Voltmeter Fuse

[valterra]

I would like to run some small gauge wire (eg CAT-5 or similar) from my batteries to an analogue voltmeter way on the other side of the house.

I think of a volt meter as a short circuit - albeit with a huge resitor in the middle to prevent current flow.  However, one failure, like the two wires in back of the meter briefly touching... and that small wire (or ANY wire) isn't  going to handle that very well.

So, I have a big pack of fuses, but the lowest rated ones are 5A.  That seems awefully high for this particular application.  I am wondering if a very light fuse (measured in mA) would be better, or is that being too paranoid?  

Silly question, but can a fuse be TOO small for this?  Technically there is no current (?) while measuring voltage.  And 1 inch of tiny wire cannot contribute much to voltage drop (thereby making the measurement inaccurate), can it?

[Flux]

An analogue volt meter could take anything from 50 microamps to perhaps 10 mA depending on the basic movement.

Any fuse that will protect your cable in event of accident is not a bad idea but it has to be rated low enough to blow to be any use. If you have thin wire and a long run of, it will have significant resistance and if the voltage is low particularly 12v you may not blow a 5A fuse. Something nearer 100mA would be better, if your wire is too high a resistance to blow that then the heat generated probably wouldn't cause a fire anyway.

There are very special fuses rated below this ( probably down to 20 mA) but these do have a lot of resistance. If you have a low grade analogue voltmeter that takes over 5mA then even for a voltmeter, on a long cable run with these low current fuses you may have enough line resistance to cause measurement error.

Flux

[zap]

Unless it's an out house, the voltage drop will be a big one running from one side of the house to the other but with an analogue meter you can adjust the needle to compensate for the voltage drop.

Using a fuse anytime you run power through a wire is never a bad idea unless the fuse hasn't been sized correctly.  The Cat5 wire would probably never carry enough current to cause too much damage but odd things can and do happen... such as the wire getting smashed and shorted 2 or 3 feet from the battery.  5amp would be too high... something around 250 or 150ma would be better.

I can't recall the smallest fuse I've seen but I'm sure there are fuses out there that would be too small for this project.

Analogue meters do use current.  Voltage doesn't do any work, it's the amps that do the work of moving the meters needle.

One inch of copper wire wouldn't add much voltage drop but a 30 foot run will.  Are you refering to the fuse when you asked about 1 inch of tiny wire?

[tanner0441]

Hi

If he has a very long cable run the resistance of the cable will affect the reading anyway.  Whats wrong with a A/D converter at the source and then a D/A converter the other end, or even a frequency counter with the A/D tweaked to suit. No volts drop to worry about and no heavy current running up the cables.

Brian

[valterra]

Thanks for the reply.  Yes, the 1 inch remark was about the fuse.

I am telling myself "duh" about current.  If the needle is moving, then SOME work is being done!  So there is SOME current!

As for how much the cable loss will affect the reading, I can certainly try it by taking readings on the cable before I even run it.  

[Flux]

If you start with a decent analogue voltmeter then the cable resistance will not be a problem. Even a 1mA movement has a resistance of 12k ohms for 12v. A few tens of ohms in the cable will have little effect.

A very poor quality voltmeter taking perhaps 10mA may cause some error. If you are prepared to dive inside you can reduce the internal series resistance to compensate. You could even get straight to the movement and add your own scaling resistor outside and compensate for the line drop. Better still get a decent meter or a 1mA movement and scale it yourself.

An analogue meter is more of a rough guide anyway so absolute accuracy is not essential. It gives you indications of trouble but is not accurate enough to do much in the way of deducing state of charge and certainly not accurate enough to set charge controllers.

Flux

[valterra]

Thanks Flux.  This is something I don't quite get.  

If I take a short piece of small wire, say #22, and short circuit the battery, then I will get lots of melted insulation burned into my fingers.  I did that once as a kid, so I know!  

