Checking PP-9 battery voltage

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manie

Senior Member
As part of the alignment project I am building a "mobile" handheld unit also. This will be battery powered and I'm thinking of using the small 9V (PP-9 ??) battery as power source. I also want to monitor this battery's voltage through an Analogue input pin (28x1 - Anal_0).

I am thinking normal voltage divider as follow:

+9V(bat)
|
|
100K
|
|----> Anal input
|
100K
|
|
0v

I am thinking 100K divider R's to reduce "wasted" current to ground to only 90uA, while getting a nominal 4.5V to the analogue input.

Any thoughts on this please ?

EDIT: The 9V battery only for its size consideration while I can then stil regulate down to 5V...
 
A

Andrew Cowan

Senior Member
I seem to remeber hearing the input impedance of an analogue input is around 20K ish. Mot sure how this would affect the setup.
 
D

Dippy

Moderator
Far too high for reliable ADCing.

Have a read of the Microchip data sheet for guidance re input impedances.

The best way is to switch the ground connection to your pot-div using a tiddly MOSFET.
Reduce your resistors to 10K, then switch the ground line. So, you montior it when you want to in code and the rest of the time it takes b-all current.

The resistance of the MOSFET will be neglible in comparison to the pot-div - and you will have to calibrate it anyway as your PICAXE will be using a regulator. (LPC2950 or 2936 are nice for low power stuff.)

This is just something to think over as I haven't seen the rest of the circuit.
Maybe put a res from MOSFET gate to ground to prevent floating.
 
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manie

Senior Member
Andrew and Dippy: thanks again. I actually have a 2N7000 mosfet in circuit already but thought the other way has less components etc.

The 2N7000 has Rdson of 1.2ohm (typical) and Gate turnon V of 0.6-2.5V(typical). Should be OK driven directly from 28x1 output ?
EDIT:
I am thinking if battery voltage gets to say 8V, then it can be considered "flat"... ?
 
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Andrew Cowan

Senior Member
You should be able to drive the FET directly. What load does the circuit draw, and what type of PP3 (I think) does it use? Rechargeable/alkaline/zinc carbide all are considered 'dead' at different voltages.

A
 
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Dippy

Moderator
Well, tbe "other way" DOES have less components etc. :)
But sadly ...

Re: Flatness.
Don't just guess - read data sheets and do some calcs.
Examples:-
http://www.farnell.com/datasheets/63901.pdf
http://www.farnell.com/datasheets/104286.pdf

I was so proud that you'd Googled the answer to BB's ripple challenge.

What sort of regulator will be using?
How much voltage 'headroom' does it need to give 5V? (i.e. it's dropout).
Then you can calculate a safe 'flat' value.

Obviously there may be other issues if it goes too flat whilst sitting in the drawer, but that's another story.

I've retired from MOSFET advice so you're on your own there.
It'll probably be OK, I haven't looked.
All I will say is that make sure you don't fall into the usuall novice trap re: Gate threshold.
I'm sure it's been mentioned to you before , but Gate threshold DOES NOT mean that is the voltage where it switches on like a happy little switch.
Gate thresholds are the voltage required at the gate to allow X microAmps or whatever to pass Drain-Source.
The Data Sheet will also have a graph so you can CALCULATE the Rds at a given gate voltage (give or take a bit).
It's all the Data Sheet just waiting for you to read it ;)

Personally, if you've got one then just stick it in a breadboard and give it a go. Two minutes testing is a lot quicker than waiting for a committee reply.
 
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manie

Senior Member
Andrew: Will probably go for Alkaline as they are more readily available and tend to last a little longer. The circuit has the following main current consumers:
1. 28x1
2. FRM010 driver chip
3. Polygon 433MHz RF module (30mA Tx and 45mA Rx or close to those)
4. 20x4 LCD with BL (will restrict BL current to 30mA, still OK then)
5. "Waste" through 5V and 3.3V regulators
6. Battery voltage now switched so negligible current.
 
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manie

Senior Member
Dippy: You Googled TOO !??

From the datasheet the threshold should be OK as given:
Gate Threshold Voltage VDS = VGS, ID = 1 mA 2N7000 0.8 2.1 3 V
Click to expand...
It seems low enough and I have used it before for switching LED's @ 20mA. It was OK then ( NO smoke !). Thanks for the datasheet link, will check detail.

GOSH !!!! Ask a question here and you're LOBBED with dinkum HOMEWORK !!! OK, slowly I'm starting to ask confirmation, not outright "how ???" but thanks still for your time guys...
 
D

Dippy

Moderator
Google? Pah!
For data sheets I just use Farnell's website.

