achris
More fish than mountain goat
- Joined
- May 19, 2004
- Messages
- 27,468
I have put this together quickly and posted here. Comments and suggestions welcome, and if enough people like I can add to the stickies.
Voltage drop test.
Most circuits are made up of a voltage source (battery, alternator), connecting cables and a load, like a starter motor, light bulb or ignition system. And most of the time we have other things in the circuit too, like fuses and switches.
Diagrammatically it would look like this.
Theoretically, in this circuit the light is the only load on the voltage source, but in practice the switch, the fuse and the connections (the large black dots, and these become very important) also impose their own load, albeit in most cases, extremely small. But if these small loads becomes excessive, the light will glow dimly, or not at all. This is where a voltage drop test works well.
If a voltmeter (your multimeter set to the DCV range) is put across the battery it will read battery voltage, and this is how most people use a voltmeter. For clarity, on the drawings I will use a red dot to signify the red (positive) lead of the voltmeter and a grey dot for the black (negative) lead of the meter. Saves having to constantly draw the leads from the test points to the meter.
We would also look for 12 volts to be across the bulb.
But what if that reading was less, like this.
The bulb would glow dimly, and you know you have a voltage loss somewhere, but how to find it? Its a lot easier than most people think. You put the meter probes across each device (switch and fuse) and each connection until you find those lost volts. Also be aware that you may be losing volts across more than one device or connection. The other thing is this test only works when the current is flowing, you must have the switch ON.
Nothing wrong with that connection.
We now walk the meter to the next device/connection.
And nothing wrong with that connection. Move along to the next device (the switch).
Now, this is a problem. A switch shouldnt show that high a voltage drop across it, but it still not all the voltage that has disappeared. From the original 12 volts we have 8.2 across the bulb and 2.1 across the switch. Were still looking for another 1.7 volts somewhere. So we keep checking.
A fuse should also not show this high, it should be just like a connection. This will also need to be investigated. But were still missing 1.2 volts. Keep checking the other connections.
And there we have it! A bad ground connection.
All these tests rely on current flowing in the circuit. If the switch was open, then all the voltage would be dropped across the switch and the other bad connections wouldnt show up.
For those with an electrical bent, it uses the formula V=I x R (Ohms' Law). The voltage is the product of current and resistance. And that is all a bad connection is, higher than normal resistance. Using practical numbers, the resistance of any connection should be 0 ohms, so the voltage would be 0 x the current (for a light bulb of 12w) 1amp, 0volts. If the connection develops a resistance (and therefore becomes a bad connection), say 1.2 ohms, then in the same circuit the voltage drop across the bad connection would be 1.2 x 1, or 1.2 volts. But if the switch is open, then the current will be 0, and any resistance across a connection times 0 will be 0 volts. Thats why we need the circuit in operation to make these tests.
As well as moving both leads to across a single device as in the drawings above, we can test multiple devices at once, but in this test we leave the red lead on the battery positive. Its also very important to be probing the actual battery post, not the terminal connected to it, lest the connection between post and terminal also be resistive Though, once you rule the terminal connection as good, you can use it.
Here we have determined a problem exists in the connections and devices between the battery positive post and the bulb end of the fuse. It would then require each device be tested as previously shown to find the individual problems. This test is used to eliminate large numbers of devices and connections in a single test. If that test had shown 0.04 volts, we then know all those connections and devices are good and dont require further testing. A bit of a timesaver.
The voltage drop test can also find completely open connections too, as the meter completes the circuit. If the switch is closed but the light doesnt come on at all we would still use the same procedure. Lets say the connection on the battery negative was very bad and not allowing any current to flow. When the meter leads were put across all the other devices it would show 0 volt, until the probes were on the last connection. It would look like this.
I hope this helps in understanding this very valuable use of a voltmeter, and helps you find problems more quickly.
Chris
Voltage drop test.
Most circuits are made up of a voltage source (battery, alternator), connecting cables and a load, like a starter motor, light bulb or ignition system. And most of the time we have other things in the circuit too, like fuses and switches.
Diagrammatically it would look like this.
Theoretically, in this circuit the light is the only load on the voltage source, but in practice the switch, the fuse and the connections (the large black dots, and these become very important) also impose their own load, albeit in most cases, extremely small. But if these small loads becomes excessive, the light will glow dimly, or not at all. This is where a voltage drop test works well.
If a voltmeter (your multimeter set to the DCV range) is put across the battery it will read battery voltage, and this is how most people use a voltmeter. For clarity, on the drawings I will use a red dot to signify the red (positive) lead of the voltmeter and a grey dot for the black (negative) lead of the meter. Saves having to constantly draw the leads from the test points to the meter.
We would also look for 12 volts to be across the bulb.
But what if that reading was less, like this.
The bulb would glow dimly, and you know you have a voltage loss somewhere, but how to find it? Its a lot easier than most people think. You put the meter probes across each device (switch and fuse) and each connection until you find those lost volts. Also be aware that you may be losing volts across more than one device or connection. The other thing is this test only works when the current is flowing, you must have the switch ON.
Nothing wrong with that connection.
We now walk the meter to the next device/connection.
And nothing wrong with that connection. Move along to the next device (the switch).
Now, this is a problem. A switch shouldnt show that high a voltage drop across it, but it still not all the voltage that has disappeared. From the original 12 volts we have 8.2 across the bulb and 2.1 across the switch. Were still looking for another 1.7 volts somewhere. So we keep checking.
A fuse should also not show this high, it should be just like a connection. This will also need to be investigated. But were still missing 1.2 volts. Keep checking the other connections.
And there we have it! A bad ground connection.
All these tests rely on current flowing in the circuit. If the switch was open, then all the voltage would be dropped across the switch and the other bad connections wouldnt show up.
For those with an electrical bent, it uses the formula V=I x R (Ohms' Law). The voltage is the product of current and resistance. And that is all a bad connection is, higher than normal resistance. Using practical numbers, the resistance of any connection should be 0 ohms, so the voltage would be 0 x the current (for a light bulb of 12w) 1amp, 0volts. If the connection develops a resistance (and therefore becomes a bad connection), say 1.2 ohms, then in the same circuit the voltage drop across the bad connection would be 1.2 x 1, or 1.2 volts. But if the switch is open, then the current will be 0, and any resistance across a connection times 0 will be 0 volts. Thats why we need the circuit in operation to make these tests.
As well as moving both leads to across a single device as in the drawings above, we can test multiple devices at once, but in this test we leave the red lead on the battery positive. Its also very important to be probing the actual battery post, not the terminal connected to it, lest the connection between post and terminal also be resistive Though, once you rule the terminal connection as good, you can use it.
Here we have determined a problem exists in the connections and devices between the battery positive post and the bulb end of the fuse. It would then require each device be tested as previously shown to find the individual problems. This test is used to eliminate large numbers of devices and connections in a single test. If that test had shown 0.04 volts, we then know all those connections and devices are good and dont require further testing. A bit of a timesaver.
The voltage drop test can also find completely open connections too, as the meter completes the circuit. If the switch is closed but the light doesnt come on at all we would still use the same procedure. Lets say the connection on the battery negative was very bad and not allowing any current to flow. When the meter leads were put across all the other devices it would show 0 volt, until the probes were on the last connection. It would look like this.
I hope this helps in understanding this very valuable use of a voltmeter, and helps you find problems more quickly.
Chris
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