Another electrical question...

[woodworker1]

It's me again.
I have 2 meters, both are older Niehoffs but never used. One is a 12v voltmeter and the other is a 12 v ammmeter. It reads to 60 amps.
Just wondering if these are worth wiring into the system with the new converter I bought.
I would like to mount them next to the converter and have a switch on them. Do I simply run each directly to the battery and put the lighted switch in the middle of the positive lead?
Do I need to put a fuse in-line for each meter?
Thanks for any information you can share...
Rob

[hertig]

Voltage is read across 2 points, across the battery is good, or across the connectors where the battery comes into the converter. Voltage is a useful check of the state of the battery. You could post a chart relating voltage to condition.

Current is read 'in line'. The best place for a current meter to get a useful reading is in the power lead from the battery. In other words, the battery lead goes to the input of the meter and the output of the meter goes to the converter. This will show how much current you are drawing, which will allow you to estimate the hours you can continue this (and stop at 50% when practical to keep battery life at a maximum). There might be a problem when you go to charge the batteries though, I'd have to do some more research to figure out how to hook it up safely.

The voltage meter can easily be switched, but it will be more difficult to switch the current meter, since it will have 'all' the power going through it.

If the meters are close, fusing is not needed and in the case of the current meter, may cause problems.

[woodworker1]

Thanks Hertig...I'm with you so far....
I'm laying out and constructing the cabinet all this will go into. I mounted the converter on 1/2" plywood and went ahead and drilled the holes for the guages. The 2 holes are about 1.5" from the converter.
If it ends up I shouldn't use the ammeter, then so be it.
I'll await your direction. (Aren't you happy to hear that!)

[hertig]

Just had a thought. Does the volt meter max out at 12v, or does it go up to say 15v? 'Normal' charge voltage is about 13.6, so if the meter won't handle that, it could be a problem.

Does the amp meter go from -60 to +60 amps or 0 to 60 amps only?

Thinking about it more, the voltage meter is 'directly' across the battery, so fusing it (close to the attachment to power) may be a good idea after all.

[woodworker1]

The volt meter goes from 10 to 16.
The ammeter goes from -60 to +60.

[TexasClodhopper]

It might not seem right, but you can switch the ammeter &quot;out&quot; of the circuit by simply &quot;shorting&quot; its terminals with a switch. In other words, turn the switch &quot;on&quot; to turn the ammeter &quot;off.&quot; It won't actually be out of the circuit, but it won't be carrying all of the load anymore. Plus, it is an easier wiring job.</p>

You could use one double pole / double throw (DPDT) switch to disconnect the voltmeter and &quot;short out&quot; the ammeter at the same time.</p>

[hertig]

Since the meter will handle current flow in both directions, it should be ok in line even during charge cycles. Texas has a good suggestion. The way I was thinking of was having 2 power supply lines, one with and one without the meter, then using a single pole, double throw (SPDT) switch to select which one to use.

In any case, make sure you use a switch which can not only handle the current, but the arc when the circuit is switched as well.

[woodworker1]

Thanks again guys!
I hate to ask, but can you hold my hand a little a walk me through this?
I'll pick up the switch...what exactly should I ask for?
So, instead of running two power leads, I'm running one. How will both meters get wired to ...
1) each other
2) the converter
3) the switch

I really am embarrased I am so ignorant with all this.

Rob

[TexasClodhopper]

Yeah, I know. I never could cut a compound miter joint! </p>

Wire it this way, and (with your zero-center ammeter) you'll be able to read load current and charge current. </p>

You really don't NEED the switch. Just leave it wired up all the time. Remember that the wires to the ammeter carry load/charge current and the wires to the voltmeter carry hardly any current at all.</p>

http://i123.photobucket.com/albums/o290/texasclodhopper/AmpVoltWiring.jpg</p>

FULL SIZE: http://i123.photobucket.com/albums/o290/texasclodhopper/AmpVoltWiring.jpg</p><a href="http://i123.photobucket.com/albums/o290/texasclodhopper/AmpVoltWiring.jpg" />

[hertig]

Since the voltmeter is connected 'directly' to the battery, I think I'd go ahead and put a small fuse before it since the wire to it probably does not have the degree of insulation the big battery wires do. If you do without the switch (or at least the switch for the current meter), you avoid the annoyance of getting a switch which will hold up to switching large amounts of current. If you did want to switch off the voltage meter only, any switch you can get should work fine.

[woodworker1]

Hey TC...compound miters I can handle. Sliding dovetails I can do
It seems like everywhere I see ammeter mentioned, I see "shunt" mentioned also. Not even sure I know what one is. Do I need to install one in line?

[TexasClodhopper]

There's no way I can know from here whether your meter was designed to use a shunt or not. I suppose there is some possibility. </p>

Basically, some ammeters are justvoltmeters with a shunt. The shunt might be built into the case or the ammeter might be designed to use an external shunt. In simple Ohm's Law terms, the shunt (a resistor) converts the current through it to a voltage across it, and the voltmeter reads that voltage. The voltage across the shunt is proportional to the current through it (twice the current; twice the voltage). The voltmeter has a scale that is printed to match the shunt, because the shunt is designed to produce a VERY small voltage (in the order of 50 millivolts); therefore not causing much interference in the circuit it is measuring.</p>

If you wire your ammeter in and it goes &quot;phfspt!&quot;, then you need something else. :o ;) </p>

</p>

[C Nash]

Tex, I really liked that last line :laugh: :laugh: :laugh: :laugh:

[TexasClodhopper]

Well, woodworker1 knows what I mean. Sometimes ya just gotta cut da board and see if it fits!

