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

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

Reply to
Default User
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Some multimeters cannot be powered from the source they are measuring. You may need a DC-DC converter to provide the necessary isolation.

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Joe Leikhim K4SAT
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Reply to
RFI-EMI-GUY

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
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At .3mA you'll use way less than that in a year in the shelter, unless you forget and leave the meter on.

Ed

Reply to
ehsjr

On a sunny day (Sun, 28 Dec 2008 00:14:38 -0600) it happened "Default User" wrote in :

You need an isolated supply. See my solution recently posted here.

Reply to
Jan Panteltje

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.

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Dave M
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Reply to
DaveM

freight

You

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?

Reply to
IanM

DaveM wrote:

The OP reminds me of the

*To a guy with only a hammer, everything looks like a nail* thing.

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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).

Reply to
JeffM

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.

  • Yeah, I've been reading texts of patents.
Reply to
Ben Bradley

yes, use a couple of diodes in series.

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Reply to
Jamie

On a sunny day (Sun, 28 Dec 2008 14:05:22 -0500) it happened Ben Bradley wrote in :

You mean like this? ftp://panteltje.com/pub/swr_pic_project_3.jpg

Reply to
Jan Panteltje

More bad advice from 'Mr Programmer'.

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Reply to
Michael A. Terrell

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.

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Reply to
Jamie

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.

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Reply to
Michael A. Terrell

un on a

is if

n easy

asing

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 power it. It works between 11V to 17V. See:

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Reply to
linnix

a

or

run on a

ow is if

an easy

reasing

Sorry, hacked.

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

Reply to
linnix

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

Reply to
Default User

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.

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Reply to
Jamie

a

increasing

Neither did the ones I've tried. He just can't admit when he is wrong, which is most of the time. Then he goes into another mindless rant, making himself look even worse. :(

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Reply to
Michael A. Terrell

a

increasing

Just shut the hell up. Everyone can see you for the drooling idiot you are.

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Reply to
Michael A. Terrell

On a sunny day (Sun, 28 Dec 2008 19:56:45 -0500) it happened Jamie wrote in :

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).

Reply to
Jan Panteltje

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