Voltage Meter (DVM/VOM)

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The digital volt meter or volt-ohm-meter is primarily used as a device to measure voltages, currents and resistance,
Meter.jpg
although some models have additional functions for measuring capacitance, transistors and diodes and even more. The DVM/VOM will likely become the most-used diagnostic tool in your toolkit, especially when you're building electronic projects. You don't need to spend a lot of money on one, but understand that a $75 DVM would probably be more accurate and have more capabilities than a $7 unit. Also know that these devices use batteries and batteries don't last forever. As the battery discharges the meter will become less accurate, which can easily cause the misdiagnosis of a problem. Replace the battery at least annually. They also don't bounce when you drop them -- at least they don't bounce very well or very many times. Take care of your DVM.


Selector Dial

Most meters have a dial that is used to select a specific mode for the meter to be in. Typical meter selections are to measure A/C voltage, DC voltage, resistance (in Ohms) and current (in either milliamps or full amps). When measuring something for the first time, select the HIGHEST range first. This gives you more margin for error and will help prevent damaging the meter. For example, if you were measuring the A/C voltage of portable generator's A/C outlet, you would select the highest range for A/C testing which might be 600 volts, and then make your first test. You can always dial down to a lesser level once you get an initial reading. Meters are more accurate when the measuring range is closer to and just slightly larger than the item being tested, so instead of seeing 120 on the DVM, you might get 120.5 volts instead. It doesn't seem that a half volt is very important and much of the time, it isn't. But when you get down to low voltage DC, a half of a volt can mean the difference to whether a part functions or not.

Dvm dial.jpg
  • In the above example, the top white portions of the dial on either side of the "OFF" mark are voltage ranges; the left side is DC (V---) while the right side is for AC current (marked with the V~). Most meters have more range options for measuring DC current than AC. The selector dial is pointed to 20 DC which means the meter is set to measure voltages up to 20vdc. Good meters have built-in protection to prevent meter damage in case the range selected isn't high enough for the voltage being measured, but it's always best to start high and work your way lower as needed. But as you gain experience, you'll find that most DC voltages on controller circuit boards fall into the <20vdc range.
  • The lower left green section of this example dial shows the Greek omega symbol, which stands for "Ohms." This is the measure for resistance. Setting the dial to the 20k mark would set the meter to measure from 0-20,000 ohms of resistance. The K represents thousands of ohms; M is millions. Setting your meter to 20k will measure most situations in this hobby.
  • The green section on the right side marked A--- stands for current. Setting the dial to the 20m mark would set the meter to measure up to 20 milliamps of current. The bottom setting, 10A would measure up to 10 Amps, a very high value. Milliamps are thousandths of an amp (.001 = 1ma). At the very top is 200u, which is microamps (.000001 = 1ua). Most digital electronics on a controller circuit board is usually in the milliamps ranges while measuring the current a light string might use is usually in the amps range or at least higher than the 200ma setting on this meter example.
  • This meter dial also has a setting for hFE (for measuring transistors) and another curious red setting down near the bottom of the dial. That's a continuity tester that emits a beep if you're testing something just to see if there's a completed circuit. Remember, for electricity to work there must be a source and return to have a completed circuit. A continuity tester can be helpful for testing plugs or wires to see if there's a break in the wire.
  • This example shows a "backlight" button next to the dial. Pressing this illuminates the display for a few seconds -- a handy convenience when you're outside at night or in other low-light situations.

Safety

You'll often measure live circuits. Sometimes those circuits carry only a handful of volts or a few milliamps of current -- harmless amounts. Other times, they might be carrying a lethal voltage or current that can knock you across the room if you're not careful. Meters usually come with a pair of "pencil-type" leads which require you to usually hold one with each hand. These leads have insulated handles -- never let a finger touch the metal part during testing! The leads generally have sharp points for a reason -- to help you zero-in on a specific test point. If you wear glasses, always wear them when you're using your meter so you don't accidentally miss the target test points. Missing a test point can cause a short circuit and damage something -- maybe even you! Also remember to use the proper kinds of test leads. If you need to attach a pair of leads to a circuit yet have one or both of your hands free, use leads that have spring clips so you can clamp them on the parts so the leads won't slip and possibly touch some other "hot" parts of the circuit and cause a short. Seeing numbers on your meter's display is good; seeing sudden flashes of light, smoke and and electrical sparks is generally bad.

Understanding 'Parallel' vs 'Series' circuits

Using your meter sometimes requires testing in a 'series' orientation, and sometimes in 'parallel.' Therefore, it's important that you understand the difference between the two circuit types. The basic difference is that in a parallel circuit, the electricity has multiple paths from the source to the return, which on a battery, are the + and - terminals; in a series circuit, there is only one path and all the electricity has to flow through it. In a parallel circuit, the flow of electricity doesn't stop if one of the paths is broken because there are other paths for the electricity to flow from the source to the return. But in a series circuit, breaking the path anywhere in the series stops the flow of electricity completely. A simple diagram is particularly helpful here:

Circuit types.jpg

How to measure Voltage

Testing the output from a battery or an AC to DC adapter (aka "wall wart") are a couple common uses for your meter. Testing is done by selecting a DC mode that's larger than the expected output of the device and then touching the meter's red lead to the + terminal (or red wire) and the meter's black lead to the - terminal (or black wire). The meter will then display the voltage between those two test points. Depending on the meter you have, reversing the leads (red to - and black to +) will either show the voltage as a negative value or a zero voltage and if you're using a meter that has a visual needle display that swings left and right, the needle will peg itself against the far left edge and remain there. Measuring a wall wart's output voltage can produce some surprising results: wall warts that say they output 5, or 9, or 12 volts often measure quite differently, and if you use them to power projects be sure to measure them beforehand!