But a LONG piece of small wire may have so much resistance that the wire actually only gets warm, but doesn't melt?  Like a  low wattage heating element?  And the long cable run might have so much resistance that less than 5A might flow through and may not blow a fuse?

Put another way, a long wire length that has about 2 ohms of resistance will allow about 6A of current to flow, which means it will generate about as much heat as a 75W light bulb?  That would need a fuse!

But a LOOONG length with 6 ohms resistance would only allow 2A to flow, generating 24 watts of heat which would be much cooler and possibly not a fire hazard, and definitely would not blow a 5A fuse!

Right?  I have always marvelled at this.  After all, light bulbs (and all other heaters) are simply a resistor in what my tiny brain still thinks of as a "short" circuit.

Now I am babbling (hey, it is MY diary! ) but I am remembering that another guy on here made a huge dump load resistor by winding small wire around a large wooden frame.  That concept relates to what I was trying to figure out.

[valterra]

I will measure the meter's resistance.  It has two of those push-on connectors on the back.  To be clear, I am worried about a short circuit... those two connectors touching, or as someone said, wire getting smashed or something.

You are right about accuracy.  I basically use them to measure the voltage directly from the wind generator, so I know if it is spinning at all.  And if it is, if it's generating a voltage higher than the bank voltage.  Nothing too exact.

Here is why I prefer analog for this purpose.  The wind speed goes up and down all the time.  With my DMM, the constantly changing display might as well be showing random numbers.

With an analog mteer, you can "see" the genny spin up and down.  When I  want an exact reading, I use a DMM.

I had to be realistic... do I really need to measure my current or genny voltage to two decimal places ALL the time?    

[TomW]

I use cat5 to run sense wires for turbine voltage and shunt monitoring from my garage / office across 100 feet of driveway plus another 20 plus feet of internal cat5 into the house. I fused the voltage sense wires with .250 amp glass fuse. The shunt leads I do not fuse.

I used doubled conductors for both sets of sense wires.

The battery voltage is 24 volts nominal the shunt voltage is possibly up to 50 millivolts. I see no drop this way as far as the analog meters can reflect, anyway. Another point where theory does not actually reflect real life.

I use good quality panel meters so this may not apply to the cheap stuff, either.

This is real world. Not theory. There comes a point where you begin to over think the plumbing. Avoid it if possible.

Just what I do here.

Tom

[valterra]

that sounds great!  I wish I could hook it up to my network and log and graph the results.  

but I think all that stuff gets a little pricey, right?

I would love the high tech stuff, especially so I could monitor it over the net.  

But part of me wants something simple and elegant that I can look at without turning it on, logging in, changing batteries, etc.

[valterra]

I agree with you Tom... too much ponderance, ay?

I posted some time ago that I was trying to find voltage loss.

My solar panels measured the same (measured to two decimal places) when measured on the panels directly and also at the end of 50 feet of #18 extension cord.

I read somewhere that using this #18 would drop so much voltage that I wouldn't be able my 12v stuff in the next room (25 feet or so) because of the drop.

that didn't turn out to be true.... not at all in fact.  

[TomW]

valtera;

I read somewhere that using this #18 would drop so much voltage that I wouldn't be able my 12v stuff in the next room (25 feet or so) because of the drop.

Yes, this is true for the high impedance [resistance] load of a meter. It would not hold true under a true load. The voltage would drop if loaded [low resistance loaded].

Like you, I don't really care about exacting precision, I just want to see an indication of whether the mill is running or not. According to my webcam tests the value I get are within the ability of the remote meters abilities compared to the "Doc Wattsons" I have installed on the turbines and solar. Even the 2 Doc Wattsons disagree with each other by 1/10th of a volt and none agree with the Outback Mate on the battery voltage. All of them seem well within reason regardless. By the way the Doc Wattsons and Mate are digital readout devices.

Just my experience with it.

Tom

[TomW]

Arghhh...

