sorry 1.1watt
These babies can draw 1amp, why cant I draw more then .22 without being very hot ?
I dont see the wattage value on the spec sheet of this chip,
|
sorry 1.1watt
These babies can draw 1amp, why cant I draw more then .22 without being very hot ?
I dont see the wattage value on the spec sheet of this chip,
Last edited by lerameur; - 2nd February 2007 at 03:02.
|
There's a wattage value buried in the 7805 datasheet somewhere. And remember, your circuit is drawing 1.1 watt, but your regulator is dissipating 1.54watt, and you are drawing 2.64watts from your battery total.Originally Posted by lerameur
The amount of temperature rise on the back of the 7805/heatsink/whatever, is directly related to the ability of that 'place' to dissipate heat.
If you've got something that just doesn't give up heat, it'll get hot...if it's very conductive, it'll stay cool.
So, with that being said, even 1 watt can feel very hot depending on what the heatsink is, if there is any heat conductive compound in between the 7805 and the heatsink, ambient temperature, etc.etc.
Think about it this way....take a 40 watt light bulb... It's hot right? All the way around it. It's also pretty big when compared to a 7805 with a heatsink. Now, take that same 40 watt bulb, shrink it 40 times, and apply 1/40th of the heat...sounds like it might be in that same ballpark. Obviously I don't know what the actual numbers would be, I'm generalizing quite a bit, I think it sounds reasonable.
And take another look at what I was saying earlier about how much power the regulator has to dissipate. Do you understand that much of it? (As in why the regulator dissipates 1.54watt when you power it with 12v and only draw .22amp from the regulator?) Remember, a linear regulator, such as a 7805, regulator by dumping extra voltage off as heat...that's the way they work (in simple terms anyways)...kinda like an infinetely variable power resistor that's always turning itself to keep the output voltage at a certain point.
|
well it dissipates 1.54watt because it can take 1watt , so 2.54 watt total from the battery. I guess I need this part: L78S05CV for now
|
You could 'preregulate' the 12v input with either another regulator ( 12v -> 7809 -> 7805), maybe a few diodes in series, etc.etc. Knock the voltage down a bit before doing the final regulation to 5v. That'll cut down on your heat.
What package is this 7805 that you have right now? TO-92? TO-220?
|
it is TO-220,
by the way I once saw a sheet with all the packages types on the digi key web site, I cant seem to find it anymore. Would you have a similar sheet?
But instead of using 7809 and 7805, I think I will use a 2amp or maybe a 3 amp regulator it wont cost more.
k
|
If you pull up a search on digikey for 'regulator', 'fixed', '5v', it should be able to show you pretty much all of the packages they have available.
Quite frankly, there's got to be something else going on. 1 watt on a TO-220 7805 isn't that much...and again, quite frankly, if the regulator isn't shutting down, I don't think it's getting too hot to worry about. The datasheet I've got for the TO-220 case shows a temp rise of 65C/watt without a heatseak (junction to air) and a max operating temp of +125C. That tells me that something just short of 2watts is safe. You're dumping 1.54watts, 1.54 x 65 = 100C. That's hot, but not really that HOT...especially if you've got a heat sink on it which will really drop that 65C/watt number to something more normal-ish.
In short, as long as it's not shutting down, you're good. If you're still worried about it, pull down a few datasheets and have a look at a thing called an 'external pass transistor'. Basically, you use the regulator to control the base of an external transistor which bypasses just enough juice to keep the regulator working and to basically do all the work for the regulator.
If not that, go ahead and parallel a couple of 7805's. Just keep in mind that 7805-A might be set at a slightly higher voltage than 7805-B, so you need to put a small value/high wattage series resistor inline with each output, or in bad cases, you'll have to isolate each regulator's output from each other with an inline diode.
Try it out...you won't hurt anything. At worst, they'll overheat and shut themselves off until they cool off...
|
Every whichaway
Hi Ken,
I'm looking at your pictures and I have to ask, are you sure you have that regulator plugged in right, because if it's backwards it will still work (kinda) and will get hot. With the metal tab against the table and the leads pointing down, the terminals are numbered 123, 1 is input, 2 ground and 3 is output.
JS
If you do not believe in MAGIC, Consider how currency has value simply by printing it, and is then traded for real assets.
.
Gold is the money of kings, silver is the money of gentlemen, barter is the money of peasants - but debt is the money of slaves
.
There simply is no "Happy Spam" If you do it you will disappear from this forum.
Members who have read this thread : 0You do not have permission to view the list of names.
Posting Permissions
Bookmarks