Using an electrical meter safely and efficiently is perhaps the most valuable skill an electronics technician can master, both for the sake of their own personal safety and for proficiency at their trade. It can be daunting at first to use a meter, knowing that you are connecting it to live circuits that may harbor life-threatening levels of voltage and current.

This concern is not unfounded, and it is always best to proceed cautiously when using meters. Carelessness more than any other factor is what causes experienced technicians to have electrical accidents.

Multimeters

The most common piece of electrical test equipment is a meter called the multimeter. Multimeters are so named because they have the ability to measure multiple variables: voltage, current, resistance, and often many others, some of which cannot be explained here due to their complexity.

In the hands of a trained technician, the multimeter is both an efficient work tool and a safety device. In the hands of someone ignorant and/or careless, however, the multimeter may become a source of danger when connected to a “live” circuit.

There are many different brands of multimeters, with multiple models made by each manufacturer sporting different sets of features. The multimeter shown here in the following illustrations is a “generic” design, not specific to any manufacturer, but general enough to teach the basic principles of use:

You will notice that the display of this meter is of the “digital” type: showing numerical values using four digits in a manner similar to a digital clock. The rotary selector switch (now set in the Off position) has five different measurement positions it can be set in: two “V” settings, two “A” settings, and one set in the middle with a funny-looking “horseshoe” symbol on it representing “resistance.”

The “horseshoe” symbol is the Greek letter “Omega” (Ω), which is the common symbol for the electrical unit of ohms.

Of the two “V” settings and two “A” settings, you will notice that each pair is divided into unique markers with either a pair of horizontal lines (one solid, one dashed) or a dashed line with a squiggly curve over it. The parallel lines represent “DC” while the squiggly curve represents “AC.” The “V” of course stands for “voltage” while the “A” stands for “amperage” (current).

The meter uses different techniques, internally, to measure DC than it uses to measure AC, and so it requires the user to select which type of voltage (V) or current (A) is to be measured. Although we haven’t discussed alternating current (AC) in any technical detail, this distinction in meter settings is an important one to bear in mind.

Multimeter Sockets

There are three different sockets on the multimeter face into which we can plug our test leads. Test leads are nothing more than specially-prepared wires used to connect the meter to the circuit under test.

The wires are coated in a color-coded (either black or red) flexible insulation to prevent the user’s hands from contacting the bare conductors, and the tips of the probes are sharp, stiff pieces of wire:

The black test lead always plugs into the black socket on the multimeter: the one marked “COM” for “common.” The red test leads plug into either the red socket marked for voltage and resistance or the red socket marked for current, depending on which quantity you intend to measure with the multimeter.

To see how this works, let’s look at a couple of examples showing the meter in use. First, we’ll set up the meter to measure DC voltage from a battery:

Note that the two test leads are plugged into the appropriate sockets on the meter for voltage, and the selector switch has been set for DC “V”. Now, we’ll take a look at an example of using the multimeter to measure AC voltage from a household electrical power receptacle (wall socket):

The only difference in the setup of the meter is the placement of the selector switch: it is now turned to AC “V”. Since we’re still measuring voltage, the test leads will remain plugged in the same sockets.

In both of these examples, it is imperative that you not let the probe tips come in contact with one another while they are both in contact with their respective points on the circuit. If this happens, a short-circuit will be formed, creating a spark and perhaps even a ball of flame if the voltage source is capable of supplying enough current! The following image illustrates the potential for hazard:

This is just one of the ways that a meter can become a source of the hazard if used improperly.

Voltage measurement is perhaps the most common function a multimeter is used for. It is certainly the primary measurement taken for safety purposes (part of the lock-out/tag-out procedure), and it should be well understood by the operator of the meter.

Being that voltage is always relative between two points, the meter must be firmly connected to two points in a circuit before it will provide a reliable measurement. That usually means both probes must be grasped by the user’s hands and held against the proper contact points of a voltage source or circuit while measuring.

Because a hand-to-hand shock current path is the most dangerous, holding the meter probes on two points in a high-voltage circuit in this manner is always a potential hazard. If the protective insulation on the probes is worn or cracked, it is possible for the user’s fingers to come into contact with the probe conductors during the time of test, causing a bad shock to occur. If it is possible to use only one hand to grasp the probes, that is a safer option.

Sometimes it is possible to “latch” one probe tip onto the circuit test point so that it can be let go of and the other probe set in place, using only one hand. Special probe tip accessories such as spring clips can be attached to help facilitate this.

