Okay, here's what I put together last night. I tried to start out as basic as possible so anyone with no knowledge of electrical circuits can get the information we need. With the help of your suggestions, I'll fine tune this later. If you're comfortable using the voltmeter, you can skip to Part 2.
Part 1 Measuring Voltage
Photo 1 shows the inexpensive digital meter from Harbor Freight Tools. Home Depot has an identical one in yellow. These can also be found at Walmart and any hardware store.
Meters in this price range commonly have three jacks for the probes. Photo 2 shows them in more detail. The black, negative probe goes in the bottom jack. That one is used for all of the functions and ranges. The top jack is used for the red, positive probe for only the highest current range. This meter can measure up to 10 amps DC. ("DC" stands for "direct current" which is used in cars and trucks, and can be stored in a battery). This meter can not measure AC current which is found in house wiring. The middle jack is used for every other function and range except the 10-amp range. The probes are plugged in to the two jacks used most often in the third photo.
Finally, in the fourth photo, we're measuring this battery's voltage. It takes four hands to hold two probes and run a camera. I only have two, so here I'm using a pair of clamps to hold the probes on the battery's posts. In the fifth photo, it's the same battery and meter, but it's a different negative probe. The probe was broken, and replaced with an "alligator clip". Now only three hands are needed.
The meter is set to the "20 Volt DC" range. That means it can measure any voltage between 0.00 and 20.0 volts. Actually it can only read to 19.99 volts.
There's two common mistakes that will not damage the meter. The first is switching the probes on the battery's positive and negative posts. The display will include a minus sign indicating a negative voltage. Switch the probes to solve that, however, we're usually just interested in the value, not the polarity, so the minus sign can be ignored. The second mistake is using the wrong scale. In the sixth photo, the meter is set to the "2000 millivolts" range, meaning the 2 volt range. It can only measure up to 1.999 volts, but with an extra decimal place of accuracy. The battery has a much higher voltage than can be measured on this scale, so it's showing a "1". Every digital meter has some means of indicating an "over range" condition. This one uses a "1". Simply rotate the range switch to the next higher range.
Now to add some confusion to this story, there's two ways to take voltage readings. By far the most common is to take a series of readings AT points in the circuit. Voltage is electrical pressure, and it is always taken in relation to some other point. Most of the time that is "ground", and for cars, that includes the battery's negative post and cable, the frame, body sheet metal, and engine that it's connected to. To say that a different way, the black, negative probe can be touched to the battery's negative post or cable, the engine, or any paint and rust-free point on the body.
Everything electrical can be compared to water in a pipe. A pressure gauge can be installed at any point in the pipe and its reading will be in relation to atmospheric pressure. Atmospheric pressure is common to any point you want to measure water pressure, just like ground is the common point for any place you want to measure the voltage. With water pressure or with electrical pressure, (voltage), we keep taking readings along the circuit until something changes. A loss of water pressure, or a loss or drop in a voltage reading indicates we just passed the defect.
The other way voltage is measured is between two points. The negative probe is not on ground.
Suppose there is a partial blockage in the water pipe. There would be full pressure before that blockage, and reduced pressure after it, but only when water is trying to flow through it. We could measure the pressure before the blockage, in relation to atmospheric pressure, then take a second reading after the blockage, also in relation to atmospheric pressure. Subtract one reading from the other to find the amount of pressure loss across the blockage. There's three places to introduce an error.
A simpler way is to take one reading, but instead of in relation to atmospheric pressure, the gauge would be between the points before and after the blockage. One direct reading would show the amount of pressure drop.
The same thing works in electrical circuits. Excessive resistance could develop, such as corrosion between a pair of mating connector terminals. Normal practice is to measure the voltage before that connector, in relation to ground, then again after the connector, again, in relation to ground. Then we'd have to calculate the difference. The direct way is to place one meter probe on each side of that connector, then take just the one reading. Besides avoiding two sources of error, the meter can usually be set to the 2.00 volt scale for more accuracy. This will be especially helpful in high-current circuits such as for starter motors. The resistance that can develop in a connection can be way too small to measure, but the "voltage drop" across it is the result of that resistance, and that can be measured. This is a story for another time.
Images (Click to make bigger)
Friday, May 19th, 2023 AT 11:45 AM