Where I said "1/10th of a volt" above I meant "1/100th" [.01].

I realized it after I hit post.

Tom

[valterra]

Exactly!  I want to see if it is turning, even if it is only creating 8 volts.

By the way, the meter I'm talking about is the cheap-o Harbor Freight model.  I have 3 of them.  Eventually they will show Genny 1, Bus Voltage, Genny 2.

Any layman, like my friends, parents and spouse, can understand that when the Genny needle goes higher than the Bus meter, then we're charging batteries.   :-)

Besides, I think it's more fun to watch the needles go up and down than it is to watch numbers on a screen.  "The cutting edge of low technology."  Where have I heard THAT before?

[valterra]

This is the harbor freight meter that I am talking about.  The word "gauge" would be more accurate.

[Flux]

That is almost certainly a moving iron or moving magnet movement and will have quite low resistance. It may be affected badly by the line resistance. Still probably good enough to tell you what you want and will tell you the mill is running but the actual reading may be way off the mark with long leads.

Flux

[valterra]

Thanks, Flux.  I will try it before running the cable and post the results.

[FuddyDuddy]

Flux is right,

"Any fuse that will protect your cable in event of accident is not a bad idea."

Where I worked before I retired we used 50 and 100 ma fuses all the time for our 4-20madc circuits. These were standard AGC glass fuses. They not only protected the wire, but they protected the circuits themselves.

A 4-20madc loop in an industrial system will work into a load anywhere from 250 ohms to 600 ohms. It that case as long as the loop is short enough, or within that range, it always works and has good accuracy.

I once did a "loop" that ran from Round Butte Dam to the Pelton Rereglating Dam. I ended up using a 1 amp fure and a 96 vdc power supply. The loop was 9 miles one way !!

That was using #9 AWG wire on the poles.

A standard 4-20madc loop uees a 24vdc supply.

I guess my question is:

How far are you going with your wires? Most GOOD analog meters have an accuracy of 2%. It sounds like the ones you have might be well below that ?

If you double up the wires on both sides, you might be okay, at least enough to get an indication as to what the genny is doing.

I too, like analog meters. As you pointed out, the digital gives a "Blur" of numbers..

Those small fuese are readily available. Try DigiKey....

[framistan]

You dont need a fuse... just put a RESISTOR in the line(in series), near the

battery.  This will only work because meters dont use much AMPS... they use

milliamps or microamps...  A digital voltmeter would work best because they

only place microamps load to the voltage being measured.  If you use a 1k ohm

or 10 k ohm resistor in SERIES near the battery i dont think you will get a

very different reading... also if the wire SHORTS out for some reason, then

all that can flow is a few milliamps.  Thats why the resistor must be near the

batteries.  other wise if you place the 10k resistor near the METER, you will

have the possibility of shorted wire causing many amps to flow... hot wires and

fires!  

IF you were going to try to run a LOAD on that wire, then you need a fuse and

good connections and thicker wires. Go to Harbor Freight Tools and get one of

their 3 dollar digital meters.  They are pretty good for dedicated uses.  

As a side note, I call this kind of voltage a GHOST VOLTAGE or FAKE voltage.  

thats because you will read FULL VOLTAGE on your meter until you put any kind

of LOAD on it.  Thats why your car battery measures full 13 volts when the

terminals are corroded.  The corroded terminal acts as a "resistor" that

passes a few MICROAMPS or MILLIAMPS to your meter.... showing OK volts. but

when you turn on your HEADLIGHTS or anything, then voltage goes to almost

ZERO.  

You mentioned you "think of your meter as a short circuit"  Thats not right.

You should think of it as a "tiny load"  of only ten thousand ohms (analog)

or one million ohms (digital).  Hope that helps.... framistan

[dnix71]

AGC fuses are 32v dc/ 250v ac dual-purpose.

http://www.galco.com/scripts/cgiip.exe/wa/wcat/series.htm?mfg=BUSS&series=AGC&line=CKTPRO&am
p;cat=FUSES&sub=MINI

There are fuses here specifically for your setup.

