3V,7.7mA supply from 12V battery

[BigBen10]

Goodday, Can anyone give me some ideas on how to design/build a 3V power supply from a 12V battery. The 3V will supply about 7.7mA to a sensor. Thanks for your anticipated replies.

 

[CocaCola]

Google 'LM317' you will need to dial in a few values to regulate (lots of online calculators) at 3V and 'might' need a heatsink (probably not with such a low current)...

It's pretty straight forward, after a few minutes of reading about the chip... The datasheet alone will guide you to the answers if you can follow along...

 

[Olin Lathrop]

Normally when stepping down so much (1/4 of the input voltage in this case), you'd use a buck converter for efficiency. However, since your load only draws 7.7 mA, the dissipation in a simple linear regulator is only 70 mW. Even something as small as a SOT23 package can handle that without issue, assuming your operating at normal room temperature.

There are lots and lots linear regulators out there. 3V is a common enough voltage that you should be able to find one fixed at that voltage, like a 7803 and many others. The thing to watch out for is to make sure the regulator you get can handle the 12V in. Some LDOs (Low DropOut) regulators work only with lower voltages. Also pay attention to what the datasheet says about caps the regulator might need on its input and output. Without the right caps, it could oscillate.

 

[BigBen10]

Thanks very much sir. I will take a look at what you both said and give u a reply

 

[BobK]

Hey Olin,

Welcome to the forum! Haven't seen you in a while

Bob

 

[Olin Lathrop]

Hi, Bob. I see you're in Worcester, but I otherwise can't tell who you are. You make it sound like we've met before. Care to elaborate on who you really are?

 

[BigBen10]

Gudday. This is work i was able to come up with. Tried two methods. Please feel free to criticize and advise on the circuits attached.
Didnt see a 7803 so i used a 7805 and applied voltage divider to get out 3V. Does it make sense?

 

[Olin Lathrop]

The voltage divider on the output is not a good idea. It raises the output impedance and causes a lot of quiescient current draw. Surely there are 3V regulators out there that can handle 12V in, but if you can't find one for some reason you can use a adjustable regulator. That seems to be what you did in your second schematic, but the resistors to control the adjustable regulator seem very low. I didn't check the datasheet to see what it really needs, but that's a lot of quiescient current as you have it.

 

[BobK]

Hi, Bob. I see you're in Worcester, but I otherwise can't tell who you are. You make it sound like we've met before. Care to elaborate on who you really are?

Hi Olin,

We were both posting actively on Microchip forum for a while.

Bob

 

[BigBen10]

Ok. Firstly the Input voltage has been changed to 9v battery. As for the resistor for the LM117. From the data sheet the formular is Vout=1.25(1+R2/R1)Vin. also from the data sheet R1 was set to 240ohm. From my calculations R2=176ohm which when i simulated got less than 3v at the output. But will still look for an LM317 calculator online and use it.

 

[Timescope]

I would use a TL431 shunt regulator for such a low current of 7.7mA instead of a 1.5A regulator.

Timescope

 

[tony ennis]

Hi Olin. Everybody knows Olin.



And I was just about to ask about power supplies