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 Posted: Apr 22, 2012 - 07:02 AM |
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Joined: Jul 20, 2009
Posts: 42
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Hello All,
My project details:
1. Atmega32A
2. Nokia 5110 LCD (Brightness varied by PWM)
3. Some switches
4. Planning to power it by AA/AAA batteries - 3/3.3V
My question is - Do I need a voltage regulator at all??
If not, can I power it directly with 2 AA batteries?
What protection I need to incorporate?
If yes, then I need to use 3 batteries. Moreover using a voltage regulator will increase the cost.
Kindly suggest
I know its more of a general electronics question, but avrfreaks has been my goto site of late 
Thanks |
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Posted: Apr 22, 2012 - 07:07 AM |
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Joined: Nov 28, 2004
Posts: 3551
Location: San Diego, Ca
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kpanchamia wrote:
Hello All,
My project details:
1. Atmega32A
2. Nokia 5110 LCD (Brightness varied by PWM)
3. Some switches
4. Planning to power it by AA/AAA batteries - 3/3.3V
My question is - Do I need a voltage regulator at all??
Nope .
Quote:
I know its more of a general electronics question...
Yep . |
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Posted: Apr 22, 2012 - 07:14 AM |
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Joined: Jul 20, 2009
Posts: 42
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thanks.
But what protection can I give so that the circuit does not get messed up if the user puts the batteries in reverse polarity? |
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Posted: Apr 22, 2012 - 10:14 AM |
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Joined: Nov 04, 2010
Posts: 2098
Location: Bristol, UK
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| A diode (early) in the power circuit will prevent cock ups. |
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--greg
Still learning, don't shout at me, educate me.
Starting the fire is easy; the hardest part is learning how to keep the flame!
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Posted: Apr 22, 2012 - 10:31 AM |
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Joined: Nov 02, 2009
Posts: 3239
Location: Zelenograd, Russia
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| Note: a series diode must be a Schottky diode with small forward voltage drop. |
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Warning: Grumpy Old Chuff. Reading this post may severely damage your mental health.
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Posted: Apr 22, 2012 - 01:48 PM |
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Joined: Aug 03, 2001
Posts: 326
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Quote:
But what protection can I give so that the circuit does not get messed up if the user puts the batteries in reverse polarity?
Most AA battery holders won't allow the batteries to be inserted in reverse or at least the circuit will not be closed.
The easiest way to prevent the voltage drop over the diode from the precious battery power is to put the diode parrallel with the circuit so it would short circuit the batteries if the polarity is wrong. Use a diode which can handle the short circuit of (disposable) batteries, or add a (poly-) fuse.
You can also use a N-Mosfet in series with the power supply which won't open if the battery polarity is reversed.
Battery contacts are also not very reliable and can open for a few milli seconds when your gadged vibrates, or is dropped. Especially if the inertia of the battery works against the battery holder spring. A beefy capacitor can buffer the battery voltage. This capacitor is also needed for high current pulses such as driving IR leds for a remote control. |
_________________
Paul van der Hoeven.
40+ projects with AVR's:
http://www.hoevendesign.com
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Posted: Apr 22, 2012 - 10:58 PM |
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Joined: Nov 11, 2003
Posts: 3841
Location: Chicago Illinois USA
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Really? I thought the voltage range on the display was too tight to run straight from AA batteries. You might be saving me some expense here;)
I've never seen a AA holder that prevented you from putting them in wrong. I'd use a logic level mosfet. Source to battery minus, Drain to device ground and gate to battery plus. |
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Discursive design,
Torby
Some days, it's just not worth chewing through the restraints.
