# I want to have a 9v battery to give a 4.5v - 5v power supply? [duplicate]

I am trying to download a program on to my PIC circuit. As you know the pic isn't supposed to operate with anything larger than 5v. I don't have any access to a battery pack which could hold 3 x AA batteries. What i do have are lods of other stuff, that may be useful. I was thinking that if i used two 330 ohm resistors to create a potential divider then had another battery snap going off from that to the pic circuit. Would that work?

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• Tape or rubberband 3 AA batteries (and some wires or bits of metal) together. Properly bent wires on a protoboard will do, too. Or wedge them into a suitable cardboard, wood or plastic box, or tube (tube may be easier as you don't ned to provide any inter-battery connections.) 9V has MUCH less capacity (amp-hours) than AA - so even if you use a switcher, you'll have comparatively poor runtime. Cost is usually worse, too. Feb 14 '16 at 1:17
• Why is this being voted negatively Feb 14 '16 at 18:06
• Google "9V to 5V regulator cap". Or build your own. Jun 21 '18 at 20:51

That's not usually a good idea for a couple of reasons.

• It wastes power. From $V = I \cdot R$ we can calculate the current, $I = \frac {V}{R} = \frac {9~V}{660~Ω} = 13.6~mA$ without doing anything useful!
• More importantly, the voltage will vary depending on what the processor load is. If the PIC has a wide voltage tolerance you might get away with it.

You really need a voltage regulator.

A quick work-around is to use a USB port as power supply. Get a cheap USB cable or broken USB device, keep the A-plug end (that goes into your computer) and cut the plug or device off the other end. The red (+) and black (-) should give you a 5 V regulated and current-limited supply. Check with your multimeter.

USB cable pintout

Red   | Vcc (+5 V)     | Connect to circuit +
White | USB Data -     | Not used
Green | USB Data +     | Not used
Black | Ground         | Connect to circuit -
Shield|                | Not used (probably connects to laptop ground)

• I'll try that. Thanks. So the potential divider won't work? Feb 13 '16 at 12:41
• Very good answer ! I did not consider a USB supply Feb 13 '16 at 12:41
• What do i do with the earth wires? Feb 13 '16 at 12:52
• I presume you mean the shield / screen. See update. If you have a USB charger you may wish to use it in preference to your laptop. Cost of replacement will be less if you have an accident. If you're only running a few LEDs or similar from your PIC it is very unlikely that you could cause any damage. Feb 13 '16 at 13:06
• @HasanImtiaz: The potential divider might work but consider what would happen if the PIC went from drawing almost zero current to drawing 20 mA. The voltage would collapse from 4.5 V to, maybe, 2 V. You need to regulate the supply. Feb 13 '16 at 13:23

You could try to use a voltage divider along with a voltage-buffer OpAmp. (if you connect the pic directly the voltage diviser output, it will not work since it will draw current)

• I don't have access to many things, which is why i am asking otherwise i would order a 3x AA battery pack, but delivery is 10 pounds and I don't want to spend that kind of money for a 50p thing. Will the potential divider work? I'm asking because i have only just started my ecpirience with electronics? Feb 13 '16 at 12:38
• voltage divider alone will not work, because its output current has to be zero, that's why I was suggesting a voltage buffer. Can't you just buy those 3 AA batteries in a shop or something ? Feb 13 '16 at 12:40
• Then how would i connect it to the circuit? Feb 13 '16 at 12:43
• What about wires ? Feb 13 '16 at 12:44
• I understand how it would work with a 9v battery. How'd i do that with aa batteries Feb 13 '16 at 12:51

Simple : Use voltage regulator L7805.(as @transistor already answered)

Output voltage : 5.2V approx(always more than 5V if input voltage 9V or more)

If cost permits, you may want to have Linear Technologies LTM4623 micro switching power supply module. This finger nail size chip can buck down 9v to 5v at 85% efficiency, just needed a voltage setting fixed resistor and two equally tiny capacitors at input and output. Great performance for battery powered loads. The only concern it costs usd10 a piece. Check their website.