# Voltmeter Question



## TX_Dj (Jul 25, 2008)

Folks,

I've been looking for a voltmeter to use on my scooter project, and being the geek I am, I don't want a "Red/Yellow/Green" SOC meter, I want something with real numbers on it.

It's been a challenge to find a voltmeter with enough scale to show the full range that my pack will have between discharge and full charge WITHOUT it just being a simple SOC dummy gauge.

I can find 0-16vdc and 18-32vdc gauges by the metric truckload, but until this morning I couldn't find something that would handle the entire voltage range.

The one I found this morning handles 0-50 vdc, which is a little overkill, but considering it has real numbers and a real scale, I decided I'd find out more about it. The specs indicate the following:

* 1mA (full scale)
* 1000 Ohm/Volt
* Includes 49.9k Ohm 1% Calibration Resistor

I'm use to volt meters you just connect tot he voltage source- I'm not familiar with this kind. Can anyone tell me how the heck I should be prepared to wire it up so it has an accurate read-out?

Thanks.


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## Zemmo (Sep 13, 2007)

I think your getting too deep into extra stuff for the scooter project. I would keep things simple unless you want it for on going use after the race.


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## TX_Dj (Jul 25, 2008)

HAH! Finally someone from NTEAA is feeling the heat of the competition.  Just kidding!

The stock 24v system's only instrumentation is a 3-color dummy LED that's roughly analogous to an SOC gauge, but mounted down near the floorboard where you can't see it while riding.

Nah, I've wanted a proper voltmeter/ammeter combo since the scooter was new. Back when I was still using it partially for my downtown commute, I had to climb the "hill" under the I-35/Victory-Hiline overpass twice daily, and had to be careful not to pull too many amps coming up the hill. As any motorcyclist knows, you want to look where you're going, not at the ground, otherwise the ground is where you're going.  So, I want to have an ammeter and voltmeter mounted on the H-bars so I can see what's going on.

I think it will help me out a lot in getting things tuned just right, even if I don't end up using them in the race.

Hey- I got your note this morning about the Fiero. Glad to know it's back on the road!


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## TX_Dj (Jul 25, 2008)

Apparently the specs that I found for that meter, based on some stuff I just read, only dictate how much current it will pull from the circuit to move the meter... not that it requires a special means of connection.

Can anyone confirm that this is correct? If it is correct, what does that "calibration resistor" get used for?


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## TX_Dj (Jul 25, 2008)

I suspect the "calibration resistor" is necessary to keep the reading within the scale of the meter.

Sensitivity = 1 V / I fsd

So that means 1/.001 = 1000, or 1 mA full scale deflection = 1000 ohm/volt.

Part ordered.


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## Gavin1977 (Sep 2, 2008)

The calibration resistor is required to set the meter to the 0 - 50v range. If you didnt use any resistor, then the meter would read full scale with 1mA passing through it. In terms of voltage this would be 0.001 * 1000 = 1V.

You can calibrate the meter to use any range you want, by choosing the correct calibration resistor. If you are only using it to read battery voltage, you could, with the addition of a couple of extra components, set it up so it shows just the operating voltage range of your batteries (Rather than reading all the way to zero)


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## ClintK (Apr 27, 2008)

I have a similar Voltmeter question (don't mean to take over your thread TX_Dj!). I pulled the fuel gauge out of my car and replaced it with a 12V nominal Voltmeter (I needed to replace it and the style matched perfectly). Worst case I use it to monitor my 12V Battery/DC-DC converter, but ideally I'd like to have it read my 120V nominal pack. Is there a simple efficient way to cut the voltage of a DC current by 1/10th? (I believe if it were AC I would just need a very simple 10-1 transformer.) Thanks!


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## madmike (Jul 11, 2008)

I've seen universal LED voltmeters that go up to 200v cheaply on ebay, but I was thinking of using a cheap micro servos and a pic to make analog style gauges.


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## Gavin1977 (Sep 2, 2008)

ClintK said:


> I have a similar Voltmeter question (don't mean to take over your thread TX_Dj!). I pulled the fuel gauge out of my car and replaced it with a 12V nominal Voltmeter (I needed to replace it and the style matched perfectly). Worst case I use it to monitor my 12V Battery/DC-DC converter, but ideally I'd like to have it read my 120V nominal pack. Is there a simple efficient way to cut the voltage of a DC current by 1/10th? (I believe if it were AC I would just need a very simple 10-1 transformer.) Thanks!


There is an easy way, it will cost you two resistors, should be fairly accurate depending on the resistance of your gauge. Throw in a double throw switch as well, and you could easily switch between the two voltages if you wanted.

Have to go out, but I'll crunch some numbers for you when I get back (Unless somebody beats me to it  )


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## EVMAN (Jul 26, 2007)

Why not just buy one of those yellow Harbor Freight digital Multimeters?
U can read a range of voltages. Just set the switch to the correct range.
They are LCD, run on a 9v battery and come with probes. And, best of all,
they only cost about $4bucks! What more do you need? afn T


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## Qmavam (Aug 17, 2008)

TX_Dj said:


> Folks,
> 
> The specs indicate the following:
> 
> ...