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Posted: Apr 23, 2012 - 03:58 AM |
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Joined: Nov 28, 2004
Posts: 3551
Location: San Diego, Ca
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| Also, you ought to be using a P-series mega device, which are designed for battery apps and use ALOT less power in sleep modes . You could use 2 batteries to get ~3V, but how long does it need to last ? ONE 3V Li-Ion or other modern type battery will have more Mah rating than a Duracell, for example . |
_________________
1) Studio 4.18 build 716 (SP3)
2) WinAvr 20100110
3) PN, all on Doze XP... For Now
A) Avr Dragon ver. 1
B) Avr MKII ISP, 2009 model
C) MKII JTAGICE ver. 1
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Posted: Apr 23, 2012 - 07:59 AM |
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Joined: Jul 20, 2009
Posts: 42
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Thank you all for your inputs.
@indianajones11,
Yes I will use PA.
I will look at the available battery options.
@Paulvdh & Torby,
Your idea seems good. I am a bit weak at mosfets. Can you please tell me whether biasing resistor at gate is required? If yes, then how much?
And what is the approx drop across drain to source?
As in how much voltage drop are we looking at between battery and device?
Thanks
PS: Just trying to save some R&D due to time constraints |
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Posted: Apr 23, 2012 - 03:03 PM |
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Joined: Aug 03, 2001
Posts: 326
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Those power mosfets are cool devices, you should check them out. I'll give you some basics, but you should really do some research and experiment a bit before you start using them.
(Simplified) they are a sort of switchable resistors. If there is less than 2V to 5V on the gate (Depends on the type of mosfet) they are "off" and the resistance is very high. The "logic compatible" mosfets need a smaller gate voltage to turn on reliably.
N-channel mosfets need a positive gate voltage.
P-Channel mosfett need a negative gate voltage.
If there is a lot of voltage (7V to 10V or more) then the resistance is very low. Some have a Rds(on) of less than a milli Ohm and can switch more than 100A.
Expect something like 50milli Ohm for a cheap mosfet. If you microcontroller needs 100mA then you will have a voltage drop of 0.1 * 0.05 = 5mV over the mosfet.
If you start increasing the gate voltage from the "off" position there is an analog region of the mosfet which can be used to turn the switch on a bit slow to limit switch noise and / or inrush currents. (But the mosfet will dissipate power and may get hot in that region.)
In your application the gate resistance, which mainly infuences the switching time in combination with the gate capacitance is not very important. It is there mostly as an ESD protection. Something between 1k to 1Meg Ohm will probably be ok.
A fun experiment with mosfets is to make a simple tap light dimmer with only:
- 12V power supply.
- 12V light bulb. (Between +12V and the Drain).
- Mosfet (BUZ10 will do). (Drain to Lamp, Source to Ground).
- Capacitor 1uF will probably do. (low leakage) between Ground and the Gate.
If you then touch the gate with one hand and the 12V with the other hand the Gate and the capacitor will be charged slowly through you body and the lamp will turn on.
If you discharge the gate to ground through your body the lamp will turn off. The impedance of the gate is so high that the amount of light from the lamp might change slowly over a period of minutes or even hours.
Some things to watch out for with mosfets:
- They have an inherent "reverse diode" and will conduct current in the reverse direction. This is the main reason you cannot use a N-channel mosfet in this application.
- Gate is sensitive to ESD.
- Gate voltage must be limited to usually <20V. (Use an extra zener?)
- Switching time mainly depends on the gate capacitances. If you want to switch them very fast you need high currents into /out of the gate (1.5A or more, for a short time(ns) ). |
_________________
Paul van der Hoeven.
40+ projects with AVR's:
http://www.hoevendesign.com
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Posted: Apr 23, 2012 - 07:17 PM |
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Joined: Jul 19, 2011
Posts: 460
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kpanchamia,
Can you please post a link to the Nokia LCD data sheet?
I somewhat doubt that you will be able to power the LCD reliably from the AA/AAA batteries because of their ever-dropping terminal voltage. They will start out at 3.0 volts and drop to 2.something after some hours of use. And that doesn't include their source resistance which will vary the terminal voltage with imposed load.
