Volt Ohm Meter

Introduction.

The technician or mechanic is like a physician. The technician or mechanic uses his test instrument to diagnose or examine the defects or trouble of the electronic equipment and appliances, while the physicians use the stethoscope to diagnose the illness of his patients.

A multitester also known as a volt/ohm meter or VOM, is an electronic measuring instrument that combines several measurement functions in one unit. A typical multimeter may include features such as the ability to measure voltage, current and resistance. There  are two categories of multimeters, analog multimeters and digital multimeters.

A multimeter can be hand-held device useful for basic fault finding and field service work or a bench instrument which can measure to a very high degree of accuracy. They can be used to troubleshoot electrical problems in a wide array of industrial and household devices such as batteries, motor controls appliances power supply and wiring systems.

With the mutitester alone, a proficient and resourceful electronic technician can perform repairing or servicing work efficiently and earn a good income.

Analog multitester 

          Resolution of analog multimeters is limited by the width of the scale pointer, vibration of the pointer, the accuracy of printing of scales, zero calibration, number of ranges, and errors due to non-horizontal use of the mechanical display.

                                                                  

Digital multitester

        

Digital multitester is an instrument used on electrical circuits and electronic equipment for testing or diagnostic purposes. They are available in both hand-held and large bench-top versions.

Digital multimeter measure a wide variety of quantities including temperature, current and voltage. Other quantities that a multimeter might measure, depending on the type, include resistance, capacitance and frequency.

Care for the multitester

          Because this test instrument is very delicate one must know how to use and care for it. This needs caution because it is expensive.

1.   Never drop this multitester it may ruin the inside mechanical parts (meter).

2.   Never expose it to water or moisture.

3.   Always set the range selectors knob to OFF position or 1000 volts if not in use.

4.   Never test VOLTAGE IF THE SELECTOR RANGE IS IN RESISTANCE.

5.   Replace the battery if used for a longer period of time.

6.   Replace fuse with the exact value if  blown off

The Scale . The most important part of the  scale are the ohmmeter scale, the voltmeter scale and the milliammeter scale.

The Ohmmeter Scale.  The ohmmeter scale is located at the top  portion.Thecalibrations of the ohmmeter scale generally starts from the right towards the left side. The numerical calibrations start from zero at the right and gradually increases towards the lazy eight at the left side of the scale.

          The Voltmeter Scale. The voltmeter scale is located below the ohmmeter scale. The calibration of the voltmeter scale is opposite to that of the ohmmeter scale. The voltmeter scale starts from number zero a the left and increases gradually towards the right.

          The rows of the voltmeter scales are the following.

                   0………….50………….100………….150.............200.............250

                   0………….10………….10…………….20..............30...............50

0………….2…………. …4………….….6................8...............10

The important thing for us to know now is this: Whether we are measuring AC voltage or DC voltage, we will be using the same voltmeter scale to read the actual voltage being measured. Also we are using the same scale when we are measuring DC current.

The Deflecting Pointer. Another name for this is deflecting needle. For simplicity we will just call this pointer. This is the part of the multitester that moves or travels across the scale when the multitester is being used to measure resistance, current or voltage. The point on the scale where the pointer stops is the reading of the multitester when used.

                                     

The important parts of the multitester are as follows:

The Rangers.  Commonly, the ranges of the multitester is divided into four. These are: The ohmmeter range, the AC voltage range, the DC voltage range and the DC current range in milliamperes (ma). The usual location and uses of these ranges are as follows:

to measure or check the amount of DC voltage.

DC Current Range. DC current range is located at the lower left portion of the range. This is usually marked DCA, 50, 2.5,25.0.25. The DC current range is used when measuring and checking the amount of the DC current in milliamperes.

Ohmmeter Range. The ohmmeter range is located at the lower right portion of the range. This range is usually marked Rx1,Rx10, Rx1k and Rx100k. This range is used when the multitester is used to measure or check the resistance and also when the multitester is used to check the continuity of the circuit.

AC Voltage Range. The AC voltage range is located at the upper right portion of the range. The AC voltage range is usually marked ACV,1000: 250: 50: and 10. This voltage range is used when the multitester is used to measure or check the amount of AC voltage.

DC Voltage Range. The DC voltage range is located at the upper left portion. It usually marked DCV, 1000: 250: 50: 10: 2.5: 0.25: and 0.1. This voltage range is used when the multitester is used

The range Switch. The range switch is located at the center of the range. The range switch is used to adjust the multitester to the correct function we desire.

The Zero – Ohms Adjust. The zero-ohms adjust is used only when the multitester is functioning or working as an ohmmeter. Example. The multitester is adjusted at Rx1 ohmmeter range. Now, if we change the adjustment from Rx1 to other ranges such as Rx10, Rx1k or Rx100k we used the zero-ohms adjust control, to adjust the multitester so that its deflection pointer is pointing at zero on the scale. Remember, we have to do this every time we change the range of the ohmmeter.

The zero-ohms adjust is used only when the multitester is adjusted at ohmmeter range, it is not used when the multitester is at voltmeter and milliammeter range.