[kotr]

One word of caution!! The amp meter carries ALL the current the whole unit needs, make sure you use a wire the same size as the wire from the battery to the charger!! The volt meter is not as important cause it only monitors the voltage.

[kotr]

PS using to small of a wire will cause heat, enough to cause a fire, that's not good!!

[TexasClodhopper]

Well, actually that isn't exactly correct. The wire size for the appliances doesn't have to be the SAME size as the wire to the charger. The charge rate could be quite small and in most cases the charge current changes over time, so we're really talking about an average current.</p>

Ya know, in 40 years, I've never seen a fire caused by a burned out 12 VDC circuit. I've seen a lot of burned up wiring, but I guess the battery tends to die before the fire starts.</p>

I have welded a wrench to a battery before! Good thing the battery was mostly bad or it could have exploded.</p>

[bufordtpisser]

woodworker1 - 2/20/2007 4:52 PM

Hey TC...compound miters I can handle. Sliding dovetails I can do
It seems like everywhere I see ammeter mentioned, I see "shunt" mentioned also. Not even sure I know what one is. Do I need to install one in line?


If you are going to wire the ammeter directly into a load carrying circuit, it is a really good ideal to either wire in a shunt or a dropping resister. That way when the circuit is turned either on or off, the insurge and or backfeed current will not fry the meter. TC's comments about not knowing whether or not your meter was shunted was valid. But the use of a second shunt will not have an appreciable effect on the actual measurement of amperage. The shunt or dropping resister will just keep you from frying the meter in the case of an over amperage condition.

[hertig]

Really? I would think that if you change the resistance between the meter leads, the voltage (due to current) across those leads would change to compensate (we will assume the current is essentially constant). And so if the reading is driven by voltage,as would be implied by the need for a shunt, the reading will be changed even if the current is the same. Last I heard, it was a law that Voltage = Current x Resistance. Or has the ACLU overturned that one too?

If there is an internal shunt, it will have a resistance calibrated to provide a 'current' reading which is accurate. If you put another resistance in parallel with that resistance, the value will change. For instance if you have 2 ohms there already, and put another 2 ohms in parallel, the effective resistance will be 1 ohm, or half what it 'should' be. Thus the voltage will be half what it was before for any particular current, and so the current reading will be 1/2 of what it should be.

[TexasClodhopper]

John has correctly stated Ohm's Law.

Also, woodworker1 has a +/- 60 amp meter. Nothing is going to hurt that meter without melting wires! I can't imagine a "surge" like that in any of his 12 volt DC circuits.

[bufordtpisser]

hertig - 3/23/2007 11:30 PM

Really? I would think that if you change the resistance between the meter leads, the voltage (due to current) across those leads would change to compensate (we will assume the current is essentially constant). And so if the reading is driven by voltage,as would be implied by the need for a shunt, the reading will be changed even if the current is the same. Last I heard, it was a law that Voltage = Current x Resistance. Or has the ACLU overturned that one too?

If there is an internal shunt, it will have a resistance calibrated to provide a 'current' reading which is accurate. If you put another resistance in parallel with that resistance, the value will change. For instance if you have 2 ohms there already, and put another 2 ohms in parallel, the effective resistance will be 1 ohm, or half what it 'should' be. Thus the voltage will be half what it was before for any particular current, and so the current reading will be 1/2 of what it should be.

That was my mistake. What I should have said was that if you do not know if there is an internal shunt, wire one in and then test the guage with a meter to see if the reading is correct. The use of the dropping resister is after the guage, and therefore will not have an effect on the reading. As far as 60 amps not being an issue in a DC circuit, I have had readings many time that amount on DC circuits. And even though the wire were very hot at the time, other components failed long before the guage crapped out. I have also seen severe damage in circuits where the current drawing device was shutdown suddenly and backfeed current literally melted components. Did you ever wonder why most circuits on a vehicle had 20 to 30 amp fuses on wires as small as 14 guage?? The resister that I was suggesting was merely to stop back feed. The proper way to do amperage measurement is to use a meter that is not a part of the circuit. Either using a clamp on meter, or using a pickup coil that the wire passes through that will allow an amperage reading without the meter being an active part of the circuit. I was not trying to rain on anyones parade, I merely did not accurately explain my answer. I have 10 years as a senior service tech for an electric motor and generator shop under my belt and sometimes I guess that I just assume that others that are trying to do these types of projects know more than they actually do. I will try to remember that in the future.

[TexasClodhopper]

No harm done or meant, Marvin. We're all amateurs here about something, so we have varying levels of understanding and many levels of expression. It's hard to know how much detail to put into an answer.

I'm sure woodworker1 would have trouble explaining to us how to make a double-jointed backbone miter dovetail joint in particle board, now wouldn't he?

[woodworker1]

Uh, when it comes to wiring something, trust me...you can't take any knowledge for granted with me.
Thanks for all the replies.

[TexasClodhopper]

Hey! What about the "double-jointed backbone miter dovetail joint in particle board"?

[hertig]

Why would you want to do anything with particle board? That stuff is inferior for most applications. And can give off noxious fumes to boot.

[TexasClodhopper]

Gotcha! ;) :laugh:

[woodworker1]

Yeah, I agree. I don't even use it for counter top underlayment anymore. Too heavy and makes too much of a mess.