Let's see what that Nokia data sheet says. |
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Posted: Apr 24, 2012 - 02:12 AM |
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Joined: Jul 20, 2009
Posts: 42
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Posted: Apr 24, 2012 - 03:59 AM |
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Joined: Nov 11, 2003
Posts: 3841
Location: Chicago Illinois USA
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2.7v to 3.3v
I'm worried that the battery voltage would vary more than that between fresh out of the package and dead. |
_________________
Discursive design,
Torby
Some days, it's just not worth chewing through the restraints.
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Posted: Apr 24, 2012 - 07:08 AM |
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Joined: Jul 20, 2009
Posts: 42
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| Do you think the LCD/Sparkfun module will have an inbuilt voltage regulator? |
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Posted: Apr 24, 2012 - 01:22 PM |
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Joined: Dec 11, 2007
Posts: 6841
Location: Cleveland, OH
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Quote:
Do you think the LCD/Sparkfun module will have an inbuilt voltage regulator?
Go to the SFE web site, locate the module, and have a look.
The nice thing about SFE's boards is that they post the schematics for all of them.
JC |
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Posted: Apr 24, 2012 - 02:39 PM |
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Joined: Jul 19, 2011
Posts: 460
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kpanchamia,
1. The Nokia display is spec'ed to operate from 2.7 to 3.3 volts. That's good news for your application, as it is nominally within the operating range of your desired battery configuration. And the supply current is very low ( a few hundred micro-amps even when using the internal LCD bias generator as I suspect you are intending to do). That's good too!
2. So, to answer your original question about needing a voltage regulator the simple answer is NO - at least not for a while. That "while" being until your two AAA/AA batteries run down enough to cause the voltage to drop below 2.7 volts. What happens then? I don't know the answer because I have never used this Nokia LCD or the controller IC it incorporates. So you should test this on a bench supply for actual operatng voltage low-limits and under voltage symptoms. My experience with other LCD of this type is that when you get below a certain supply voltage they "lock-up" in one way or another and will never recover until subjected to a complete power-off-on cycle.
3. If you are incorporating this into a saleable product, take my advice and spend a lot of time and effort perfecting your battery & supply voltage strategy. Poor battery performance is one of the leading product killers, mostly caused by end-user aggravation. I have designed many battery-operated devices. If you need more advice on battery selection, design, testing, ask me. |
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Posted: Apr 25, 2012 - 05:43 AM |
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Joined: Jul 20, 2009
Posts: 42
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Well chuck-rowst,
A little background - I am on a job with a company currently. I wish to design a product so that I can get the ball rolling and start my own business. Having said that internet is the only source for my guidance in the little spare time I get. Answers like these, are of tremendous value to me.
Thanks a ton 
Alrite back to topic,
I am awating PCB. Should get it in 2-3 days. Will test on it and post the feedback.
How will it be if its like,
Battery -> MOSFET (for reverse protection) -> Zener -> device
? |
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Posted: Apr 25, 2012 - 02:30 PM |
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Joined: Jul 19, 2011
Posts: 460
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kpanchamia,
What does the zener do in your cryptogram? |
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Posted: Apr 25, 2012 - 03:58 PM |
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Joined: Nov 11, 2003
Posts: 3841
Location: Chicago Illinois USA
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| I think I'd go for one of those boost-buck converter chips to get a steady 3v whatever the battery does. Then, when the thing quits working, the battery is DEAD. |
_________________
Discursive design,
Torby
Some days, it's just not worth chewing through the restraints.
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Posted: Apr 25, 2012 - 04:53 PM |
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Joined: Jul 19, 2011
Posts: 460
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Hmmm....
So, the "zener" is just a "place holder" in your cryptogram to represent a voltage regulator or controller of some sort? Is that right?
Those "boost-buck converter chips" are not free and they usually need a handful of "friends" around them to actually do what they do - and those friends aren't free either.
Since this is for a saleable product, why not just use three batteries and a cheap (i.e. to you, the manufacturer ) dissipative series-pass voltage regulator, and let the end-user bear the cost of the extra batteries for the lifetime of the product? |
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