I'm trying to build a power supply for an AVR project....
I need +5V regulated for the AVR and +12V for other crap.
Yes, I could just use a 7805 and 7812, but since I want to drive this off of batteries (hopefully?), that's looking like +18V out of two 9V batts. That would be a LOT of heat off the 7805, and lots of wasted power from the batts.
Or does someone have suggestion on a battery to use that might be closer to the minimum voltage. +15V if I recall...
The problem with the 7805 and 7812 is that they are linear regulators. This means that if your source voltage (for example) is 15V and your 5V circuit draws 100 ma, then the 7805 is going to be dissipating the difference: (15V-5V) * 100 ma. You might want to look at a simple switcher; you can get them in small SOIC packages and they will even boost the input voltage from your battery level up to 12V. You can also expect 75%-90% efficiencies from them.
Dave
You could connect the 7805 to just one of the 9V batteries with the 7812 connected to the pair. The down side is that the one battery driving both the +5V and +12V is going to have a higher load and deplete out faster.
Don
Use two independent switchers. 9V battery gives 12v with step up. And, it give 5V with step down. If currents are low, you can do it for around $2US per supply.
Jim
Your device will be much more economical to operate if you design it
to run with a pair of AAs driving the switchers. The pair of AAs are cheaper,
pack more energy than a 9 volt rectangular, and occupy the same amount
of space.
Tom Pappano
Tulsa, Oklahoma
Agree with Tom!
Jim
Yes, use a step-up (boost) switch mode converter with dual outputs (5V and 12V) and run it from two 1,5V batteries.
If it is possible you should consider running the Microcontroller circuit at a lower voltage than 5V, for example 3.3V to save energy.
Maxim-IC.com, National.com, Linear.com etc. makes a lot of IC's for this purpose.
Try to check out these dual output boost converters from Linear Technology for example:
http://www.linear.com/SelectorGu...
LT1944 or LT1944-1 might be good choices.
Linear regulated power supplies are not good for battery applications.
Anything I do battery powered gets a switcher. If you set it up right, as was mentioned you can get up to 90% efficiency which ain't bad.
And even in projects with line power, I'm going to switchers if noise (or price) isn't some real driving factor. Linears are just little resistors for all practical purposes and heat up everything around them besides wasting all that power.
If you really need maximum lifetime and can trade off price and board real estate, you can drive the efficiencies even higher by oversizing the passives around the switcher IC. Most data sheets will discuss this.