When assembling controllers or other electronic projects, you'll often need to verify that voltages are within spec before putting that item into use. If voltages are too high, it could damage the electronic parts and if too low, the parts may not function properly. For example, measuring the output of a voltage regulator or the supply voltage to a PIC chip on the circuit board are common, everyday events in the DIY world as you build your gear.

For example, the 7805 voltage regulator, a 5vdc regulator which is frequently used in the TO-220 style case is pictured below. The +/- measurement test points are marked here in red (+) and black (-). Notice that a voltage regulator has TWO positive points: one is the input voltage which might be 10 or more volts and the regulated, 5vdc output side. The other leg is marked in black for it is common to both the input and output sides of the regulator, and it is called the "ground." In DC electronics, "ground" and the negative point are usually synonymous. Also notice that the metal tab at the top of the regulator is marked in black, showing that it shares the ground leg. So touching the meter's black lead to the metal tab is the same as touching the ground leg.
7805regulator.jpg
Here's another example, taken from a section of the datasheet for the PIC16F688 microcontroller chip, one of the common PIC chips that are used in many Renard controllers. The + voltage pin is marked in red; on a chip it's commonly called the Vdd or Vcc pin; the ground pin is marked in black and it's common name may be marked either Vss or Vee. Measuring voltage at these two pins, you'd expect to see 5.0vdc (or very close to it) for the chip to function properly. This chip can tolerate up to 6.5vdc and if the measured voltage was greater than that, the chip would likely be damaged.
Pic16f688.jpg
  • Note: Not all electronic parts share the same +/- orientation and positioning, and the only way to know which pins/legs are what polarity is to consult the part's datasheet. Datasheets are easy to locate via an Internet search tool and most parts supply companies usually provide direct links to them from their online catalogs.
  • DC voltages are always measured between a positive test point and ground. (This is usually a 'parallel' circuit test although in some situations, it could turn out to be a series measurement.)
  • AC voltages are always measured between the 'HOT' and the 'common' lines, but never between 'HOT' and 'ground.' For more information on homeowner AC wiring, visit your local hardware store or home improvement center and pick up a booklet on the subject.
  • CAUTION: On circuit boards where both DC and AC voltages are present, NEVER test from one to the other. If you do, you may very likely experience the unwanted sudden flash of light, smoke and sparks. So only test from DC to DC or from AC to AC, and never across DC and AC.

How to measure Current

Current is a measurement of the quantity of electricity going through the circuit. To do this, the meter must be put in series with the circuit; current measurement is a series type of connection. To accomplish it, you "break" the circuit and splice the meter into the break so the electricity must flow through the meter to complete the circuit:

Current measurement.jpg
  • CAUTION: Remember to set the meter to its highest current setting before connecting the meter into the circuit or the meter may become damaged. A common meter maximum value is 10A (amperes). If you think that the current may be more than the maximum setting for your meter, don't use it; go buy something else that has a higher limit.
  • CAUTION: On circuit boards, measuring current sometimes means you must un-solder or remove a part to splice the meter into the circuit. Applying the meter's leads otherwise such as you'd do for measuring voltage creates a 'parallel' measurement which is not an accurate method for measuring electrical current and may create a short circuit.

How to measure Resistance

Resistance is a ability of an electronic part to slow the flow of electricity through a circuit, and while all electronic parts have some sort of resistance, the most common part used for this is the "resistor." Resistors are often used in various combinations to fine-tune the flow of electricity to make a circuit react a specific way.

Resistor.jpg
Set the meter to measure a mid-range value, such as 20k ohms. No harm will come to the meter or the part if the setting is too high or low; if the setting is too low, the meter won't be able to measure it and you may see an error message. If the setting is too high, you may get a zero value because the meter's readout doesn't have enough decimal points to display the value.
Touch the meter's leads to points on opposite sides of the part. This lets a small flow of current from the meter's battery to flow through the part and the resistance to that flow will be displayed on the meter's readout. Depending on the measurement range you selected, you must interpret the reading relative to the selected range. Example: if the meter is set to a 20k range and the readout shows .672, it means the part has 672 ohms of resistance.
  • Most resistance testing should be performed with the circuit powered OFF and disconnected from all power.
  • Don't willy-nilly poke around a completed circuit board to test things for fun. The meter uses a battery and not every part on the circuit board will be tolerant of the sudden jolt of electricity; some parts can be damaged.
  • CAUTION: Use great caution when testing a part in a live AC circuit so you don't accidentally get a shock. And in general, be careful so you don't accidentally create a short circuit when the object being tested is powered on.

How to measure Circuit Continuity

Because electricity always needs to have a completed circuit for the electricity to flow and because everything that's electrically conductive has at least some kind of resistance, you can use your meter to test for this, too. One example might be if you are making a cable and want to verify that the connector on one end makes a connection on the other end.

Set the meter to a low resistance value, such as 200 ohms. Touch the two ends of the cable and watch the meter readout: it should start changing and eventually give you a measurement. If the meter readout never changes, it means the two ends you're touching are not connected to one another and you do not have continuity between the two.
*CAUTION: Do not use this technique on a live, powered circuit.