Branch: Electrical 3rd sem Group members: Naisargi (130450109011) Garvish (130450109012) Himika (130450109013) Aakib (130450109014) Harsh (130450109015) Branch: Electrical 3rd sem SVMIT

Galvanometric Recorders

Galvanometer The galvanometer is the "classic" device to measure current. Based on the fact that a wire carrying current in a magnetic field feels a force.

The current flows through a coil in a magnetic field. The coil experiences a torque proportional to current. The movement of the coil is "opposed" by a spring. The deflection of the needle is proportional to current

Principle The principle of galvanometric recorders is same as that of moving coil instruments. When current carrying conductor is placed in the magnetic field, force is exerted on the conductor. F = NBIL…..Newton where, N = is number of turns of coil B = magnetic flux density I = current flowing through coil L = length of conductor cutting the flux

The direction of this force is given by Fleming’s left hand rule. Statement:- Keep the three fingers i.e. first finger, middle finger and thumb of left hand mutually perpendicular to each other. Then, first finger points the direction of magnetic field. Middle finger points the direction of current flowing through conductor. Then, the thumb indicates the direction of force exerted on the conductor.

In conventional current moving coil instruments the controlling torque is produced by the spiral springs but in case of galvanometric recorders the controlling torque is produced by the current flowing through the suspension of coil. The pointer is attached to the suspension with a pen at its end. As a coil moves as per the deflection torque produced by the quantity under measurement an ink trace is produced on the chart.

R = length of pointer θ = angular deflection of the coil y = displacement of the pen Then displacement of the pen y is given as y = R sinθ The relationship between current and displacement is non-linear since the angle is proportional to the current through the coil.

The error due to non-linearity can be limited to 0 The error due to non-linearity can be limited to 0.5% if the angular displacement is restricted to + or – 10º. The curvilinear paper is used for the recording since the pen draws curve instead of a straight line. The figure on left shows the curvilinear paper used in galvanometric recorders.

Drawbacks But it suffers for the drawback that there are difficulties in interpolation for points between the curved lines.

To overcome this difficulty rectilinear charts are used rather than curve linear charts which are also known as knife edge recorder. It consist of heat sensitive paper which is impregnated with a chemical that shows a marked colour when heated with a contact.

Galvanometer A typical galvanometer has a "full-scale-current" (Ifs) of 10 A to 10 mA. The resistance of the coil is typically 10 to 1000 . How can we use a galvanometer to measure currents higher than its full scale current? Divide the current, so that only a well understood fraction goes through the coil Measure how much goes through the coil Rescale by the known fraction

The current I divides itself between the coil and the shunt Rsh = "shunt" resistance The current I divides itself between the coil and the shunt I = IC + Ish By Ohm's law, Vab = IC RC = Ish Rsh Ish = IC (RC/Rsh) I = IC + Ish = IC (1 + RC/Rsh) If RC and Rsh are known, measuring IC is equivalent to measuring I

Furthermore, I is still proportional to IC, which is proportional to the deflection of the needle. Thus, by "switching in" different shunt resistances I can effectively change the "range" of my current measurement

Example I = IC (1 + RC/Rsh) Galvanometer, RC=10 , Ic=1 mA What shunt resistance should I use to make the full scale deflection of the needle 100 mA? I = IC (1 + RC/Rsh) Want the "multiplier" to be 100 (i.e. 1 mA  100 mA) 1 + RC/Rsh = 100  Rsh = 0.101  Here: RC and Rsh in parallel Equivalent resistance Req = RCRsh/(RC+Rsh) = 0.1  Small, much closer to ideal ammeter (R=0)

Galvanometer as a Voltmeter Move the shunt resistance to be in series (rather than in parallel) with the coil. Remember that an ideal voltmeter has infinite resistance, so we want to make the resistance of the device large! IC = Vab/(RC + Rs) The needle deflection measures IC and, knowing RC and Rs, measures Vab

Example Galvanometer, RC=10 , Ifs=1 mA What resistance should I use to make a voltmeter with full scale deflection of the needle Vfs = 10 V? IC = Vab/(RC + Rs) RC + Rs = Vfs/Ifs = 10 V / 1 mA = 104  Rs = 9,990  Here: RC and Rs in series Equivalent resistance of voltmeter = RC + Rs = 104  (large!)

Galvanometer as a resistance meter IC = /(RS + R) From the needle deflection, measure IC Then, knowing the emf and RS infer R In practice RS is adjusted so that when R=0 the deflection is maximum, i.e. Ifs = /RS

Application A major early use for galvanometers was for finding faults in telecommunications cables. Galvanometer mechanisms were also used in exposure mechanisms in film cameras. A galvanometer mechanism is used for the head positioning servos in hard disk drives and CD and DVD players. These are all of the moving coil type, in order to keep mass, and thus access times, as low as possible

Mirror galvanometer systems are used as beam positioning or beam steering elements in laser scanning systems. The newest galvanometers designed for beam steering applications can have frequency responses over 10 kHz with appropriate servo technology.

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