7805 will handle 1A at 12V input fine, so long as you use a big enough heatsink.
But you'll be better off using a cheap switch-mode DC-DC converter like the eBay LM2596 units,much higher efficiency, no heatsink needed (only downside is the noise which could be an issuewith sensitive low-voltage analog signals).
i have a 12V DC adaptor, can i then use a 7809 *and* a 7805 to drop the voltage over two 'regulators' - ie. dropping from 12V to 9V and then subsequently to 5V.
Okay, looks like i'll be going with that option - how about the current, those converters will pass the input supply current through with no losses ?
A DC DC converter swaps current for voltage. If you feed 12V in and take 5V out at 1A then the input current at 12V will be a lot less. If there were no losses then the input current at 12V for 5V 1A out would be 5/12 * 1 = 0.42A (This is where some kind person tells me I got the maths wrong!)
...the 7805's will tell you if it's okay, they have automatic shut down for over-current and over-heat.
It doesn't work like that. The short answer is don't worry, it will do what you need.A DC DC converter swaps current for voltage.
...Just connect it up, it will be OK.
i see, good to know !Thanks, that puts my mind at ease I see - I kinda figured that the power (wattage) is the limiting factor.Conversely, with a boost converter, we can then expect a *drop* in the current ?(eg. from a 5V@1A supply, we won't get 1A if we boost it to 9V)Okay - i shall proceed with confidence !
i see, looks like i still can't do the test yet then, no heatsinks yet.just to be sure though, from the datasheet;it says Maximum Power Dissipation is PD=(TJ(max)- TA) / *0JA (*0 is actually theta)that becomes PD=(150-33)/19 = 6.16Wso; it won't handle 7W, right ?
Just to be clear a 7805 is not a converter like Perry is suggesting but a regulator, which drops voltage leaving current the same and dissipating the extra energy as heat, not very energy-efficient of course.
You may use a resistor to drop the excess voltage. Or a few diodes. Use the linear regulator as the last step.
...Yes, if you use a boost converter then the output current will be lower than the input current.
(I'd better hide in case it goes BANG!)
That's using a PCB as heatsink or nothing as heatsink, using a block of copper that 19 becomes 3, and the power handling is 30W fora 90 deg rise above ambient....
Perhaps cut up a couple of aluminium cans, open them out and arrange three or four layers together with a hole through them, use a washer to hold them together where you screw them to the tab of the 7805. The separate layers can fan out as fins on the heatsink.
Stop fiddling with linear regulators, and buy a switching (buck) converter.
Don't you have a car cigarette lighter style USB charger (same thing).Leo..
oh - you mean like phone chargers that plug into the mains AC ?