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

[sougata]

Hi All,

If you are using a simple shunt regulator to power your PIC and the LCD (not the backlight) then while driving outputs high would require your shunt regulator to use a lower value resistor. That means to keep the zener under conduction with all the loads. When the loads are less the zener will act as the load. Keeping in mind that it is battery operated it results in higher quiscent current. I recommended using the LEDs,Buzzer and the backlight to be driven from the unregulated supply. This is only possible if you are using an inverted logic. And setting PORTB = 0 or PORTB = $FF takes the same amount of labor and code. Also the PIC and the LCD would be a fair enough steady load to dimension your resistor-zener shunt regulator for maximum supply efficiency. I hope I have made myself clear or am I missing something??? And using a cap with buzzer is always a good practice. To find out the root of the problem I suggested the experiment to find out exactly whats going wrong and where. Even if the problem is solved with software I would strongly recommend to use a cap as Steve mentioned. The buzzer is a killer.

[charudatt]

Hello Sougata,

I have a similar requirement, but without the LCD. Same 6V operation and a Piezo buzzer.

Its basically a code lock operating on 6V. A couple of flashing LED, Sounder (piezo) and a Solenoid.

I have a design before me which does not use even a series diode to drop the voltage, So I presume , the PIC can take that, till the Alkaline battery drops down to safer level on its own. Seeing this , I feel a series diode would be good.

I used a GP PNP transistor with zener at its base to regulate the voltage. Don't know if that's the right way.

Any help ?

[Ron Marcus]

Check the new TPS60002(xxx1 for 3.3 volts) from TI. It will take 2.5 to 5.5 volts input, and put out a regulated 5VDC up to 800 mA. It will allow you to wring every drop of juice out of four Alkalines. It's a tiny 3x3 package, but works like a champ.

[T.Jackson]

A shunt-based approach is possibly THE worst avenue you could take for a battery powered circuit. Zener diodes require at least 5mA of current at all time in order to maintain satisfactory regulation. As you can see, this is a highly undesirable attribute, particularly for batteries. Go with the suggested low dropout regulator - clearly your best option.

[charudatt]

IMHO - I feel a series diode dropping the voltage by 0.7V would be the most ideal. Still I would try those LDO Ron has suggested.

I also plan to use a TC54 voltage detector circuit to measure Low voltage on the PIC.

Thanks RON for that reference.

Thanks to all

[T.Jackson]

Just the series diode alone will be inadequate. Four fresh new cells, series connected, with no load will be around 1.7 - 1.8 V ea. So 4 x say 1.75V = 7V. As the batteries begin to discharge to about 85% capacity remaining, then you'll have your 1.5V p/cell. You need more than just a diode unfortunately.

[Luciano]

Its basically a code lock operating on 6V. A couple of flashing LED, Sounder (piezo) and a Solenoid.

Any help ?

Hi,

In order to ensure long battery life, your circuit must use almost no current in standby.
The PIC has a wide operating voltage range and has a sleep mode for power saving.
To drive the LEDs, piezo and solenoid, make the I/Os sink current. Drive the
solenoid with a PNP transistor. If you use four 1.5V cells in series, just connect the
PIC after the third cell (4.5V). The rest of the circuit (solenoid) will use four cells. (6V).
A diode in series on the battery minus terminal will be the simplest form of battery-reversal
protection.

Best regards,

Luciano

[BobP]

Hi,

As I am about to build a circuit powered from 4 x 1.5v cells I just looked at the maximum ratings for the 18F2525 and it says it can take a max of 7.5 volts! So running a 2525 from 4 x 1.5v cells should be ok. Check the datasheet for the PIC you are using.

But personally I would put a diode in series with the +ive supply. It’s surprising some of the kit I have seen where the user has 'overcome' the plastic mouldings preventing incorrect battery fitting!!!

Bob

[Luciano]

I just looked at the maximum ratings for the 18F2525 and it says it can take a max of 7.5 volts!
So running a 2525 from 4 x 1.5v cells should be ok. Check the datasheet for the PIC you are using.

Hi Bob,

The voltage 7.5V is not the maximum operating voltage of the chip.

The 7.5V visible in the "Absolute Maximum Ratings" page 323, is
the voltage where you can go without physical damage to the chip.
The PIC18F2525 will operate reliable only when the voltage is between
4.2V and 5.5V.

This note is visible on page 323:

NOTICE: Stresses above those listed under “Absolute Maximum Ratings” may cause permanent
damage to the device. This is a stress rating only and functional operation of the device
at those or any other conditions above those indicated in the operation listings of this
specification is not implied. Exposure to maximum rating conditions for extended periods
may affect device reliability.

See the datasheet page 326, parameter No. D001 for the operating voltage.

Datasheet:
http://ww1.microchip.com/downloads/e...Doc/39626b.pdf

Best regards,

Luciano