7805 alternatives

Hello,

This is a doubt i have from some time now:

Let´s say i have a unit loaded with a 6v battery pack, how can I feed Arduino 5v from it? Idea is to get rid of the 7805 and use the alternative instead.

Tried a simple tension divider scheme with no luck, what is the best option?

Thanks

What’s wrong with the 7805?

The dropout voltage for the 7805 is around 2V. With a 6V battery pack, you are gonna lose too much through the voltage regulator to get the full 5V. The board may run, but it could cause problems.

You should look into boost regulators. Or even buck-boost regulators for wider voltage inputs.

Nothing wrong with the 7805 itself but in the datasheet it says a minimun voltage of 7v for proper operation (line regulation?).

And my battery is 6v, less than the minimum required… if i intepreted the datasheet in the right manner.

Thanks

Very thanks kiltjim, im going to look for those.

Cheers,

Rodrigo

Well one simple method is to wire a series diode or two between the battery positive to the Arduino. Each diode will drop about .6 volts. A simple 1N4001 would work well.

Lefty

I’m not sure how much you want to spend… but if it’s a setup worth spending money on… you can get a switching regulator that can give you up to 95% efficiency with batteries… and has even a lower-dropout voltage than LDO regulators.

http://www.robotshop.us/Dimension-Engineering-Switching-Regulato-2.html?utm_source=google&utm_medium=base&utm_campaign=jos

They also have variable regulators with a trimmer pot built in, so you can just attach the input… ground and output, then control the voltage with the pot on top.

Well one simple method is to wire a series diode or two between the battery positive to the Arduino. Each diode will drop about .6 volts. A simple 1N4001 would work well.

Lefty

But would the power still be unregulated and undependable? :-?

its coming from a battery

Well one simple method is to wire a series diode or two between the battery positive to the Arduino…

Sounds interesting but TchnclFl reminded me that Ni-Mh batteries will drop tension very sooner before it being discharged.

I'm not sure how much you want to spend..

Those options sounds great CaptainObvious, thanks. Sad its the difference between the amount you want spend and the amount you can spend. I know you´re just being gently, understood, Im just kidding ;D From what I know will must import those with high taxes (incredible 80%), but it´s a good solution for some final setup but prohibitive for prototyping.

What about Lefty solution using Lithium ion batteries? Sounds reasonable? So maybe i dont get the exacts 5 volts all the time but it ends reliable ?

Cheers,

Rodrigo

its coming from a battery

Precisely…Ergo, lack of regulation and decoupling, am I right? :stuck_out_tongue:

a battery + load is a pretty stable and calculable thing

Precisely…Ergo, lack of regulation and decoupling, am I right?

Battery voltage stability/regulation is not too bad, but it depends on what kind of battery you are using and your application. Ni-Cads and Nimh have pretty good flat discharge voltage until very near the end of their charge. Li are more like lead-acid in that the voltage decreases linear with charge capacity remaining. All would power a AVR processor chip fine as long as the voltage is not over or under the max and min specifications for the chip.

If you are using analog inputs the A/D conversion accuracy will be effected some with falling battery voltage unless you utilize the internal 1.1vdc band gap as the A/D reference or other external regulated voltage reference for the AVCC pin.

Decoupling should be OK with just the normal .1mfd bypass caps that are normally wired right at the AVR chips power pins, extra decoupling when using batteries should not be required, but that might change if you are also powering some external components (servos, motors, solenoids, etc) along with the processor chip.

Lefty

whay not a low dropout regulator? They’re not expensive.

A good alternative may be MIC2941, employed in ladyada’s adjustable power supply:
http://www.ladyada.net/make/bbpsup/
It drops “only 40mV - 400mV”.

A good alternative may be MIC2941, employed in ladyada’s adjustable power supply:

Looks like a nice design. I would not install the LEDs if using battery source as it just wastes some energy. Here is the schematic of the design:

Lefty

LM2931 or LP2950, only 100mA supply, though.

-j

Looks like a nice design.

Hey, agree !
Im going to see if i can find these parts!

Thanks

How much current do you require? The dropout voltage is dependent on output current so the 7805 may be work in your application. I do not recall the change
for a 7805 but for some devices it can be quite large.

Another linear regulator to look at (in a TO-220 package) is the NCP1086 which has a dropout of 1.4V max (1.05V typical) at 1.5A. The NCP1086 can be purchased as fixed 3.3V or adjustable (to 5.5V).

If you can do SMD there are lot more options.

(* jcl *)


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Be careful using LDO regulators on a PCB designed for non-LDO regulators. The pinout is often different (LDO regulators tend to have the heatsink tab and center pin connected to Vout, while the 7805 has them connected to GND.)