Remember that meter test leads are part of the whole equipment package and that they should be treated with the same care and respect that the meter itself is. If you need a special accessory for your test leads, such as a spring clip or other special probe tip, consult the product catalog of the meter manufacturer or other test equipment manufacturer.

Do not try to be creative and make your own test probes, as you may end up placing yourself in danger the next time you use them on a live circuit.

Also, it must be remembered that digital multimeters usually do a good job of discriminating between AC and DC measurements, as they are set for one or the other when checking for voltage or current.

As we have seen earlier, both AC and DC voltages and currents can be deadly, so when using a multimeter as a safety check device you should always check for the presence of both AC and DC, even if you’re not expecting to find both! Also, when checking for the presence of hazardous voltage, you should be sure to check all pairs of points in question.

For example, suppose that you opened up an electrical wiring cabinet to find three large conductors supplying AC power to a load. The circuit breaker feeding these wires (supposedly) has been shut off, locked, and tagged. You double-checked the absence of power by pressing the Start button for the load. Nothing happened, so now you move on to the third phase of your safety check.

First, you check your meter on a known source of voltage to see that it’s working properly. Any nearby power receptacle should provide a convenient source of AC voltage for a test. You do so and find that the meter indicates as it should. Next, you need to check for voltage among these three wires in the cabinet. But voltage is measured between two points, so where do you check?

The answer is to check between all combinations of those three points. As you can see, the points are labeled “A”, “B”, and “C” in the illustration, so you would need to take your multimeter (set in the voltmeter mode) and check between points A & B, B & C, and A & C.

If you find voltage between any of those pairs, the circuit is not in a Zero Energy State. But wait! Remember that a multimeter will not register DC voltage when it’s in the AC voltage mode and vice versa, so you need to check those three pairs of points in each mode for a total of six voltage checks in order to be complete!

However, even with all that checking, we still haven’t covered all possibilities yet. Remember that hazardous voltage can appear between a single wire and ground (in this case, the metal frame of the cabinet would be a good ground reference point) in a power system.

So, to be perfectly safe, we not only have to check between A & B, B & C, and A & C (in both AC and DC modes), but we also have to check between A & ground, B & ground, and C & ground (in both AC and DC modes)! This makes for a grand total of twelve voltage checks for this seemingly simple scenario of only three wires. Then, of course, after we’ve completed all these checks, we need to take our multimeter and re-test it against a known source of voltage such as a power receptacle to ensure that it’s still in good working order.

Using a Multimeter to Check For Resistance

Using a multimeter to check for resistance is a much simpler task. The test leads will be kept plugged in the same sockets as for the voltage checks, but the selector switch will need to be turned until it points to the “horseshoe” resistance symbol. Touching the probes across the device whose resistance is to be measured, the meter should properly display the resistance in ohms:

One very important thing to remember about measuring resistance is that it must only be done on de-energized components! When the meter is in “resistance” mode, it uses a small internal battery to generate a tiny current through the component to be measured.

By sensing how difficult it is to move this current through the component, the resistance of that component can be determined and displayed. If there is an additional source of voltage in the meter-lead-component-lead-meter loop to either aid or oppose the resistance-measuring current produced by the meter, faulty readings will result. In a worse-case situation, the meter may even be damaged by the external voltage.

The “Resistance” Mode Of A Multimeter

The “resistance” mode of a multimeter is very useful in determining wire continuity as well as making precise measurements of resistance. When there is a good, solid connection between the probe tips (simulated by touching them together), the meter shows almost zero Ω. If the test leads had no resistance in them, it would read exactly zero:

If the leads are not in contact with each other or touching opposite ends of a broken wire, the meter will indicate infinite resistance (usually by displaying dashed lines or the abbreviation “O.L.” which stands for “open loop”)

Measuring Current with a Multimeter

By far the most hazardous and complex application of the multimeter is in the measurement of current. The reason for this is quite simple: in order for the meter to measure current, the current to be measured must be forced to go through the meter.

This means that the meter must be made part of the current path of the circuit rather than just be connected off to the side somewhere as is the case when measuring voltage. In order to make the meter part of the current path of the circuit, the original circuit must be “broken” and the meter connected across the two points of the open break. To set the meter up for this, the selector switch must point to either AC or DC “A” and the red test lead must be plugged in the red socket marked “A”.

