Test Equipment Application
VOLTAGE DROPVoltage is gradually used up in pushing a current through a circuit. Each electrical unit, including the connecting wires, has some resistance. The electrical pressure that is expended in forcing electrons through these resistances is called "voltage drop." All circuits have a normal voltage drop. As long as the voltage drop is normal, the circuit will function properly. However, whenever voltage drop exceeds the maximum amount specified it is a problem. Normal voltage drop for a connection should be approximately 0.1 volt.
VOLTAGE MEASUREMENT
Unlike the low resistance ammeter, the voltmeter has high internal resistance. That is why it is not used or connected like an ammeter. There are two ways to connect and use a voltmeter, depending on what you want to test in the circuit.
PARALLEL CONNECTION
Voltage Available. To find the voltage available at any terminal in a circuit, connect the voltmeter from the circuit terminal to ground.
DIRECT VOLTAGE DROP CONNECTION
Direct Voltage Drop Reading 0.2 V
Open Circuits - Resistance (Direct Voltage Drop). Electrical problems may be encountered on the ground side of a circuit due to incomplete ground contact. Use of the voltmeter connected in series at a load connector (0 volt reading), then connected in parallel to a separate ground (12 volt reading), would confirm this problem. This type of connection will give actual voltage drop readings.
VOLTMETER - USE
A voltmeter is connected in parallel with a circuit and it reads directly in volts. In parallel the meter draws only a small current, just enough to sample the voltage. That is why you can "short" right across the battery terminals with a voltmeter without damaging it.
However, never try to check actual circuit voltage by putting the meter in series. The voltmeter hookup should always parallel the circuit being measured, because you do not want the high resistance meter disrupting the circuit in a series connection.
CLOSED CIRCUIT VOLTAGE
Closed Circuit Voltages
The voltage at "A" is 12 volts positive. We drop six volts over the 1.0-ohm resistor and read 6 volts at "B." We drop the remaining voltage in the fan load and read zero positive at "C".
Voltage should not be observed at the ground side of a circuit. If voltage is present on the ground side of a circuit this voltage indicates a current back feed or short (wires touching) from another circuit. Fuses should be removed until voltage reading is 0 V. This will identify the circuit that is the cause of the short or back feed. If voltage is present at the power and also the ground sides of a circuit, current can not flow. The load device cannot function.
OPEN CIRCUIT VOLTAGE
Open Circuit Voltages
Now we read the voltage in the same circuit as above but with no current flowing because there is an open at point "X." The voltage at "A," "B," and "C" will all be 12 volts positive.
Remember, there is no voltage drop across a resistor or load if there is no current flowing.
USING A VOLTMETER (DIRECT VOLTAGE DROP)
To check the same circuit to determine if a resistance exists within the conductor:
Connect a voltmeter between the ends of the conductor, the Positive (red) lead to the supply end, and the Negative (black) lead to the load end. Switch on the current supply; a reading of approximately 0.1 volt would be an acceptable figure. A higher reading would confirm, as the ohmmeter does, that a higher resistance than the natural resistance of the conductor exists.
You may be wondering how the voltmeter gives a reading under these conditions. All test instruments have resistance within their structure; in the case of a voltmeter, that resistance is very high. In the application of the voltmeter as illustrated, the voltage in the circuit will take the least path of resistance, that is the conductor.
If a resistance higher than the natural resistance exists within the conductor, then the voltage will divide to flow through both the conductor and the voltmeter, with the meter then showing a higher reading.
USING AN OHMMETER (NO POWER)
If one wishes to check for resistance within the conductor, ensure that the supply is switched OFF or disconnected, then connect an ohmmeter between the ends of the conductor and note the reading. In the case of a good conductor it should read approximately Zero (ohms). A higher reading would confirm that an unwanted resistance exists.
AMPERAGE - USE
Current flow in a circuit is determined by the total resistance of the circuit and voltage. Current checks are useful for finding specific circuit problems. Examples of effective current tests would include starter amperage draw tests or charging amperage. Amperage is also useful in diagnosing chassis wiring problems such as partially cut wires or improperly crimped wire terminals (see Fuse & Wire Amp Ratings for approximate circuit amp flow).
You measure current draw or flow with an ammeter. But unlike the parallel voltmeter, you must connect the ammeter in series with the load to read the current draw. That means disconnecting the load and reconnecting with all the current going through the meter. Polarity must be followed, the red lead going to the positive side. Inductive type ammeters are clamped around the wires and measure amperage by means of the magnetic flux that is proportional to current flow.
Always use an ammeter that can handle the expected current since excessive current can damage a meter. Also, never connect an ammeter across a circuit (parallel) or you may damage the meter or the circuit.
MEASURING CURRENT FLOW
CIRCUIT RESISTANCE MUST LIMIT CURRENT FLOW
TOTAL CURRENT EQUALS SUM OF BRANCH CURRENTS
In electrical troubleshooting it is often necessary to measure current flow. Since the ammeter must measure all of the current flowing in a circuit, the ammeter has very low resistance. Because it is a low-resistance instrument, it is very delicate and must be used and connected with care.
An ammeter is always connected directly into a circuit in series:.. it becomes part of the circuit. The circuit resistance must be high enough to limit current flow to an amount less than the maximum capacity or rating of the ammeter. In other words, the resistance of the units in the circuit must protect the ammeter against an overload.
Never connect an ammeter across a circuit or full battery voltage will push excessive current through the meter and damage or destroy it.