Serin serout problem

[lerameur]

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

[skimask]

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?

[lerameur]

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

[skimask]

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...

[Archangel]

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

[websmith]

I come across many LCD's with incandescent globes inside them not LED's for the backlight, and designing the resistor for a 2.4V led will result in a whack of over-current. Just increase the series resistor to the backlight pins untill the display is only as bright as it really needs to be (try 150 ohms), this will certainly get you out of trouble with the regulator, and as they already advised you, feed it from the 'raw' side of the regulator not from the 5V regulated side. The backlight is completely isolated from the 5V rail of the LCD.

[lerameur]

I actualy tried the 150 Ohm, and it gets hot just as fast.
NO the regulator is installed correctlt, with the 12v pin (1) on the outside, far away of the IC.
I cant play around too much with the circuit now, I will try it as is see, or get myself a 2amp regulator. Have to go to the store anyway to get an Lm331.
thanks
ken