So, I thought I'd just have a look.

ST 2N7000 Typ 2.1V for ID= 250 microAmps.

Fairchild same as you read.

ON is also at 1.0mA

Vishay gives values for 250microAmp and 1.0mA

Hopefully, the 'threshold' is now more meaningful to you - and, above all, it shows that you must read fully. AND be careful when comparing all sorts of devices like-for-like.
(Especially if data sheets are changed as the years pass).
So many people just skim through, read 'Threshold' and think that's where it switches on fully.
But I know you didn't just skim through manie ... ;)

And the graphs are useful too - again something that people don't study.
But there we are. I'm offically retired (emphasis on 'tired') from MOSFETs unless paid handsomely.

HOMEWORK?
:) It's your project and WE'RE doing your homework :)
 
B

BeanieBots

Moderator
manie, it's all been covered now (again!) but here's some more homework for you. This is real world stuff...

You know the capacity of your batteries.
You know the average current draw of your circuit.
You know the current draw of your now correctly calculated divider.
You know how much extra it will COST to add a FET to turn off the divider.

How long will it take for that extra cost to be justified?
Alternatively, just show how much longer it will run on each battery.

PS. I have lost track of how many times I've posted the requirements of potential dividers for PICAXE ADC work. R1.R2/(R1+R2) < 10k
(varies from PIC to PIC but <10k is a safe bet)
 
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manuka

Senior Member
Will probably go for Alkaline as they are more readily available and tend to last a little longer. The circuit has the following main current consumers:
1. 28x1
2. FRM010 driver chip
3. Polygon 433MHz RF module (30mA Tx and 45mA Rx or close to those)
4. 20x4 LCD with BL (will restrict BL current to 30mA, still OK then)
5. "Waste" through 5V and 3.3V regulators
6. Battery voltage now switched so negligible current.
Click to expand...
Manie- while appreciating you may intend just occasional portable service with this circuitry, IMHO a 9V alkaline (typically of 300 mAh capacity at modest current drains) will probably give miserable service life with such a load. While C-Zn/Alk.Mn 9V batteries abound, their "bang for buck" is terrible, & falling terminal voltage often means discarding while they still hold considerable energy. I rarely specify them these days, especially since their cost (~US$3) can rival the entire electronic parts BOM in simple circuits! Additionally 3-3.3V supplies are now well established, meaning that even 5V circuits are considered "quaint".

Grab something rechargeable I say -AA NiCd/NiMH or ex cell phone Li-Ion perhaps? Stan.
 
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manie

Senior Member
Stan: After looking at datasheets for the so-called "best" Duracel, I've arived at the same conclusion. Duracel claims 550mAH capacity but this is NOT really reflected in their service life graphs. Drop-off to < 7V occurs after about two hours worth of use, even if used "pulsed" (intermittently) .

Will HAVE to consider NiCd as the minimum. Since I'm fabricating the Hand-held enclosure I can make the size as required. PCB size = 138mm x 71mm. 'AA' size NiCd batteries are 50mm x 12mm (roughly) so I could fit 8x in the length and thus get 9.6V.

What I remeber from RC aircraft days, these NiCd's have about 1000mAH capacity and voltage is maintained well with a fairly sharp drop-off when empty. I will also look at after-market Cellphone batteries, they will probably have a better usage life than NiCd's and the size/weight is appealing. I require 5V for the Axe and LCD parts and 3.3V for the RF module so I might have to get 4x for a "serial/parallel" battery pack.

It NEVER stops though, does it ? Now for a constant current slow charger too..........

Any more thoughts ?

THE TRUTH IS OUT THERE......
 
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manie

Senior Member
Well BB,Dippy,Stan,Andrew,Wolfgang,John,EC and SO MANY others, over the past 15-18 months I've started out knowing the terms Chip/IC/Cap/Resistor/Volts/Amps/Watts and knew Ohm's Law. In between I've learnt of Faraday and almost understand his law/s. I've met Kirchoff and ditto for his laws. I've also met BB and now appreciate HIS law.... RTFM etc, also Dippy and his law... read the datasheet ! All of you have contributed so much to my learning about Picaxe and electronics.

Fact is, I've crammed so much in over this 18 month period, sometimes you know "it is there' but too many facts just flood the grey matter, that is when we ask, and when given an answer or direction to look, I at least, feel quite stupid for asking.

All I can say, again and again, is Thank You all, I do appreciated it. Every time you guys have helped me to get something going that is more complex than my knowledge and ability allows for, it is like making that first LED flash from a chip you have programmed yourself !
 
