How to drop from 6V to 5V a "clean" way?

[flotulopex]

Hello,

I want to power my circuit with four 1,5V batteries.

I don't find a 5V voltage regulator that would do the job unless it is fed with at least 10V.

What would do the trick best?

[jblackann]

You have a few options that you could use. It all depends on what you are trying to do with your application. Is this something you want to produce or is this something you are working with at home?

You could use a Zener diode rated at 5 volts. A zener is used to clamp the voltage.
You could make a voltage divider and by selecting the correct resistor values get an output voltage of 5V.
You could use a diode in series with your batteries, not a bad idea as it will give you polarity protection. This should drop the input voltage down by 0.7V.
You could purchase a low dropout voltage regulator. They are alittle more costly than standard voltage regulator. You can find them labeled as LDO's. Take a look on digikey.com and see what you can find.

[Luciano]

Hi,

Use the voltage regulator LP2950.

75 μA quiescent current
100 mA output current

http://cache.national.com/ds/LP/LP2950.pdf

The LP2950 and LP2951 are micropower voltage regulators
with very low quiescent current (75 μA typ.) and very low
dropout voltage (typ. 40 mV at light loads and 380 mV at
100 mA). They are ideally suited for use in battery-powered
systems. Furthermore, the quiescent current of the LP2950/
LP2951 increases only slightly in dropout, prolonging battery
life.

Distrelec = 640722 (CHF 1.02)

Best regards,

Luciano

[flotulopex]

Thanks a lot!

I'll have a look at those LDOs I didn't know about.

The problem with the divider and the diodes is that they will follow the batteries voltage drop.

I'll have a look at DISTRELEC.

Thanks again.

[sougata]

Dear flotulopex,

LDOs are a good choice as suggested by Luciano. Depending on your application if you are using a nanowatt PIC then your PIC may work within the range of 2V-5.5V or 2V-6.0 volts. If you are using CMOS logic as well then this should not be a problem. Also for alkaline batteries the cell voltage is 1.5V, but if you are using a rechargeable solution with Ni-Cd or Ni-Mh then your cell voltages are 1.2 volts per cell. So let us know more about your app.

[flotulopex]

I have made a kind of timer - more or less something like the ones for the chess players but for up to 5 players. My mother just drives me crazy when we play Rami and she wins but she takes almost triple time for reflection....

So to the project, there's a PIC, an LCD display, 5 LEDs, a piezo and a few external pull-up resistors.

On my breadboard, I had no problem to get the timer work properly. I have a lab power supply so no trouble here.

When I made the prototype, my thought went first to the power supply. Due to the boxe size contraints, I choose to go with four UM4-AAA batteries (I didn't want to go with accus and a charger).

After some tests, I had to modify quite a few things such as:
- external 10k pull-ups (the internal ones look to be to weak because both buttons just switch from time to time even if nobody presses them)
- addition of 100nF cap between Vdd and Vss (I never needed before) and another one between LCD and Vss (same)

Actually, the circuit consumes around 50mA when the display is ON and one LED is activated (normal operation). Strangely, when I start the timer, the current drops to 20mA. I'm still invastigating this.

I also used another display for the final version than the one I use on my breadboard and this display is extremely sensible to power fluctuation. If I have 4,8V then the display will dimm a lot and if the power is over 5,4V, the display will just go black (the contrast setting is very sensible).

I think the LDO is effectively the best solution in my case.

Grazie Luciano ;-)

[sougata]

Hi,

LDO would remove all headaches perhaps. By the time you get it consider this.

1. Use a simple 5.1V Zener shunt regulator for your PIC and LCD.
Both of them are not that current hungry. (Beware not the backlight)

2. Power the LEDs, Buzzer (and the LCD backlight if you are using) from the unregulated supply. (Batt +ve)

3. For lighting up LEDs or Buzzer invert your logic and drive the cathodes while the anodes are connected to Batt +ve through necessary limiting resistors.