Is 3 Watts really too much to ask?

Sorry, forgot the link:

buy a usb car charger

or

With respect, if you're using a bare 78xx part in an automotive environment, heat dissipation is the least of your worries.

You can get around the heat issue, though, by bolting the regulator to a largish piece of metal in the vehicle. I seem to remember bolting a

A better solution is so use a switching converter. Look on eBay for buck converters. I like the designs from Pololu Electronics -- reliable, well built.

Please look up "alternator load dump" and ISO 16750. The automotive electrical environment can be quite exciting.

The L7805 won't go up in smoke, because it has Thermal Overload Protection. Also, you can do the finger sizzle test. If saliva on your fingertip doesn't sizzle when you touch it, it's under 100C. Since Rth-JC=5C/W, the junction temp should be under 115C, which is under the part's 125C spec limit.

You could use a 78xx regulator to drop the minimum voltage and help protect the switcher! They're very rugged.

The LM78xx is hard to kill even from _intentional_ abuse.

Would this help?

$9 and you use it like a 7800 regulator.

One of George's customers managed to find a way to blow one up. He connected a large coil to the output, which drew enough current to put the regulator into thermal limit. When it tried to turn off, the resulting inductive kick blew the regulator up from overvoltage.

Cheers

Phil Hobbs

ND/2352143

Hmmm. I'm seeing other suggestions here for going down the switcher route, too. It just seems totally OTT for such a simple task. I don't doubt a buck converter or some other DC-DC down converter topology of some sort would work best, but I have a ton of these 5v regs which I've had for years and years and would like to use some of them up!

Thanks, Win! That test could be very useful. I'm still skeptical, because although it may well pass today, it'll be a very different story come July/August.

Can you use a bigger heatsink? That little piece you're showing is too wimpy to dissipate 3W in an adverse environment. The regulator may not blow up but you're likely to experience shutdowns.

Mebbe put it in an die cast box and use that as the heat sink ?

Remember that a 7 watt night light glows from the heat! If I get what you are saying you are using a heat sink about an inch square. That's not like ly to be big enough.

nough to boil water. The fact that you can't touch it doesn't mean so much

Get a larger heat sink. Any piece of metal will do. I was going to make m y own car charger and thought about using the metal of the ash tray for the heat sink.

You can also use a dropping resistor. Try a 20 ohm power resistor dropping 6 volts at 300 mA cutting the 3 watts to more like 1.2 watts. The resisto r can get a lot hotter than the regulator with no ill effect. Don't keep t hem close though.

Here's a couple of 8 ohm parts.

Amazon:

You can probably find these in single quantities too. Handy to keep a few around.

It does seem to be a *very* 1970's solution to a trivial problem though. I recall having a similar problem with powering a Robert's radio in that era. Painting the shiny aluminium box made an enormous difference.

There are countless cheap USB 5v 1A converters certified to work in an automotive environment which is hostile when turning the engine over.

Not sure "too hot to touch" is a good measure, because a good conductor of heat will feel too hot even at a modest temperature.

If a single bigger heat sink is not an option, you could use a 7809 or

Sylvia.

There are cute little 3-pin potted switching regulators that drop into a 7805 footprint.

I hadn't come across those. Cursitor might be interested in

Check this one out: