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Convert 12-14 vdc to 9 vdc for a device only needing 0.3ma...

D

Default User

Jan 1, 1970
0
Hi,

I've got a battery box with a 115AH deep cycle battery for power in a
tornado shelter.

I had a thought that it would be nice to mount one of the cheap harbor
freight $2 multimeters to it to monitor the voltage. They usually run on a
9 volt batter and consume a nice and small 0.3ma. What I don't know is if
12-14 volts would fry it or if it would do fine on that. Is there an easy
(read cheap) way to drop the 12-14 volts down to 9 volts without increasing
the power consumption very much?

Thanks,

Alan
 
E

ehsjr

Jan 1, 1970
0
Default said:
Hi,

I've got a battery box with a 115AH deep cycle battery for power in a
tornado shelter.

I had a thought that it would be nice to mount one of the cheap harbor
freight $2 multimeters to it to monitor the voltage. They usually run on a
9 volt batter and consume a nice and small 0.3ma. What I don't know is if
12-14 volts would fry it or if it would do fine on that. Is there an easy
(read cheap) way to drop the 12-14 volts down to 9 volts without increasing
the power consumption very much?

Thanks,

Alan

Yes, but. (Actually, 2 "buts")
The first "but": Those meters might not be able to monitor the
same source that provides power to the meter, so you might need
a dc-dc converter with an isolated 9V output.

The second "but": You don't need to worry about minimizing
power consumption. At .3mA those meters use very little power, so
even using a 7809 voltage regulator won't be a significant drain
on a 115 AH battery. If you don't have (and don't want to buy)
a 7809, you can use 5 1N400x diodes in series to drop the voltage.
That will work, if the meter is capable of measuring its own source.
You can test that with a 9V battery installed, measuring the 9V
battery.

Finally, why do you care? Just mount the meter and use it
when you want to. It will last for a loooong time in tornado
shelter use - probably close to shelf life. Buy a new 9V
battery once a year and put it in the shelter if you want a
little insurance. The shelf life is easily far in excess of
1 year (Duracell says 7 years) and a 9V battery should be able
to provide .3 mA for hundreds of hours. Duracell shows
250 hours at 2 mA draw. Typical capacity is 500 mAh per
http://www.techlib.com/reference/batteries.html
At .3mA you'll use way less than that in a year in the
shelter, unless you forget and leave the meter on.

Ed
 
I

IanM

Jan 1, 1970
0
DaveM said:
I recommend that you opt for an analog voltmeter instead of a digital unit. You
can buy a 0-15 volt meter, or fashion a voltmeter from a 0-1ma or less analog
meter. Requires no outside power source and has negligible effect on your
battery.

Well for lead acid battery monitoring applications, you never need to
measure below 10V (battery DEAD or nearly so) and you need to be able to
read up to just over 16V for equalising. The nearest available analog
meter scale would be 0-20 probably graduated in 2V intervals, each
sub-divided in five 0.4V increments. This is *USELESS* for monitoring a
lead acid battery in normal use. Other likely scales are worse. If one
adds a circuit to turn it into a suppressed zero voltmeter and one makes
a custom scale 10-16 graduated in volts each subdivided into 0.1V
intervals, it will be as usable as a cheap digital meter.

The simplest method of meeting the OP's requirements would be to take
the cheap digital meter, keep its existing battery, jumper out the
range switch and add a 'push to test' SPDT switch that both connects the
meter's own battery and the 12V battery to be tested. Simple, cheap,
fairly accurate and easy to use/read. What more could you ask?
 
J

JeffM

Jan 1, 1970
0
Default User said:
[...]115AH deep cycle battery
[...]one of the cheap harbor freight $2 multimeters[...]
to monitor the voltage.[...]
Alan
DaveM said:
I recommend that you opt for an analog voltmeter

The OP reminds me of the
*To a guy with only a hammer, everything looks like a nail* thing.

http://www.google.com/search?q=site...cs.com+panel-meter+-LCD+-LED&filter=0&num=100
A text search there for uA or 1mA will give likely candidates.
An 8.2V zener and a calibration resistor in series
will give a useable range of ~8V - 15V.
(The meter movement's scale will have to be re-labeled).
 
B

Ben Bradley

Jan 1, 1970
0
Hi,

I've got a battery box with a 115AH deep cycle battery for power in a
tornado shelter.

I had a thought that it would be nice to mount one of the cheap harbor
freight $2 multimeters to it to monitor the voltage. They usually run on a
9 volt batter and consume a nice and small 0.3ma. What I don't know is if
12-14 volts would fry it or if it would do fine on that. Is there an easy
(read cheap) way to drop the 12-14 volts down to 9 volts without increasing
the power consumption very much?

Others have adequately answered "How do I run a 9V multimeter off a
12V 115AH deep cycle battery and measure the voltage of said*
battery?"

But it appears what you really want to ask is "How do I measure the
charge left in a discharging 12V 115AH deep cycle battery?" This
requires monitoring the current in addition to the voltage, and
integrating (perhaps even weighting a higher current draw as more than
a simple linear increase, as a higher current results in a lower AH
rating) with respect to time. I'd think there would be such devices on
the market, even calculting the time left on the charge at the current
rate of discharge, but I haven't seen such. Perhaps researching it
would be fruitful.

Or you could program a microcontroller to do this very thing. Most
manufacturers have "starter kits" for $100 or less. It's easy enough
to get current draw down to 0.3 mA with most microcontrollers
available today (especially doing this, the processor would run to
measure voltage and current, do some calculations, maybe update a
display, running for a cuple of mS then dropping back to wait or stop
mode for 50mS, reducing the average current draw proportionately. Or
just run it full out and pull a couple mA from the huge battery -
that's still probably a lot less than the equivalent self-discharge
current on the thing.
Thanks,

Alan

* Yeah, I've been reading texts of patents.
 
J

Jamie

Jan 1, 1970
0
Default said:
Hi,

I've got a battery box with a 115AH deep cycle battery for power in a
tornado shelter.

I had a thought that it would be nice to mount one of the cheap harbor
freight $2 multimeters to it to monitor the voltage. They usually run on a
9 volt batter and consume a nice and small 0.3ma. What I don't know is if
12-14 volts would fry it or if it would do fine on that. Is there an easy
(read cheap) way to drop the 12-14 volts down to 9 volts without increasing
the power consumption very much?

Thanks,

Alan
yes, use a couple of diodes in series.


http://webpages.charter.net/jamie_5"
 
J

Jamie

Jan 1, 1970
0
Michael said:
More bad advice from 'Mr Programmer'.
Screw you.
I just happen to have 2 of those cheap meters the poster is
referring to, that was given to me in a grab bag. I gutted one of them
to use
as a display in a junk project, the unit can read it's own source just
fine if that is what every one is so worried about.

If you want to talk about some one giving bad advice, take a look
in the mirror assole.

Unless they have changed the design of those 3..5 dollar DMM's
at the freight, it'll work just fine..

So go screw your self Terrell until you have something
worth while to say, with out vile and excrement rolling out your pie hole.

I don't give a shit how disabled you are and how that makes you feel
that it give's you the ok to be a royal asshole with just about every
one you intersect with.

There are a lot of disabled people out there on their last leg, you
aren't anything special.

fringing ignorant old basted, thinks the world owes him something.


http://webpages.charter.net/jamie_5"
 
L

linnix

Jan 1, 1970
0
The proper question is whether an isolated power source is necessary.

Couple of resistors will do.

Thank you for your opinion.
  Screw you.

No need to get emotional.
    I just happen to have 2 of those cheap meters the poster is
referring to, that was given to me in a grab bag. I gutted one of them
to use
as a display in a junk project, the unit can read it's own source just
fine if that is what every one is so worried about.

Mine can't. I guess some works and some don't.

I just hatched a DC-DC converter and couple of 100 ohms resistors to
power it. It works between 11V to 17V.
See: http://linnix.com/meter.html
 
L

linnix

Jan 1, 1970
0
The proper question is whether an isolated power source is necessary.




Couple of resistors will do.




Thank you for your opinion.




No need to get emotional.


Mine can't.  I guess some works and some don't.

I just hatched a DC-DC converter and couple of 100 ohms resistors to

Sorry, hacked.
power it.  It works between 11V to 17V.
See:  http://linnix.com/meter.html

Pay no attention to the date on the pictures. They were taken today.
 
D

Default User

Jan 1, 1970
0
Hi Everyone,

Thanks for the ideas on the voltage regulator or diodes. I will check to
see if it can read its own source or not.

I wasn't looking for any sort of battery analysis, just the voltage and the
reason I don't want to leave it on 9V is that I'd like to just leave the
meter on all the time which would be no big deal to a 115AH battery.

Thanks,

Alan
 
J

Jamie

Jan 1, 1970
0
Michael said:
I think the world owes you an attitude adjustment. Not all cheap DMM
can measure their supply voltage, even though you continue to foam at
the mouth while posting your illiterate crap with little or no
punctuation or proper capitalization. It looks like you should be using
crayons while the attendant wipes up your drool.
Do you now have comprehension problems? Was my post that unclear to you?

You must be among those ignorant saps that represents the greater
whole of the moron society this country is quickly getting polluted
with.

Go dig a hole while you still can. Cause when its time for you to
go, I doubt many will notice.


http://webpages.charter.net/jamie_5"
 
L

linnix

Jan 1, 1970
0
Just to rain a bit on your parade,
I opened mine, and tried to measure its own 9V battery.
With the + lead on the battery + it says 6.3V or so.
the minus lead on the battery minus causes
the display to show over range at the20 V..
So, no, it cannot measure its own supply, unless you think 6,2V is 9V and use only one lead.
here is a picture of the inside of mine:
ftp://panteltje.com/pub/cheap_meter_inside.jpg
Note the the beeper, very nice for cable testing.
My older one did not have the beeper, and also cannot measure its own
battery (as any 7106 based chip, still have a 40 pin DIL 7106CPL in
the box somewhere, I think that is what is used in those meters,
or a variant of it).

Yes, my fuke meter is SW7106A (QFP 40), but the Harbor Freight chip is
42 pins. The chip itself can do single ended input, but not wired for
so in the meter. Adding isolated DC/DC converter works, but the
converter needs 20mA to run. I will try to rewire the board without
the converter. For now, I can test drive it (in my car) with the
converter. See:

http://linnix.com/meter.html
 
E

ehsjr

Jan 1, 1970
0
linnix said:
Yes, my fuke meter is SW7106A (QFP 40), but the Harbor Freight chip is
42 pins. The chip itself can do single ended input, but not wired for
so in the meter. Adding isolated DC/DC converter works, but the
converter needs 20mA to run. I will try to rewire the board without
the converter. For now, I can test drive it (in my car) with the
converter. See:

http://linnix.com/meter.html

Do you have a schematic of your converter? Wondering why
it takes 20 mA to run. Jan has a circuit (haven't tried
it, I have my own) that may take less - mine provides
two isolated 9V outputs at up to 10 mA each, so it
draws a lot. There was also one I stumbled on where the
guy uses a cmos 555 that provides 1 mA so it may be low draw.
I'll post a link to it if I find it again.

And - I tried one of my Harbor Freight cheap meters and
it won't measure its own source, either. If you mod
yours successfully, please post.

Ed
 
E

ehsjr

Jan 1, 1970
0
linnix said:
Yes, my fuke meter is SW7106A (QFP 40), but the Harbor Freight chip is
42 pins. The chip itself can do single ended input, but not wired for
so in the meter. Adding isolated DC/DC converter works, but the
converter needs 20mA to run. I will try to rewire the board without
the converter. For now, I can test drive it (in my car) with the
converter. See:

http://linnix.com/meter.html

Here's the link I mentioned in the other reply:
http://www.discovercircuits.com/DJ-Circuits/9v-isolated supply.htm

Ed
 
J

Jamie

Jan 1, 1970
0
Jan said:
Just to rain a bit on your parade,
I opened mine, and tried to measure its own 9V battery.
With the + lead on the battery + it says 6.3V or so.
the minus lead on the battery minus causes
the display to show over range at the20 V..
So, no, it cannot measure its own supply, unless you think 6,2V is 9V and use only one lead.
here is a picture of the inside of mine:
ftp://panteltje.com/pub/cheap_meter_inside.jpg
Note the the beeper, very nice for cable testing.
My older one did not have the beeper, and also cannot measure its own
battery (as any 7106 based chip, still have a 40 pin DIL 7106CPL in
the box somewhere, I think that is what is used in those meters,
or a variant of it).
Sorry, the design must of changed. I got mine about 5 years ago.
M-830B. I still have 3 more of them lying around here. I got my hands
on a box of them at a grab and give away as part of an event at a meeting.

I looked at your picture, mine does not look like yours.
Mine do not have continuity beepers and the fuse is just above the
battery and the board goes all the way to the top of the meter covering
the LCD..

But it looks the same on every where else how ever except for the
color..
Oh well.


http://webpages.charter.net/jamie_5"
 
M

Mark Zenier

Jan 1, 1970
0
You need an isolated supply.
See my solution recently posted here.

If it's a 7106 or 7126 type chip, I think that you can use a 3 resistor
divider to measure it's own voltage. But you need to go in and isolate
the COMMON virtual ground terminal from the IN-LO terminal.