If you use wire long distance, through walls or ductwork, the code and common sense requires "plenum rated" wire.

http://www.firefold.com/Categories/Cat-5E-Products/Cat-5E-Bulk-Cable/Cat5E-Solid--Plenum.aspx?gclid=
CPPOpNnu25UCFSCcnAodjlMhXg

[dnix71]

On the first linked page there is a page 2 and 3 for even smaller fuses. (bottom right of the window list)

[wooferhound]

From what I understand, you can't measure the voltage that the genny is putting out if it is connected to a battery. The battery holds the voltage down no matter where you are trying to measure the voltage, even if you try to measure the AC at the outputs of the generator coils themselfs. I even think that the diodes will have enough reverse leakage that you will still be reading the battery voltage on the generator side of the rectifiers when the genny is not spinning.

[valterra]

I have read that, too, lots of times.  In reeal life, from what I have seen, the voltage IS held down, but not nearly like you would think.  

Possibly that is due to the fact that my batteries are charged.  They will be dumping (14.45v) and the measured voltage from the meter pegs the 16v analog volt meter.  

Even when my batteries are "less" fully charged, like 12.6-13.8v range, that meter still gets pegged some times.  

On my solar panels, they measure in the 20s when not connected to the batteries, and around 16(?) volts when hooked up.  So there is a real example of voltage being held down in action.

[wooferhound]

OK here is how I did my remote meter . . .

The meter I used is a VU meter removed from an old cassette deck. While it was connected to my 12 volt battery, I tried different Hi-Ohm resistors on it until the meter movement went 3/4 of the way to 'pegged out', ended up using a 68,000 ohm resistor.





I have a 110 foot run of cat 5 cable going from my battery to the room that I use the power in, 2 conductors are connected to the battery through a 250ma fuse to the meter.

I am only concerned about 2 voltages: 14.7vdc full charge, and 12.4vdc where the battery starts to sulfate to much. So I measure the voltage directly from the battery terminals and when I see the voltage reading 14.7 I run into the house and put a mark on the meter representing that voltage. Then that night as the battery is being drained I keep making measurements till I see a reading of 12.4 on the terminals, then run into the house and put another mark on the meter representing that voltage. This gives me perfectly accurate readings on the remote meter. As long as the meter needle is between those 2 marks, then I am happy.

This cassette deck meter also has a backlight which I can turn on with a pushbutton switch. I have a 120 ohm resistor in series with the backlight and it glows nicely when I push the button. I use 2 more conductors of the Cat 5 wire for LED indication of Stage one of my dump controller, plus 2 more conductors for Stage 2 dump indication. There is a spare pair of conductors in the wire.

Way Low Tech and works wonderfully !

[Ungrounded Lightning Rod]

If anything makes the wire draw enough current to melt the insulation (like a flaw making a short near the load end) the short will "run up" the twisted pair toward the battery, pulling more and more current as it gets closer, until it sets the cable on fire.

So be sure the pair is fused.

For the one across the shunt, make sure it's connected to the metering terminals of the shunt.  You don't want a connection failure on the shunt to try to push the whole load current through the cat5.

I assume the shunt is in the ground run.  If not I'd fuse (both sides of) it anyhow:  A short to ground elsewhere will still make it overcurrent, so you want a fuse to protect the cat5 wires.

Don't forget that the pairs can short to each other, not just between the halves of the pair.  So enumerate your possible shorts and make sure you're protected for all of them.

[Opera House]

In situations like this, I use PTC resetable fuses. They look like a ceramic capacitor.  The ones I use are solid state devices that have less than  0.1 ohms resistance and trip at 4.6 amps.  They turn into much higher resistance as they heat up to about 100C and can tolerate a 40A surge for a few seconds.  I tried selling them on ebay, but no one there was smart enough to figure out how useful they are.