voltmeter

[meledson]

Yes, you can use an ammeter as a voltmeter. The best range to use (to use as a voltmeter) would be low-range ammeters (ex: µAmp range) - I'll explain the reason later.

You can connect the ammeter the same way as a voltmeter, but with one tiny detail ... that is to insert a precision resistor in series to the meter. 'Di ko mai-drawing, so ganito lang: (+) battery to (+) ammeter, (-) ammeter to resistor end, opposite resistor end to (-) battery. Actually you can insert the resistor on the other side of the meter, series naman siya.

Now, according to Ohm's law ...
voltage = current * resistance
V = I*R

Pick a fullscale voltage mark, say you want 16v to be the max reading of your ammeter. If your ammeter's dial range is 0-100 µAmp, then you'll know what resistor to use ...
R = V/I
R = 16v / 100 µA
R = 160kΩ

To know at what µA mark the other voltage readings would be, use Ohm's law again. Like at 14volts, the ammeter reading would be ...
I = E/R
I = 14v / 160kΩ
I = 87.5µA
And so on for the other voltage readings ...

Now if you're going to use a high-current ammeter, medyo mahihirapan kang maghanap ng high-wattage[SIZE=4][SIZE=2] precision resistor. But if you are to use a [SIZE=4][SIZE=2]µ-ammeter as a voltmeter and an ammeter, too, bitin naman yung range ng ammeter because of the large current involved. [/SIZE][/SIZE][/SIZE][/SIZE]

Ooops ... sumobra pagka-OT

[SIZE=4][SIZE=2][SIZE=4][SIZE=2]HTH[/SIZE]
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Ammeters are actually voltmeters but magkaiba ang connection nila.

Voltmeters use voltage dividers (resistor in series). Low voltage kasi ang coil (analog) so mag series ka ng mataas na resistor. The higher resistance, the higher the voltage that can be measured. Ideal Voltmeters have infinite resistance. Para di maka affect the lugar na nag measure ka ng load.

Ammeters have shunt resistors ( resistor in parallel to the coil). Ammeters measure the voltage across the low resistance. The lower the resistance, the higher the current that can be measured. Ideal Ammeters have zero resistance, para walang effect the circuit.

Let us say gusto mo na i-measure the current na lumalabas o pumapasok sa battery, you will place the ammeter in series with the battery. (What would happen if you have a 160KOhm resistor in series with the battery?) Old school automotive analog meters measure frpm -60 to +60, sa iba -30 to +30 lang.

BUT, you do not actually insert an ammeter (for automotive applications) in series. you use a current transformer. a current transformer is a coil that you place around the wire you want to measure the current. This is the principle used by hand held clamp meters.

In my opinion, it will be much easier to monitor the voltage then the car is running, turned off and cranking the engine. Voltage no load (patay lahat pati makina) sould be at least 13.8 volts and better. Cranking the engine consumes a lot of current, the voltage should not go to less than 11 volts (I am not sure on this) When the car is running, the voltage should be higher than voltage no load. This signifies that it is being charged.

Sorry, haba na e..