          The Scale. The most important  part of the scale are the ohmmeter scale, the voltmeter scale and the milliammeter scale.

          The Ohmmeter Scale.  The ohmmeter scale is located at the top  portion. The calibrations of the ohmmeter scale generally starts from the right towards the left side. The numerical calibrations start from zero at the right and gradually increases towards the lazy eight at the left side of the scale.

          The Voltmeter Scale. The voltmeter scale is located below the ohmmeter scale. The calibration of the voltmeter scale is opposite to that of the ohmmeter scale. The voltmeter scale starts from number zero a the left and increases gradually towards the right.

The rows of the voltmeter scales are the following.

                   0………….50………….100………….150.............200.............250

                   0………….10………….10…………….20..............30...............50

0………….2…………. …4………….….6................8...............10

The important thing for us to know now is this: Whether we are measuring AC voltage or DC voltage, we will be using the same voltmeter scale to read the actual voltage being measured. Also we are using the same scale when we are measuring DC current.

The deflecting Pointer. Another name for this is deflecting needle. For simplicity we will just call this pointer. This is the part of the multitester that moves or travels across the scale when the multitester is being used to measure resistance, current or voltage. The point on the scale where the pointer stops is the reading of the multitester when used.

Using the Multitester as voltmeter

          A voltmeter is an instrument used to measure voltage. The two kinds of voltmeters are the AC voltmeter and the DC voltmeter. The AC voltmeter is used to measure AC voltage while the DC voltmeter is used to measure DC voltage

Using the Voltmeter to measure the voltage in the circuit is not an easy as memorizing its definitions. In doing this, we have to be very careful because a minor mistake will mean a painful electric shock, a short circuit  and a damage               electronic  a damage electronic appliances. We must remember that when we are measuring voltage in the electronic equipment or appliances, the said equipment is connected to the main voltage source.

For our safety and to avoid damage to the electronic appliances we are repairing, we have to memorize the following guides:

1.    When measuring voltage, be sure your feet and hands are completely dry.

2.    If possible, when measuring voltage  wear dry rubber shoes.

3.    When measuring voltage. Use only one of your hands. Place the other hand inside your pocket.

4.    Do not measure voltage with the hand holding the metal chassis of the electronic equipment or appliance3 and the other hand holding the test prod of the voltmeter.

5.    Remove any objects such as ring, watches and bracelets that might accidentally short the equipment.

Using the  DC Voltage Range of the Multitester.

         

DC voltage is a type of voltage that comes from all kinds of batteries and converter power supplies. Literally Dc means direct current. But it is an accepted practice in electronics to use the terms DC voltage and DC current have fixed polarities.. These types of voltage and current have fixed or permanent positive and negative points. To measure DC voltage, the first thing to we do is adjust the multitester at DC voltage range. The DC voltage ranges divided into different ranges. You may ask why there are many numerical ranges. The answer is that each of these numerical ranges has its own use. For example if we adjust the range switch of our multitester at 250, it means that the maximum amount of voltage that our multitester is capable of testing or measuring is 250 volts. Above 250 volts the multitester will be destroyed.

          Can we adjust our range switch at the higher range right away, so that there is ni need of changing the position of our range switch if we want to measure different values of DC voltages? No this is not possible. I few set the range switch of the multitester at 250, and use it to measure voltage as low as 5 volts the pointer of the multitester will not deflect. The  actual practice is to adjust or set the range switch higher but nearer to the expected amount of the voltage to be measured. Interpreting the reading of the Multitester.

When interpreting the reading if the multitester at DC voltmeter range, there are three things we must consider.

1.    The adjustment or setting of the range switch.

2.    The voltmeter scale to be used.

3.    The reading on the scale as indicated by the deflecting pointer.

DC VOLTMETER READING

1. When operating on 0.1v range, we utilize the 10v scale and shift the two decimal places to the left.

2. If the instrument is operated on its 0.5v range, we utilize the 50v scale and move the decimal point two places to the left.

3. When the VOM is set to its 2.5v range, we utilize the 250v scale and shift the decimal point two places to the left.

4. If the DC voltmeter function of the VOM is operated on its 10v range, we utilize the 10v scale and read directly.

5. When the VOM is set to its 50v range, use the 50v scale and read the scale directly.

6. If the instrument is operated on its 250v range, we read 250v scale directly.

7. To read values on the 1,000v range we utilize the 10v scale and shift the decimal point two places to the right. t{ x m g �w1 ��* entally short the equipment.

Using the  DC Voltage Range of the Multitester.

         

DC voltage is a type of voltage that comes from all kinds of batteries and converter power supplies. Literally Dc means direct current. But it is an accepted practice in electronics to use the terms DC voltage and DC current have fixed polarities.. These types of voltage and current have fixed or permanent positive and negative points. To measure DC voltage, the first thing to we do is adjust the multitester at DC voltage range. The DC voltage ranges divided into different ranges. You may ask why there are many numerical ranges. The answer is that each of these numerical ranges has its own use. For example if we adjust the range switch of our multitester at 250, it means that the maximum amount of voltage that our multitester is capable of testing or measuring is 250 volts. Above 250 volts the multitester will be destroyed.

