My son want to make a simple DC voltmeter on 0-10V range by Convert a galvanometer to voltmeter. The Galvanometer DC1mA is cheap and easy with one resistor only.
Anyone’ve used all meters:
Voltmeter : for measure voltage; in volts unit
Ammeter ; for measure current; in Amperes unit
Ohmmeter; for measure resistance; in ohms unit
We think should start with a galvanometer as Analog meter cheap, somewhat less precise than digital types.
Best for finding the precise value of a voltage, current or resistance.
Figure 1 The Galvanometer DC1mA
In Figure 1 the galvanometer very sensitive meter which is used to measure tiny currents, usually 1mA or less. It is a main display inside all meters.
A galvanometer is a type of sensitive ammeter: an instrument for detecting electric current. It is an analog electromechanical actuator that produces a rotary deflection of some type of pointer in response to electric current through its coil in a magnetic field. As Figure 3 is the internal structure galvanometer
Full scale voltage of the galvanometer
How it works
We want to use the galvanometer to measure the voltage of DC power supply 10V.
But, Can we use it?
We need to know the full scale voltage of the galvanometer before.
From principle V = I x R
Now we know the value I = current of meter DC1mA
But we do not know the resistance (R) Best way is measured directly is 234 ohms only.
Thus; V = 234 ohms x 1mA = 234mV or 0.234V
Figure 4 the galvanometer certainly damage with high voltage
We can measure the voltage, when making the meter is equal to the voltage 10V only.
The voltage is difficult to understand Because invisible image.
I explained the voltage with a length of wood as Figure 5
On the left is wooden sticks length 10 cm as the source(DC power supply Input)
On the right is the small wood sticks length 0.234 cm only
How to makes both wood the same height as 10 cm (compared to 10V)
My son said very easy, just find another wood, braced it to the same height.
This Wood, we compare is a devices connected in series.
Which the best devices is resistor because cheap and easy to use.
How long is this wood?
10cm-0.234cm = 9.766cm
We compare the voltage with a length of wood.
Thus the voltage of Vs = 9.766V
How to find RS
When we know level voltage, then the following order of Rs lets’ find the resistance.
Figure 6 Try to find resistance of RS
Rs = Vs / Is
We know the value Vs = 9.766V
When is a serial circuit Current flows through all devices are equal is 1mA (as galvanometer).
Rs = 9.766V / 1mA = 0.9766K
But this resistance does not sell I used 10K instead as Figure 7
How to test
He trys the simple voltmeter as video below.
The build the voltmeter is easy. It would be helpful to learn of your electronics.
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