MeasurementsSeries Ohmmeter

Dr.Othman Al-smadiDone by: Moath Qteishat Mohammed Al-hadid

Analog ohmmeter

An ohmmeter is an electrical instrument that measures electrical resistance, the opposition to an electric current. Micro-ohmmeters (microhmmeter or microohmmeter) make low resistance measurements. Megohmmeters (aka megaohmmeter or in the case of a trademarked device Megger) measure large values of resistance. The unit of measurement for resistance is ohms (Ω).

Ohmmeters:

are used to measure resistance and to check continuity. An ohmmeter is connected in parallel with the resistance being measured. The ohmmeter range which allows a midscale indication should be selected. Resistors are used to allow an ohmmeter to have several ranges.

The original design of an ohmmeter provided a small battery to apply a voltage to a resistance. It uses a galvanometer to measure the electric current through the resistance. The scale of the galvanometer was marked in ohms, because the fixed voltage from the battery assured that as resistance is decreased, the current through the meter would increase. Ohmmeters form circuits by themselves, therefore they cannot be used within an assembled circuit.

The Movement

Analog series ohmmeter circuit: In series ohmmeter the resistors are used in series with the resistance being measured. Series ohmmeters have the 0 indication on the right of the scale.

Shunt ohmmeter:internal range resistors are in parallel with the resistance being measured, A shunt ohmmeter will have the 0 indicarion on the left side of the scale.

The following SAFETY PRECAUTIONS should be observed when using an ohmmeter.

Reenergize and discharge the circuit before connecting an ohmmeter.

Do not apply power while measuring resistance.

Switch ohmmeters OFF, if a setting is provided, or to the highest range and remove the meter leads from the meter when finished measuring resistance.

A MEGOHMMETER (MEGGER) used to measure very

large resistances, such as the insulation of wiring. To use a megger, isolate the resistance being measured from other circuits, connect the meter leads, turn the hand crank, and note the meter indication. Normal insulation will indicate infinity.

The following SAFETY PRECAUTIONS should be observed when using a megger.

Use meggers for high-resistance measurements only.

Never touch the test leads while the handle is being cranked.

Deenergize and discharge the circuit completely before connecting a megger.

Disconnect the item being checked from other circuitry, if possible, before using megger.

Design #1: Here: R1 :current limiting

resistance. R2 : zero adjust

resistance. E: internal battery Rm : movement

resistance.

In this design Ix depends on Rx , hence follow the steps:

1-Let Rx=0 (S.C) . For this condition adjust R2 until the movement indicates a full scale current ,at this current position mark this pointer place as a zero ohm.

2-let Rx=∞ Ω (O.C), the current in this circuit will be zero, therefore the movement will indicate a zero A , mark this pointer place as infinity ohms.

For intermediate markings . Place known Rx & mark the readings as observed .

Design #2: Half scale deflection

Req=Rh

Req=R1 + R2Rm

R2+Rm

To produce half scale current

I=0.5I(fs)

I=0.5E/Rh

→R1=Rh-((R2xRm)/(R2+Rm))

Notice that when Rx=0 we get a full scale deflection

I =E/Rh Let Rx=Rh design for R1, even

though,R2 is the one responsible for the main scale markings, we adjust R2 to get a full scale deflection.

To produce a full scale deflection:

Make Rx=0 It will be 2Ihfs =E/Rh

I2=It -IMfs

Also:ER2 =Em

R2I2 = Rm Imfs

R2 = Rm Imfs /I2

R2 = Rm Imfs

It –Imfs

It = E/Rh

R2 = RmImfsRh

E-ImfsRh

Ex:1) A 100Ω basic movement

is to be used as an ohmmeter requiring a full scale deflection of 1mA and internal battery voltage of 3V . A half scale deflection marking of 2k is desired. Calculate:

i. value of R1 and R2

Rm=100Ω , Imfs=1mA E=3V , Rh=2KΩ It=E/Rh

It=3/2K=1.5mA I2=1.5-1=.5mA R2=100*1/.5=200Ω

R1=Rh - R2Rm

R2+Rm

R1=2k-(200*100)/(200+100)

R1 =1.33KΩ