SparkFun Forums

[boelkw]

Hey I'm making some turn signals for an old 1936 buick and need to find a way to make a regulated 5v DC for the ICs and LED's I'm using.

All of the tutorials and such for breadboards only seem to be putting out 5v from around 7-12v input at 1A. I need mine to run from around 3-8 volts and put out 5v. I've looked at the LM 7805 but cannot figure out how to run from 3-8V.

Ideally looking for around 1-2A, 5V DC, 3-8V DC input

Any help would be appreciated thanks!!

[leon_heller]

Look at buck-boost converter chips.

Leon

[MichaelN]

Why would the input be as low as 3V on a car? Even a nominal 6V battery should give more than that, even when cranking the motor. Also, you may not need a full 5V - can your circuit accept a lower voltage?

A "buck" (step-down) circuit is a lot simpler to implement, but if you really need those input and output voltages, look at a "buck-boost" circuit as Leon suggested.

[boelkw]

i wanted the range just for a saftey factor i guess, i'll take a look at those buck-boosts, thanks alot

[boelkw]

now that i really think about it, i don't think i need to be able to go down past 5v.

Are there any 5v regulators that can handle being at 5.5-6.5 volts? All the ones I've looked at seem to want a nominal voltage of around 7volts.

thanks alot

[leon_heller]

LDOs can go down that low.

Leon

[UhClem]

Are there any 5v regulators that can handle being at 5.5-6.5 volts? All the ones I've looked at seem to want a nominal voltage of around 7volts.

LM2940

[boelkw]

that one will work well even though the spec sheet says it want 7v input?

[riden]

Where do you see the datasheet say 7v max? Page 3 of the datasheet says a maximum of 26v. Naturally, you will want to keep the input voltage well below that for long life.

[MichaelN]

The dropout voltage of the LM2940 is typically 0.5V @ 1A according to the datasheet.

Should go without saying, but check that the power dissipation in the regulator is OK (ie, suitable heatsinks). With 1A current in an automotive situation, I'd be looking at switchmode (buck) chips instead.