          Can we adjust our range switch at the higher range right away, so that there is ni need of changing the position of our range switch if we want to measure different values of DC voltages? No this is not possible. I few set the range switch of the multitester at 250, and use it to measure voltage as low as 5 volts the pointer of the multitester will not deflect. The  actual practice is to adjust or set the range switch higher but nearer to the expected amount of the voltage to be measured.

Measuring AC voltage.  AC voltage is the type of voltage that comes from AC voltage generators such as those supplied to  us by power supply companies. The electricity supplied to our homes by MERALCO is AC voltage.

What is the difference between AC voltage and DC voltages? The difference between AC and DC voltage is: AC has no fixed polarity. On one instant one point is negative and the other is positive and vice versa. On the other hand, DC voltage has permanent polarity. DC has permanent or fixed negative and positive points.

To use the multitester to measure AC voltage, adjust the multitester at AC voltage range. As we said before, AC voltage has no fixed polarity. So  to check the AC voltage with the mutitester the black test and red test prod can be connected on either side of the circuit.How to interpret the reading when Measuring AC Voltage.

In interpreting the reading on the multitester, when measuring the AC voltage, the method is the same as the method used in measuring AC voltage. The scale used in the multitester for the reading of AC voltage is also the same scale used for DC voltage

The AC voltage range, however, is different. Thus, when measuring the AC voltage the range switch must be adjusted at AC voltage range.

1.     When measuring voltage, ( AC and DC voltages) adjust the range switch at the higher range.

2.    When measuring voltages (AC and DC) be sure your feet and hands are completely dry.

3.    When measuring Ac voltage adjust the range switch of the multitester at AC voltage range.

4.    When measuring voltages (AC and DC) use only one of your hands. This can be achieved by connecting one of the test prods permanently to the circuit, and voltage measurement can be done by one hand only.

5.    Always be very careful when measuring voltage (AC or DC).

6.    Remove any objects such as ring, watches and bracelets that might accidentally short the equipment.

AC VOLTMETER READING

1. When operating on the 10v range, we utilize the 10v use the red scale and read the scale directly.

2. If the instrument is operated on its 50v ac range, read the red scale 0 – 50 scale directly.

3. When the VOM is set to its 250v ac range, use the red 0-250 scale and read it directly.

4. To read voltage in the 1000v ac range, read the red 0-10 scale and move the decimal point two places to the right.

Using the multitester as Ohmmeter.

          Before going further, we must make it clear that when we adjust the multitester at ohmmeter range, we are using the battery inside the said multitester.because the ohmmeter has its own battery power supply of the equipment whose parts or components must be tested by the ohmmeter, should be switched off or must be plugged off before using the ohmmeter  on it. If we fail to do this our multitester at ohmmeter range will surely be damaged.

Interpreting the multitester reading at Ohmmeter Range.

The important things to consider when interpreting the multitester reading at ohmmeter range are the range on which the range switch is adjusted, the numerical calibration on the ohmmeter scale indicated by the deflecting pointer.

Now at this point, we change the adjustment of our range switch. From Rx1 we switch to Rx10. After switching at Rx10, we connect the red test prod and the black test prod, then rotate  ( clockwise or counterclockwise) the zero ohm adjust until the deflecting pointer point at zero. After making this adjustment, we are now ready to use the multitester at ohmmeter range. Why do we have to make a zero ohm adjust? The reason for doing this is to obtain accurate test result.

OHMMETER READING

         

1. When the VOM is set to its Rx1 position, we read the scale directly.

2. If the instrument is operated on its R x10 range, we multiply the scale reading

by 10.

3. We multiply the scale reading by 1,000 if the Rx1k range is in use.

4. If the VOM is operated on its Rx10k range, we multiply the scale reading

by 10,000.

DC CURRENT METER (AMMETER)

          AMMETER is an instrument used for measuring electric current. Its basic units of measurement are milliampere and ampere represented by the capital letter A. The most frequently used measure of current in electronics is the milliampere (mA). Always start with the highest range to confirm approximate voltage present.

          The milliampere and ampere ranges are generally used to measure circuit current, charging and discharging current of batteries, maintenance and inspection of DC power equipments and power transistor circuit. Use the figures along the black arc (or black scale) just below the mirror marked DCV. A in common with DC voltage measurement. We observe that the 50uA position is also marked 0.1v. this switch position double duty on the DC current and DC voltage function. When the instrument is operating on its DC current function, we disregard the 0.1v markings.

DC CURRENT METER READING:

(NOTE: observe correct polarity of the instrument when measuring DC current. Be very careful against overloading your meter. If you are in doubt of approximate reading, reading always starts with the highest range. If the pointer deflects left across zero point, just reverse the connections; no damage to the meter if this occurs.)

1.    When operating on the 50uA range, we utilize the 0-50 (black) scale and read the scale directly

2.    To read current in the 2.5mA range, read the black 0-250 scale and move the decimal point two places to the left.

3.    If the instrument is operated on its 25mA range, read the black 0-250 scale and move the decimal point just once to the left.

When the VOM is set to is 0.25A range, read the black 0-250 scale and move the decimal point three to the left.