Thread: embarrasingly easy newbie question

hi, whats the easiest way to power a 9v circuit from a 12v supply? or any other sub 12v circiut be it 2v, 3v etc

can it be powered with a voltage divider circuit, would that take into account fluctuations in the 9v circuit? or would i have to use a voltage divider consisting of a resistor and zener diode. or does it always have to be a voltage regulator?

Theoretically you can obtain a lower voltage with a simple voltage divider (using resistors), but the problem comes when you put charge on that end of the circuit... you'll place more components on that end and the behaviour of the whole circuit will change, and there is no sureness you'll always get those 9V. (Unless you include it on your initial design and never change it).

If you're building a power supply and you're going to place different charges on it, you should use a voltage regulator so you assure you'll get 9V always no matter what you put on that end. The regulator isolates both circuits, the power supply and the charge circuit.

LM7809 regulator. No other parts really needed. Cost you about $1 or less.

thanks guys, what sort of voltages do they come in? and are they 3 terminal +,- and output or is that too simplistic?

Yep... that simple.

For further reference, check the datasheet: http://www.datasheets.org.uk/search....809&sType=part

thanks, found them in local elecronic store catalogue, what about resistor and zener diode together? or will that have the same problem as the divider? just for the extra info. Im trying to learn as much as i can :-D

78xx will produce 1 amp of current at a regulated voltage, much more then a voltage divider type setup. You can get them in +/- 5,9,12,15 volt ratings and maybe even some in between. With a resistor or two they can be made to adjust voltage over their rated regulation and still produce the same current. Only thing I could suggest is perhaps a block of aluminum to cool it. If the current is low it will handle it without. Also they are very hard to kill. Over current or heat will just make them shut off for a bit.

Their is also a CMOS version that can regulate say 5.5 volts volts down to 5.0 volts. The voltage drop is very very low. I'm trying to recall the part number.

most of the circuits i'm thinking about run of pp3 batteries for quite a while, so dont think the currents that high, just want to run some off car/bike batteries to be fitted in/on vehicles :-D


you've all been really helpful, thanks :-D