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resister size question


By TheCasualTraveler, Section Controls
Posted on Wed Sep 10th, 2008 at 02:09:34 PM MST
a little help with my ciphering

     Like many folks here I use those cheap Harbor Freight digital multimeters. I have found them to be fairly accurate when compared to each other and to my other analog meter. So far I have ruined 3 of them by trying to measure volts with the leads still connected to measure current or other stupid mistakes. At $2 a pop it's no big deal but I hate waste and so want to hardwire them into my bank where I don't have to mess with them. Also, changing the 9 volt battery costs more than the meter.

     The question: Will a 4700 ohm resister be the best choice to run this 9 volt meter off my battery bank which varies in voltage from 12.4 to 14.8 volts? I did a quick test and with my bank sitting at about 13 volts and using a 4700 ohm resister it showed the voltage across the meter at 9.6 volts with the meter turned on and working ok.

Thanks, Andy
resister size question | 14 comments (14 topical, 0 editorial)

Re: resister size question (3.00 / 0) (#1)
by Norm (peppysue@suite224.net) on Wed Sep 10th, 2008 at 08:35:28 AM MST
(User Info)

Don't know about resistors....It's easier for
me to think: a small battery pack of 8 - AA Nicads
and taped to the back of the multi-meter....a
little home-brew solar charger scavenged from
some lawn solar lights charges up the other
multi-meters could be likewise set up....
(I already have similar setups for Cordless drills, small mini
-grinders sabre saws,etc.)
    Fun on a 'fixed income' budget!

( :>) Norm


Re: resister size question (3.00 / 0) (#2)
by jaskiainen (jaskiainen74inhottestmailcompany) on Wed Sep 10th, 2008 at 08:45:28 AM MST
(User Info)

Hi Andy!

That 4.7k resistor seems ok to me.
I think that digital multimeters are not that accurate for the input voltage.
If you try with old battery and start to get some strange measurements and
after that measure your 9V battery voltage it propably be nearer 6-7 volts.

Any voltage between 8-10 for your multimeter should be just fine...
Best regards jaskiainen



Re: resister size question (3.00 / 0) (#4)
by TheCasualTraveler (a.miklos@yahoo.com) on Wed Sep 10th, 2008 at 08:53:39 AM MST
(User Info) http://thecasualtraveler.com/wind.htm

Thanks,
     Your right, when the battery gets low on these meters they really act squirlly, mostly with the numbers on the display going up and down flashing high and low and because I always forget to turn them off I get a lot of low batteries.
Andy
[ Parent ]


Re: resister size question (3.00 / 0) (#10)
by Norm (peppysue@suite224.net) on Wed Sep 10th, 2008 at 09:28:33 AM MST
(User Info)

Too bad they don't have an automatic shut-off
like even the cheapest $1 calculators, besides
some of the $1 calculators are even dual (batt.
and solar) powered....I imagine it would even be
possible to have a cheap multi-meter powered with
a button battery like the $1 calculators!
( :>) Norm
[ Parent ]


Re: resister size question (3.00 / 0) (#3)
by ghurd on Wed Sep 10th, 2008 at 08:50:45 AM MST
(User Info)

Don't do it.
The meter will pop.
Takes some fancy stuff to run a meter from the battery powering it.
http://www.fieldlines.com/story/2008/5/3/1306/72467
G-


Re: resister size question (3.00 / 0) (#5)
by ghurd on Wed Sep 10th, 2008 at 08:53:40 AM MST
(User Info)

Lemme try that again.
Takes some fancy stuff to measure the battery that powers the meter measuing the battery.
Easiest to use 2 batteries.
G-


[ Parent ]


Re: resister size question (3.00 / 0) (#8)
by TheCasualTraveler (a.miklos@yahoo.com) on Wed Sep 10th, 2008 at 08:59:16 AM MST
(User Info) http://thecasualtraveler.com/wind.htm

Thanks for the link to that thread Ghurd. I missed it in May and didn't find it in a search.
Andy
[ Parent ]


Re: resister size question (3.00 / 0) (#9)
by Norm (peppysue@suite224.net) on Wed Sep 10th, 2008 at 09:09:29 AM MST
(User Info)

Like my comment above...but you would really
need a 7809 voltage regulator to go along with
it?