 Hi TX,
I'll try to teach you how to figure this out, first an equation. E=I x R , where E is Voltage, I is current and R is resistance. 
Using the meter specs you gave, 1ma (FS) and 1000 Ohm/Volt we can prove the meter spec. 1v=.001 x 1000 ohms (note .001 = 1ma). So if we put 1 volt across the meter 1ma of current will flow because the meter has
1000 ohms of resistance. 
Now if you want to make the meter measure 50 volts full scale, you need to rearrange the equation to solve for the resistance. That would be R=E/I.
So if R=50/.001 then R=50,000 ohms. BUT, there is some additional resistance
in the circuit, (the meter), remember it has a resistance of 1000 ohms. So you need to subtract it from the 50,000 ohms. 50,000 - 1000 = 49,000 ohms. If you put 49,000 ohms in series with the meter it will measure FS when you apply 50 volts across resistor/meter circuit. I know your resistor
is 49.9k ohms, this will cause a very minor error in the reading. 
A series circuit is as it sounds the components are connected in series.
Where ^^^^ is the resistor and M is the meter, and * are the battery connection points, *---^^^^---M------*.
If I didn't make it clear please ask and I'll try again.
Qmavam


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## Qmavam (Aug 17, 2008)

ClintK said:


> I have a similar Voltmeter question (don't mean to take over your thread TX_Dj!). I pulled the fuel gauge out of my car and replaced it with a 12V nominal Voltmeter (I needed to replace it and the style matched perfectly). Worst case I use it to monitor my 12V Battery/DC-DC converter, but ideally I'd like to have it read my 120V nominal pack. Is there a simple efficient way to cut the voltage of a DC current by 1/10th? (I believe if it were AC I would just need a very simple 10-1 transformer.) Thanks!


 We don't have enough information to give you the exact value needed to make you 12v meter read as a 120v meter. But if you have some resistors we could get the info we need. Resistors are available at Radio Shack. 
Let's start with the meter reading 12v on a battery, now put a resistor, say 10,000 ohms in series with the meter and remeasure the battery, the meter should read lower, maybe 6.5 volts. Now, find the resistor that gives you exactly 6v. When you get the resistor that makes the meter measure 1/2 of Full Scale (6v in this case) you have found the resistance of your 12v meter. Let's assume you end up with a 12,000 ohm resistor.
Using the equation I = E/R (where I = current, E = volts and R = resistance), find the current through the meter at FS; I = 12 / 12,000 so I = .001 amps. To find the total resistance needed to make the meter read 120v at FS, rearrange the equation to R=E/I or R=120/.001 or 120,000 ohms. So you would need a total resistance 120,000 ohms but remember your meter is 12,000 ohms so you must subtract 12,000 from 120,000 to get 108,000 ohms. So you need a series resistor of 108,000 ohms. 
Meters come with different FS current values so your resistor could be 108,000 ohms or 1,080,000 ohms or 2,160,000 ohms or any value, depends on the meter. Your first trip to Radio Shack you might get a 10k a 100k and 1Meg resistors to see which one gets you closest to 1/2 FS. Then get an assortment near the value that got you closest to 1/2 FS.
If you have further questions please ask.
Qmavam


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## TX_Dj (Jul 25, 2008)

Qmavam, Gavin1977, Thanks for the extra info. This is more or less what I found (in less detail) elsewhere on the web. For some reason I had misunderstood at first that the specs given were telling me that I needed a shunt or similar that would convert an observed voltage into an amperage or resistance... but realized later it was only describing the sensitivity of the gauge, and had assumed that the resistor was what is required to make the gauge read correctly.

49.9k 1% does seem a little off, considering what you mentioned, Qmavam. I'd expect the extra 900 ohm will make it read just a few tenths low at any given time, at worst... give or take. I can live with that, and will check the output of the meter vs. my DMM to see exactly how far the deviation really is. If it's enough for me to fret about, I can just get a 49k 1% resistor from a local electronics supply house, I'm sure... but it doesn't sound like it will put it too far off from accurate for what I want to accomplish.

Thanks!


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## Qmavam (Aug 17, 2008)

TX_Dj said:


> 49.9k 1% does seem a little off, considering what you mentioned, Qmavam. I'd expect the extra 900 ohm will make it read just a few tenths low at any given time, at worst... give or take. I can just get a 49k 1% resistor from a local electronics supply house, I'm sure... but it doesn't sound like it will put it too far off from accurate for what I want to accomplish.
> Thanks!


