car deck and volt meter setup trigger

I bought a 12v volt meter, like this one here except it takes 12v in to run, it has 4 wires, 2 for the power source, and 2 for the reading source. If I hook the power source wires up to the switched 12v source that goes to the deck, and the 2 wires that read voltage from the battery to the constant red voltage battery wire going to the deck, will that correctly read my voltage. I figure when the deck turns on cause of the switched 12vs going on, it would turn on the voltimeter, and it would read from a 12 volt source that should be right from the battery. If that should work, do I need to fuse the wires going to this little meter since its coming off a bigger gauge source, or is it not needed cause of the small wires and little chance of a overload to it?
This seems to be the best way for it to work, but let me know, essentially I want the voltmeter that I will install into the car to look all nice to come on with the car is turned on, or the deck is turned on with the key in the accesory, and I figure if that switched voltage wire would be perfect.

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Viper 5902 - Ordered
Kicker Dual CVX 15's - Installed
Kicker ZX 1500.1 Amp - Installed

I forgot to post the volt meter, looks like this on here
Link

Except it takes 12v in to turn on and run

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Viper 5902 - Ordered
Kicker Dual CVX 15's - Installed
Kicker ZX 1500.1 Amp - Installed

AFAIK you cannot use it.
Although not specifically stated (albeit implied), its 5V power source must be independent of the voltage it is sensing.
Hence it needs a dc-dc converter or isolator, else a separate battery to run (as per DMMs).

Powering from a linked source (even through a 5V regulator) will probably destroy it.

As to fusing - voltmeter sense wires are normally not fused since voltmeter protection is no required (their sense inputs are hi-impedance; if they go low impedance, something is wring anyway).
Most merely use thin wires that do no damage if they burn out, else they insert a resistor in the +ve (or both) sense wires (like fuses - near the source - ie, the voltage being measured).

Since these meters typically have >100MOhm input impedance, any resistor large enough to limit short circuit current is fine.   
EG - limit short cct current to 12mA...
12V @ 12mA = 12/12 = 1 k-Ohm = 1k = 1,000 Ohms (V=IR => R = V/I).
1k in a 100M circuit means ~1k/100M = 0.001% error.
So even a 1M resistor is only a 1% error (ie, 1M/100M).
[ PS - why pick 12mA? 12/12 = 1. Also 12mA x 12V = 0.144 Watts, or worst case P = VxV/R = 15Vx15V/1k = 0.225W so a 1/4Watt or larger resistor is fine. I'd use a 1/2W resistor for extra mechanical strength - even if using a 10k or 100k or 1M or 22k or 390k etc resistor. ]

And if being used for a vehicle voltmeter, I'd suggest only ONE decimal point - eg, 14.3V or 14.4V, not 14.35V because the bobbling 1/100th Volt digit will probably drive you nuts, and is essentially useless anyway (except for battery condition monitors where 0.01V resolution per 2V cell is required - but that's a resolution of 0.05V for a 12V battery anyhow...)
That can be set on those voltmeters if they have a 200V range, else you design your voltage divider (to their standard 200mV input) as if it's reading 120V instead of 12V, and select the DP accordingly.

Alas those modules are a pain - I have seen many great displays but they don't have independent supplies (isn't that ambiguous - they have separate supply pins but are not independent).
Though it only takes a small audio transformer and oscillator circuit and 3-terminal regulator or similar to isolate its supply....

that is the one I actually bought right here, 12 dollars, he puts the 8 in front when he is out, so your saying I need resistors or something?

LINK

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Viper 5902 - Ordered
Kicker Dual CVX 15's - Installed
Kicker ZX 1500.1 Amp - Installed

this is some information on the panel, except it takes 12v in for a power source
Link

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Viper 5902 - Ordered
Kicker Dual CVX 15's - Installed
Kicker ZX 1500.1 Amp - Installed

Ebay links do not work here.

aww man, that pdf link works at least that shows the specs, its actually a 12v input for power, but I do not know if it needs to be isolated or what

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Viper 5902 - Ordered
Kicker Dual CVX 15's - Installed
Kicker ZX 1500.1 Amp - Installed

For eBay links, just post the item number. (You then eBay & Search it.)

The pdf states 5V input. I was going to suggest that maybe a 12V supply had been added.
BUT I just read in the pdf...
!!! The power supply and the signal being measured can have the same GND or the power converter can be a separate one, (not that I understand why the later comment is included!!)

So it seems you can use it.
Nice to see that they finally put isolating converter on board - they are so cheap and part of most DVM chipsets etc.

You could confirm with the seller to make sure....


But if it were required, no - resistors would not do it - it has to be a DC isolated supply - ie, through capacitors or a transformer. Hence why they used to be such a pain!

its ebay item "360201378485", I hope I can just hook up directly without a mess of wires, I mainly want it to know if I need to start my car up if I drain it to low from running my big amp, or to see if i am dropping my voltage too much with my amp, I have a kicker 1500.1, and 2 cvx 15's, they broke my mirror today, they are that loud, it fell down

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Viper 5902 - Ordered
Kicker Dual CVX 15's - Installed
Kicker ZX 1500.1 Amp - Installed

I hope you got a bargain - it's currently listed for USD $812.00!!

The new pdf is the same as the old except it specifies 12V DC input power.
It still has the "can common the GNDs" hence an independent supply is not required.

So power it any way you want....


It doesn't state its load current (why provide useful information like that?), but they are usually up to 100mA at 5V - typically ~50-70mA.

he just puts the 8 infront when he is out of stock, so if it says ground can be the same, its stating the positive can be the same to then? would load current just tell me how much it will draw, not that it would effect wiring correct? also, does this need a fuse? or should I just wire the thing right into it?

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Viper 5902 - Ordered
Kicker Dual CVX 15's - Installed
Kicker ZX 1500.1 Amp - Installed

The chip that does the voltage sensing must be supplied independently (isolated) from the voltage it is sensing - it cannot be have any "reference" between the two voltages.
Hence it needs to be isolated - aka galvanically isolated - meaning that current cannot from one to the other.

If this unit can share GNDs (which is the most common implementation), then is follows that it can share +ve too. (It doesn't have to, but it doesn't make sense to do it other than by full isolation. Then again what I consider sensible isn't always obvious to or reflected by others!)


But I suggest asking:
- is the (12V) supply independent of its sensing circuit - ie, you can connect the sensing wires to the 12V supply wires/terminals?
- what is its max current consumption - eg, for a display of 18.88?
- what fuse is recommended for its +12V supply?


Normally these are not fused because there is not much to protect - if a fault occurs, then it's dead.
You would just ensure the power distribution itself is protected (ie, a fuse to protect the wire that supplies +12V to it; normally the vehicle's gauge/meter circuit is used (typically 5-10A fusing)).

The sense inputs need no fusing. But if it is desired, it is better using in-line resistors (ie, 1k to 1M Ohm, 1/4 - 1/2W ratings as explained earlier).