Diodes for P.I.C

[Screech2]

hi guys, I've been a member for 10 years, and lost my passwords and emails so had to reregister. Now I'm back to 1 post

anyway........

I want to power a PIC with 4 * AA batteries).

I noticed the AA batteries are not exactly 1.5v. Anyway I am getting about 6.4 volts with four "AA" batteries.
so, I need a voltage drop.

I am using a silicon diode, of 0.7 voltage drop.
6.4 volts -0.7 volts gives 5.7volts, but, that may be still too high for the pic. I think up to 5.5volts is the max.

I read somewhere that germanium diodes have a voltage drop of about 0.3 volts. Can they be used?
I would like to add this diode to the silicon one in series for a whole 1 volt voltage drop.

I am using a general purpose 1 amp 1n4004 silicon diode.
I need a germanium one too. Could you please recommend one.
I have Jaycar electronics shop round the corner. Here is their list of diodes.
**broken link removed**
I am running a 5volt stepper motor with the pic. The whole circuit uses about 120 milliamps.
thanks guys.

 

[4pyros]

i would use a low dropout 5 volt regulator to get the most out of your batteries.
will your pic run down to 3 volts?

 

[Screech2]

Thanks Pyro, I forgot about those low drop regulators.
but I still would prefer the least number of components, so a regulator and 4 capacitors is not what I want at the moment, unless there is no choice.

 

[4pyros]

OK well you should at least use a zener to keep the voltage below 5 volts
what is the min voltage of your pic?
with a small load on your batteries what is the voltage, may be lower under load

 

[Screech2]

Minimum voltage for the pic is I about 3 1/2 volts I think.
Not sure about the voltage under load.
The Zener may be the way to go.

 

[JimW]

What is the variability of your circuits current draw? Is it just the PIC? I would consider putting a red LED in series with the PIC power pin. You might need a resister in parallel with the PIC to make sure you draw a minimum amount of current. Check out the VI curves of the LED you choose, but it should drop around 2 volts.

A voltage regulator is the way to go, but if you want simple and cheap. You can use the LED (and it doubles as a power-on indicator!) to drop the voltage. Hopefully this is not for production or for a critical application.

JimW

 

[4pyros]

You should just use three batteries, then you will not need anything else.
Some people simply put a metal plug in the forth slot, like a aluminum rod cut to length.

 

[Screech2]

Power draw is only 100 milliamps for the 5 volt stepper.
The pic will just send signals to the stepper.



I like the idea of the power on led in series with a resistor in parallel to the pic for led to light.
and I like the zener idea.

I also like the 3 batteries or 4 rechargeable batteries.
It is for production.
I would like minimal parts if possible,
Don't know what to do?

 

[skimask87]

Which PIC are you using?

 

[Screech2]

IT'S AN 8 PIN
12F683

 

[skimask87]

I've ran the 12F683 directly from 4 alkaline cells. The datasheet says that's out of spec, but it'll take it.
If it was me, I'd run the PIC off the battery pack with one diode. The battery pack might have 6.4volts on it when the batt's are fresh/brand new and with no load, but put a slight load on them, the voltage will drop a bit. Power the stepper directly off the battery pack. The 12F683 will run down to 2volts @ 8Mhz. With one diode inline, that'll put your battery pack at 2.7v before the PIC quits working (in theory anyways)...and by that time those alkalines will be far past dead as far as your stepper motor is concerned.

Or just run the whole thing off 4 NiMH or NiCAD batteries and call it good.

 

[Screech2]

I've ran the 12F683 directly from 4 alkaline cells. The datasheet says that's out of spec, but it'll take it.
If it was me, I'd run the PIC off the battery pack with one diode. The battery pack might have 6.4volts on it when the batt's are fresh/brand new and with no load, but put a slight load on them, the voltage will drop a bit. Power the stepper directly off the battery pack. The 12F683 will run down to 2volts @ 8Mhz. With one diode inline, that'll put your battery pack at 2.7v before the PIC quits working (in theory anyways)...and by that time those alkalines will be far past dead as far as your stepper motor is concerned.

Or just run the whole thing off 4 NiMH or NiCAD batteries and call it good.

the problem is it's for production, and I don't know what batteries people will use with it.

 

[Screech2]

I'm using 4 energizers max batteries. (Alkaline)
6.25volts @ no load, and
6.20 volts @ load

with the 1n1004 diode, I am reading 5.6volts at the pic.
is this safe?

 

[skimask87]

Plug it in, run it for a few weeks, see what happens. Yes, that's out of spec, but not by much. Would I do it? Ya, probably.

As far as knowing what batteries people will use with "it".....well, that's up to you. Tell them what they will use with it. If they don't use what you say, and it don't work, it's their fault.

 

[MrAl]

Hi,

I would not use a zener because a zener regulator is a shunt regulator and that will eat up battery power.

Schottky diodes have less voltage drop than regular Si diodes (about 0.5v) so you could try two of them.

You can find Low dropout regulators that dont require much power to operate and all you need is one capacitor on the output.

Another idea is to run the PIC from three AA batteries and the stepper from four AA batteries. That way the PIC gets a safe voltage and the stepper gets the voltage it needs too.

At 100ma current draw the voltage will not stay high for very long anyway when using AA batteries.

 

[4pyros]

Will your motor run off 6.4 volts OK or will that need to be regulated too?

 

[Screech2]

I've decided what I'll do.

For now, I use two, general purpose 1 amp silicon diodes in series (1n1004 )should drop the voltage down 1.2 volts (6.0-6.2 to 4.8- 5.0 volts), and that will power everything.
I'll see how that goes.
I was over thinking this and want to put this topic to rest.

Thanks for your help guys, and I learnt something from you too.

Screech