Sorry, I 'm correcting the line in brackets below:

[Error-That 49.9K resistor will cause the meter to read 0.6 v to high at 50v and 0.3v high at 25v. Error]

Correction; That 49.9K resistor will cause the meter to read 49.1v at 50v and 24.6v at 25v.
Regarding a shunt (or parallel) resistor; a parallel resistor is used to increase the current range of a current meter. Using your 1ma, 1000ohm/volt meter,
if you put a 0.1 ohm resistor in parallel with the meter and caused 10 amps to flow through the circuit the meter would read FS, because 10amps x .1 ohm = 1volt. Your meter reads FS with 1 volt across it.
Qmavam


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## TX_Dj (Jul 25, 2008)

Makes sense. I see I had it inverted as far as whether it would read high or low, and it was more than just a few tenths. 

I will double check against my trusty DMM, just in case the specs of the meter sensitivity are off and in case the resistor is accurately sized.

At the end of the first stage charge, and throughout the second stage charge, I will see between 43.2v and 44.1v, with this shifting down to 40.5v to 41.4v during float. 0.3-0.6 deviance from accurate isn't much in this case, but I'd like it to be as accurate as possible.

Thanks again for the info, it's nice to learn new things.


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## piotrsko (Dec 9, 2007)

If you want it exactly accurate use a trimming potentiometer in series. probably 0 to 10K perhaps multi turn definately LINEAR taper not audio. dial in the resistance with a dmm. lock setting screw with nail polish paint job

side notes: temp compensation??
Radio shack parts are usually +- 10%
more digits in DVM is more precision, especially multiplied 
with a range extender


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## TX_Dj (Jul 25, 2008)

Ok, temp compensation may be where I have to draw the line. I'm not trying to make it laboratory accurate, just moderately accurate, within 1-3 tenths over the whole scale, give or take a tenth... which it'll be pretty close to anyhow.

Trim pot does seem like a decent idea, if I can find, say, a 48k 1% resistor, and a 0-5k trim pot, I could dial it until the meter and the DMM agree close enough... but I think perhaps it will be close enough anyhow... not rocket science, just want to know for certain the difference between the upper voltage of my float charge and the lower voltage of the first CC/CV stages.


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## ClintK (Apr 27, 2008)

Qmavam said:


> We don't have enough information to give you the exact value needed to make you 12v meter read as a 120v meter. But if you have some resistors we could get the info we need. Resistors are available at Radio Shack.
> Let's start with the meter reading 12v on a battery, now put a resistor, say 10,000 ohms in series with the meter and remeasure the battery, the meter should read lower, maybe 6.5 volts. Now, find the resistor that gives you exactly 6v. When you get the resistor that makes the meter measure 1/2 of Full Scale (6v in this case) you have found the resistance of your 12v meter. Let's assume you end up with a 12,000 ohm resistor.
> Using the equation I = E/R (where I = current, E = volts and R = resistance), find the current through the meter at FS; I = 12 / 12,000 so I = .001 amps. To find the total resistance needed to make the meter read 120v at FS, rearrange the equation to R=E/I or R=120/.001 or 120,000 ohms. So you would need a total resistance 120,000 ohms but remember your meter is 12,000 ohms so you must subtract 12,000 from 120,000 to get 108,000 ohms. So you need a series resistor of 108,000 ohms.
> Meters come with different FS current values so your resistor could be 108,000 ohms or 1,080,000 ohms or 2,160,000 ohms or any value, depends on the meter. Your first trip to Radio Shack you might get a 10k a 100k and 1Meg resistors to see which one gets you closest to 1/2 FS. Then get an assortment near the value that got you closest to 1/2 FS.
> ...


Very doable  I've got extra resistors lying around and will take care of it this weekend! Thanks!



Gavin1977 said:


> There is an easy way, it will cost you two resistors, should be fairly accurate depending on the resistance of your gauge. Throw in a double throw switch as well, and you could easily switch between the two voltages if you wanted.
> 
> Have to go out, but I'll crunch some numbers for you when I get back (Unless somebody beats me to it  )


You were beaten to the punch but I appreciate it nonetheless!


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## Qmavam (Aug 17, 2008)

TX_Dj said:


> Makes sense. I see I had it inverted as far as whether it would read high or low, and it was more than just a few tenths.
> 
> I will double check against my trusty DMM, just in case the specs of the meter sensitivity are off and in case the resistor is accurately sized.
> 
> ...



Sorry, I made an error in my math, I corrected it in the original post, but I'll put it here also.

That 49.9K resistor will cause the meter to read 49.12v at 50v and 24.56v at 25v.
Qmavam


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## EVplasmaman (Jul 26, 2007)

What you really want is a volt/amp meter. Thats what i put on my scooter. It tells you the voltage, current, amp hrs and watt hrs. It's a little unit from AstroFlight and as long as you are under 60 volts and 70 amps it will work nicely and for $50 its hard to go wrong. 

http://www.astroflight.com/index.php?main_page=product_info&cPath=2&products_id=17

The only thing is if you want to mount it up on your handle bars you will have to run the power wires up there. Or if you are in to hacking things you can do what I did which was run the LCD on a long ribbon cable so you can keep the "brain" down by the other power electronics. I'll do a post of it on my blog if anyone is interested.

-Jos Goble


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