that link stinkbutt! That's pretty neat, who knew a few pennies could beat out a real heatsink... My regulator has been functioning just fine, but I may need to try this out anyway!

[richms]

It may be fine now, but box it up with less airflow etc and it may fail.

I have used a long bolt with alternating nuts and big washers as a heatsink once, because it is what I had lying around.

[pstemari]

Just make sure you get old (pre 1983?) pennies. The new ones are made out of zinc, not copper.

My usual solution for heatsinking TO-220's is bolting them to the case, using insulating washers if necessary. You can use an old-fashioned terminal strip (the phenolic ones with solder jugs) to attach the legs.