how to get 9 volts 300ma

[moldmaker]

I have a 12 volt lead acid gell cell
battery at rated 4 ah.
what i would to do is regulate the voltage and current down to 9 volts 300ma.
i was thinking about using a 9 volt voltage
regulator but i am afriad that if i use it
that it will suck the juice out of the battery
when it drops the voltage down.
also when the battery drops below 2 volts
the regulator will be wont work anyway<p>does anybody have any suggestions<p>thanks
moldmaker<p>[ July 30, 2004: Message edited by: moldmaker ]</p>

[Chris Smith]

Most devices are self limiting on the current side, except for some raw devices like LEDs etc. <p>So the question is are you just needing 9 volts, and the device will limits its own draw, naturally?<p> If Your reading the values off the label on a walk man or transistor radio then the answer is yes, something like a walk man or what ever has its own way to regulate it self and will only draw what it needs in terms of current, but not voltage.<p> Then all you have to do is place a regulator in line, 9volts, and the rest is history.<p> Your device may or will start to fail when the battery drops below about 10.2 volts or so unless [Regulator drop out voltage or loss of 1.2v from battery in the regulator] its not sensitive to voltage. Things like transistor radios that use 9 volt batteries can run as low as 5 volts or so, before they fail?<p>[ July 30, 2004: Message edited by: Chris Smith ]</p>

[L. Daniel Rosa]

The mixed company and no name 78xx regulators usually pull about 5mA above the load requirements. Three volt overhead is a worst case scenario, at 300mA it will be less.<p>If you don't have a 7809, a lower voltage device can be progrmmed for it with a zener (or similar device) between te common pin and ground or a resistive divider.

[jollyrgr]

Unless you are biasing a semiconductor you don't need to regulate current. Just exactly are you wanting to do? If you are planning on powering some electronic device that runs on a nine volt wall wart and want to use the lead acid battery a simple three terminal regulator will do. If you are worried about draining the battery by the idle current of the regulator you can put three to five silicon diodes in series (something like the 1N4001, 1N4002 etc.) and let the voltage drop of these devices drop the voltage down. <p>A "12 volt" lead acid battery will be slightly higher than 12 volts when fully charged. The 1N400x series of diodes can easily handle one amp of forward current. The "standard" voltage drop of a silicon diode is about 0.6 to 0.7 volts. Unlike a regulator that will continue to draw current even after the device is shut off, the diodes in series will not draw any current when the load is turned off.

[moldmaker]

im sorry
i should have been more specific
i was just trying to match the voltage that is on my wall wort
what im trying to do is run a digital depth probe in my machine shop
its kind of a pain whenever I have to measure parts
from one part of the shop to the next part of the shop
then you have to find an outlet to plug the thing into
its not worth listing to someone complain about tripping over the extension cord<p>thanks
moldmaker

[Chris Smith]

Run the standard 9 volt regulator, and your device will only draw the current it needs.
Batt volttage should be above 10.2 in theory.

[toejam]

i bet a small nine volt battery would do unless you intend to use it in a constant duty mode. Than it would be easier to use. I doubt the probe uses 300 ma.

[moldmaker]

i tried using a 9 volt battery but it would only
last for a day

[toejam]

an lm 7809 would work with no problem less you hook it up wrong.