Theoretically (I never got around to trying this), if your circuit can
isolate the IN-LO input (pin 30 on the dip version) from COMMON (pin
32), and you toss the existing voltage dividers and run a resistor
chain from the full V+, one between IN-HI and IN-LOW, and one from
IN-LO to V-/ground, both IN-HI and IN-LO stay in the common mode range.
(Doing this to Chip-on-board construction like the Harbor Freight meters
may be a challenge).

The ratio of R(IN-HI,IN-LO) to the sum of the other two resistors is
either 1:99 or 1:9 depending on if the circuit is set up with a 100
millivolt or 1 volt reference.

At that low a current, I'd use a zener/resistor for the supply.

meter battery V+
6?-20 V |
-------+----/\/\/\--+---|<--------gnd
| 3.9 kohm 8 V zener
|
| 7 Mohm 1 Mohm 2 Mohm
+-----/\/\/\----+----/\/\/\----+----/\/\/\-----gnd
| |
| |
Chip IN-HI Chip IN-LO

Beware, if you try to measure one of these multimeter's own battery,
hooking the negative probe to the + battery terminal might burn out the
chip (depending on how much resistance is between the negative/ground
probe and the chip's COMMON input). The COMMON input is a shunt regulator
that sets a virtual ground to 2.8 volts (nominal) below the V+.

Mark Zenier [email protected]
Googleproofaddress(account:mzenier provider:eskimo domain:com)
 
T

Tom Bruhns

Jan 1, 1970
0
Hi Everyone,

Thanks for the ideas on the voltage regulator or diodes.  I will check to
see if it can read its own source or not.

I wasn't looking for any sort of battery analysis, just the voltage and the
reason I don't want to leave it on 9V is that I'd like to just leave the
meter on all the time which would be no big deal to a 115AH battery.

Thanks,

Alan

A simple way to do this that takes less power and is very easy to hook
up is to just put an analog moving-coil meter across the battery. You
don't need a whole lot of accuracy, since the battery voltage will be
a function of temperature as well as state of charge anyway. You can
easily expand the scale of a meter by putting a zener diode in
series: get a reasonably tight tolerance 10 volt zener, a 100uA
meter, and 50,000 ohms of net resistance (including the meter itself),
and you have a meter that reads from 10V to 15V, and you should be
able to visually resolve to less than 50 millivolts. You can use the
meter's mechanical zero to calibrate one voltage accurately, and if
your resistance is adjustable, you can calibrate the slope too.
Typical current would be 40 to 60 microamps. Doing the same with a
0-500uA or 0-50uA meter would make it even easier to read without any
re-writing of the scale. If you really think you need finer
resolution, you could give it a 2V range from 12V to 14V, or
whatever. I'm aware that analog meters aren't as easy to find as they
used to be, but they're still available.

Cheers,
Tom
 
L

linnix

Jan 1, 1970
0
That's what I had in mind, but I haven't got too far yet.
Yes, that solution you describe requires 5 extra components.
The little DC / DC converter I use has 9 components,

I use 1 DC/DC converter and 2 resistors. Should add a zener across
the converter input as well.
The problem is the 20mA current to run the converter, for on-line
monitoring.
I have been using it in my car. It reads 14V with engine on and 12V
(plus low battery indicator) off.
My car battery dies after a few days off anyway.
A typical example, have this setup here with _one_ PIC to drive a LCD,

I also have an AVR driving a LCD and reading the battery. It draws
3mA and requires no isolation.
But the $2 AVR is much more expense than the 20 cents 7106.
 
L

linnix

Jan 1, 1970
0
20 mA from a 40 Ah car battery, that takes 50 x 40 = 2000 hours to discharge,
or 83.3 days.
Would only be a problem if you drive once per month, but it can be behindthe key no?



Yes the PIC (16F690) is also about 2$, 1$62 in a different package:
 http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en023112

Turtle wax is more expensive :).
or are you planning large quantities?

If I have my way, it will be in every car battery and dash board.
Also, there will be audio input jack and usb power outlet in every
junk box (that come with all car) they called the radio.
Auto makers are decades behind the curve.
 
R

Rich Grise

Jan 1, 1970
0
I carefully checked if the meter was still reading correctly after that
battery test. Did not connect the + lead to the battery -, so was lucky. I
have one problem with these meters, the range selection switch looks the
same from where the arrow is pointing and 180 degrees opposite. I marked
the old yellow one I have with black, but as this one is black I will have
to get some special white[?] paint to mark that switch.

http://www.google.com/products?q=wite-out ;-)

Cheers!
Rich
 
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