((At the present time for my needs I merely use
2 for a $1 9v batteries )(even cheap batteries
there are better and really crappy and cheap !))
Lucky so far one brand that I get last about a
week when left on all the time....after that start to get unreliable readings
( :>) Norm
[ Parent ]



Re: resister size question (3.00 / 0) (#11)
by ghurd on Wed Sep 10th, 2008 at 09:35:55 AM MST
(User Info)

I think the $3 yellow HFs read OK with a
battery between 7 and 11V.
Not sure about the red ones.

A 7809 isn't fancy.
It worked for resistors and
batteries other than the one powering the 7809.

It could not read the voltage of
the battery powering the 7809.
Took about a half a nano-second to
know that was a very bad idea!  LOL

[ Parent ]



Re: resister size question (3.00 / 0) (#6)
by electronbaby (roy<at>windsine.org) on Wed Sep 10th, 2008 at 08:55:39 AM MST
(User Info) http://www.windsine.org

I dont think it will last too long if its a cheap meter, and I dont think it will be very accurate while reading the voltage from the same point it is being powered. Usually there needs to be separation between the power supply and the voltage or current being read (with cheapo meters).

Have Fun!! RoyR KB2UHF


Re: resister size question (3.00 / 0) (#7)
by richhagen (richhagen (a t) Juno.com) on Wed Sep 10th, 2008 at 08:58:34 AM MST
(User Info)

V = I*R  where V is the voltage drop across the resistor, I is the current drawn by your meter, and R is the value in Ohms of the resistor.

It depends upon how much current your multimeter draws over the voltage range it will see.  If I were determined to use just a dropping resistor, I would find out how low of a voltage it can take and still function - since I don't want to failure test the high voltage level - and size the resistor, calculate the voltage across the meter at the highest voltage if it were to draw the same current and see if the value was reasonably within 10% or so of the meters voltage.  The problem would be that if conditions existed where the meter drew almost no current, there would be almost no voltage reduction across the dropping resistor.  

You might pick up a 3 pin 7809 voltage regulator from Radio Shack, E-bay, or some other place, and have better results for your meter.  The standard 7809 should handle the low current draw of the meter up to the maximum voltage your 12V. system would see with no problems, and in a T-220 package, likely without additional heat sinking, but read the specifications.  You can also add a switch so that it is only powered when you need it to reduce the mA's of draw when the meter is not in use.  Rich
'A Joule saved is a Joule made'



Re: resister size question (3.00 / 0) (#12)
by Flux on Wed Sep 10th, 2008 at 10:39:02 AM MST
(User Info)

As the others have said, you can power it from a 12v battery as long as you intend to use it to measure things isolated from that battery. If you attempt any connection to the powering battery you will be in trouble.

A series dropper resistor may work if the meter current doesn't change with reading or load. otherwise a resistor /Zener arrangement or a true regulator will be needed. It may actually survive 12v but don't take my word for it.

Flux

[ Parent ]



Re: resister size question (3.00 / 0) (#13)
by FishbonzWV on Thu Sep 11th, 2008 at 06:26:17 AM MST
(User Info)

Andy,

I use one of the yellow HF meters at the farm for monitoring the bank.
What I did was put a momentary push button on the face of the meter and wired it into the lead of the battery. When I need to check the voltage, just push the button and the meter turns on. This way I can leave the selector switch on the 20v level.
The battery has been in for 2 years now.
The newer red HF meters have an on/off switch right where I put mine on the yellow one.
I would post a pic but the meter is at the farm. Sorry
I have some extra switches if you need one.

Fishbonz



Re: resister size question (3.00 / 0) (#14)
by veewee77 on Thu Sep 11th, 2008 at 02:33:47 PM MST
(User Info)

Don't attempt to measure the voltage powering the meter with the meter being powered by the source being measured by the meter. . .kinda goes in a loop, don't it. . .

Anyway, the meter will fry.

Doug



resister size question | 14 comments (14 topical, 0 editorial)
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