Constant 4.5 V supply using batteries?

robot_marc · October 19, 2011, 6:43am

how may i get a constant power supply of 4.5 Volt

using batteries?

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WHAT I FOUND SO FAR

googling and visiting several discussion groups

i found related advice ...

- e.g. using 1 more than the required batteries

in combination with e.g. a voltage regulator, diode, resistor, DAC -

if i use 4 instead of 3 aa batteries

and the voltage drops after a while -

would a voltage regulator, resistor, DAC

still provide 4.5V from the remaining voltage?

may i ask you what you would recommend?

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WHAT I NEED IT FOR

i am using rf sender/receiver modules

with my picaxe 08m microcontrollers

&

found, that they (and the connected antennas)

work best at a constant voltage of 4.5 V

geirandersen · October 19, 2011, 7:41am

If your voltage is critical

If your voltage is critical I would go for a higher voltage source and a voltage regulator setup. If you add a big capacitor after the regulator it should take care of the voltage dips that you see.

robot_marc · October 19, 2011, 8:25am

thanks a lot …

thanks a lot for your hints & immediate replies,

Geir & Oddbot-

looking at the datasheets both, receiver & sender,

seem to operate far below 500mA

(if “Supply current” refers to " current load");

the sender is most demanding:

Suppsuply current: .9 mA, typical 11 mA, max. 22 mA

i will try both of your solutions

- using a big capacitor in combination with a regulator

(how big does the capacitor need to be?)

- a zener diode regulator (with a resistor and a zener diode)

(which values do zener and resistor need to have?)

thank you for your support!

torrentula · October 19, 2011, 2:15pm

(how big does the capacitor

(how big does the capacitor need to be?)

That depends on what you are doing but 22mA in the worst case is not much at all (thats what a standard LED draws). I don’t know how much the receiver needs but if it is as you say, that the sender needs the most current, then I guess you only need a small capacitor actually, something like 100µF. If you want to you can of course use a 470µF but that would be a little overkill in my opinion.

I can’t answer the question about the Zener Diode/ Resistor regulator because I never used a construction like that.

A voltage regulator I can highly recommend is the LM317. Have a look at the datasheet.

Connect the LM317 as shown in the datasheet (on the very first page). Then your value for R2 would be (assuming you use a 240Ohm resistor for R1):

R2 = (4.5/1.25 -1) * 240 = 624 Ohms

I don’t think you will get a 624 Ohm resistor but you can combine two resistors or use a value thats close to 624Ohms.

Further Information on LM317 calculations (I hope linking to my own website is ok)

EDIT: Maybe a linear voltage regulator like the LM317 is not considerable because the input voltage must be 2V higher than the output voltage. So you must supply the regulator with at least 6.5V, but as Geir has already said a voltage regulator gives you a constant voltage over the whole battery charge (as long as this doesn’t drop below 2V above your output voltage).

Good luck

Torrentula

robot_marc · October 19, 2011, 4:27pm

thank you -

this was a lot of good advice …

i was hinted to use the MIC39100 or LM2931 low dropout variable regulators

if i am not mistaken i may use your formula with these as well which is handy

since i would like to add just 1 additional aa battery limiting the options

for the regulators (as you mentioned - it’s less than the required 2V)

am not sure yet if will go for a larger (than 6V) voltage as recommended before

-

am still a novice to electronics and might need more experience

before i start to work with more detailed components such as

variable voltage regulators.

anyhows:

thanks so much for all your advice

and replying to my questions;

i got a lot of information to start

testing it on the breadboard now

dannyv · October 19, 2011, 4:42pm

**A: a 7805 regulator with **

A: a 7805 regulator with diode behind it. diode drops about 0,7 volt.

B: a 7803 regulator with 2 diodes between its ground pin. (voltage becomes higher 2x0,7 volt)

robot_marc · October 19, 2011, 6:38pm

thanks, danny -looking at a

thanks, danny -

looking at a 7805 datasheet

(http://www.datasheetcatalog.org/datasheets/228/390068_DS.pdf)

i tried to figure out which values i need to use for the diode in order to transform 6V to 5V

but did not succeed …

may you give me a hint, please?

(… do i have to add capacitors as well?)

voodoobot · October 19, 2011, 11:23pm

Zener diodes…good for

Zener diodes…good for VRef, not so much for VReg…I would avoid this option as it’s not suitable for long term use or good design and stability. Get an LDO or use 1.2v NiMh cells and a couple of 4001 diodes.

torrentula · October 20, 2011, 2:11am

The decoupling cap on the

The decoupling cap on the input side and the output cap make sure that the voltage regulator operates correctly. So yes, you need to add caps to basically every voltage regulator.

The decoupling cap gets rid of the noise and the output cap keeps the voltage stable, as long as it takes the regulator to stabilize the voltage again, if all of a sudden more or less current is drawn.

I’m still learning about electronics too, but as far as I can tell when trying new things at first it probably is going to fail. Then checking the circuit I notice that e.g. I didn’t pay attention to the diodes polarity^^

Torrentula

torrentula · October 20, 2011, 6:27am

I don’t know these devices

I don’t know these devices but you should have a look at the formulas in the datasheet of the specific chip, because this calculation is the one used for the LM317.

The internal voltage reference of these devices may be different etc…