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Dippy

Moderator
I told you BB!

"Drop-off to < 7V occurs after about two hours worth of use, even if used "pulsed" (intermittently) ."
- eh? What?
So, if I pulse it for 1mS every 5 minutes it'll fall to 7V? :)
You've got to use real numbers before working things out.

You haven't told us the exact total current draw.
... or how long the device will be on for and how often.
... or whether the RF module is 'on' all the time the device is 'on'.

I made a device with an 18X plus LCD+backlight and used the old 2 FET p/b on/off method.
It was not used very often and only for a few minutes at a time.
An Alkaline PP3 lasted 12 months. For that application the PP3 was perfect (and fitted the box :) )

I have made auto sensing devices with PICs and RF Tx. Again PP3 alkaline and 9 months service use and 14 months with lithium PP3. Again, it's work duty cycle was low.

So, horses for courses, and if the box is small then you may be limited to a PP3 or smaller.
There is NOTHING wrong with PP3s if the application is appropriate.
You MUST dig out a calculator and take measurements to determine suitability.
(Though, I must add, if this device is on for significant periods of time and often I would deffo consider something more manly than PP3 - but then without info we can only guess).
Do the calcs.
I did myself a spreadsheet to calc battery lifes. You input 'on time' , 'times on per day' 'quiesecent current' 'current on load' and it estimates battery life. It was pretty good even if I say so myself.
 
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manie

Senior Member
Dippy, i will for reasons of getting something going use PP3 at first, if the total average draw allows it some time before dropping output, then good. In the meantime, I will allow for recgarheable batteries (later on) on the PCB and in the enclosure. The charger can sit with the ATX SMPS and the hand-held can be placed on it like remote landline phones.
 
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manuka

Senior Member
Manie : You're not in RSA wine growing country are you ? Don't be distracted by those imperial commands just after your responsive Post, but focus on bundling up a few crates of the Cape's finest & PM us for delivery details. Stan.
 
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manie

Senior Member
Stan you old........ Fair Cape is 1700+ km's away (1000 miles for the "IMPERIAL" commanders). Which raises the question, is it Imperial or "Draconian" ??? One thing for sure:

If EVER I make some money with these projects, you guys MAY JUST receive something by parcel post. Import taxes and duties are off-course your own concern........
 
B

BeanieBots

Moderator
manie, you're comming along nicely. Don't let us put you off;)
However, as Dippy points out, you need to give us NUMBERs to work with... before we tell you where to find the answer:p

FWIW, a Duracel PP3 may well be ~500mAH but a typical rechargeable PP3 is only ~250mAH.
The cheapest solution is likely to be AA cells which are typically ~2.5AH these days.
Also, don't forget self discharge. Unless you use 'special' cells, they will self discharge in about 3 months even if you don't use them at all.
 
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manie

Senior Member
Dippy:
So, if I pulse it for 1mS every 5 minutes it'll fall to 7V?
Click to expand...
See my post again. You should INTERPRET quotes around a word as having some OTHER meaning, or is getting to that time where new "spectacles" are required ? In this case "pulsed" meant just what the brackets said (intermittent) = on = off = on == off for short but USEFULL periods. We were discussing a HANDHELD unit, now what would I do with a handheld unit that pulses for 1mS every 5mins ???:rolleyes:;):D
 
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Dippy

Moderator
Haha. Well, I was obviously teasing a bit, but you must understand the ambiguity of such a statement.

Intermittent - Yes, it means on-off-on-off (everyone knows that) but it does NOT define a time period or regularity or even a percentage 'duty'.
Hence my teasing comment. I was just trying to make you think.

Here we are again: "SHORT but useful"... useful for what? Sending 1 byte of data over an RF link or sending 100 bytes or what? Daft, yes, but don't you get the point?

"You should INTERPRET quotes around a word as having some OTHER meaning.."
- Yes, I probably need to change my spectacles, but I obviously definitely need to change my Crystal Ball :)

Quotes around word usually (other than a quotation) mean something-like or, a-bit-like-this or, in-effect. Everyone accepts that.
"Pulsed" means pulsed - by definition "intermittent" . Slightly tautological wouldn't you say ;) And, yes, the "often-ness" is unspecified as is the duration.

People often use quotes when there isn't quite the right word/phrase kicking about or if they can't think of the right word/phrase.

If you'd actually put in some kind of guide e.g. I'll use it every few minutes for a minute at a time , then at least it goes from total guesswork for us lot to something fairly meaningful.


Anyway, joking and kidding aside, I'm sure you'll get it sorted. As long as you know what you're doing then it'll be fine.
 
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