The following illustration shows a meter all ready to measure current and a circuit to be tested:

Now, the circuit is broken in preparation for the meter to be connected:

The next step is to insert the meter in line with the circuit by connecting the two probe tips to the broken ends of the circuit, the black probe to the negative (-) terminal of the 9-volt battery, and the red probe to the loose wire end leading to the lamp:

This example shows a very safe circuit to work with. 9 volts hardly constitutes a shock hazard, and so there is little to fear in breaking this circuit open (barehanded, no less!) and connecting the meter in line with the flow of current. However, with higher power circuits, this could be a hazardous endeavor indeed.

Even if the circuit voltage was low, the normal current could be high enough that an injurious spark would result at the moment the last meter probe connection was established.

Another potential hazard of using a multimeter in its current-measuring (“ammeter”) mode is the failure to properly put it back into a voltage-measuring configuration before measuring the voltage with it. The reasons for this are specific to ammeter design and operation. When measuring circuit current by placing the meter directly in the path of the current, it is best to have the meter offer little or no resistance to current flow.

Otherwise, the additional resistance will alter the circuit’s operation. Thus, the multimeter is designed to have practically zero ohms of resistance between the test probe tips when the red probe has been plugged into the red “A” (current-measuring) socket. In the voltage-measuring mode (red lead plugged into the red “V” socket), there are many mega-ohms of resistance between the test probe tips, because voltmeters are designed to have close to infinite resistance (so that they don’t draw any appreciable current from the circuit under test).

When switching a multimeter from current- to voltage-measuring mode, it’s easy to spin the selector switch from the “A” to the “V” position and forget to correspondingly switch the position of the red test lead plug from “A” to “V”. The result—if the meter is then connected across a source of substantial voltage—will be a short-circuit through the meter!

To help prevent this, most multimeters have a warning feature by which they beep if ever there’s a lead plugged in the “A” socket and the selector switch is set to “V”. As convenient as features like these are, though, they are still no substitute for clear thinking and caution when using a multimeter.

All good-quality multimeters contain fuses inside that are engineered to “blow” in the event of excessive current through them, such as in the case illustrated in the last image. Like all overcurrent protection devices, these fuses are primarily designed to protect the equipment (in this case, the meter itself) from excessive damage, and only secondarily to protect the user from harm.

A multimeter can be used to check its own current fuse by setting the selector switch to the resistance position and creating a connection between the two red sockets like this.

A good fuse will indicate very little resistance while a blown fuse will always show “O.L.” (or whatever indication that model of multimeter uses to indicate no continuity). The actual number of ohms displayed for a good fuse is of little consequence, so long as its an arbitrarily low figure.

So now that we’ve seen how to use a multimeter to measure voltage, resistance, and current, what more is there to know? Plenty! The value and capabilities of this versatile test instrument will become more evident as you gain skill and familiarity using it.

There is no substitute for regular practice with complex instruments such as these, so feel free to experiment on safe, battery-powered circuits.

source:https://www.allaboutcircuits.com/textbook/direct-current/chpt-3/safe-meter-usage/

See more:

Why You Should Have a Multimeter When Doing Any Electrical Work

Guide to digital multimeter safety

The bedroom is not only a place to sleep but also a private space for you to relax, watch movies, read books after a tiring day of work. And to choose a bedroom light that is suitable for the bedroom space but still has to meet the function when used is not easy.

The following article, Ceiling Lights will introduce to you how to choose the best bedroom lights today and help you gain more experience in using bedroom lights.

1. How to choose the most suitable bedroom lighting?

First: According to the color of the bedroom

When choosing a bedroom light, you need to pay attention to the color of the walls and other furniture in the bedroom so that the light is balanced. Normally, the bedroom does not require strong light, mainly warm colors (most commonly yellow) with low luminous flux to create lightness and relaxation, helping the eyes to relax and go easily. into sleep. Besides, the light in the room must not create glare angles that cause inhibition to the eyes.

Therefore, you should choose a night light that can be adjusted to many different light intensities to create favorable conditions for everyone to use.

Second: Location of bedroom lights

The arrangement of night lights is very important because a mistake that many people often make when arranging night lights is to install them right above the bed or on the ceiling, especially the type of chandelier or lantern that often needs to be installed. rack bar design. These are not good positions because violating the taboo in feng shui is “the crossbeam over the bed”.

Installing night lights in these locations both creates a feeling of insecurity and increases psychological pressure, which can easily cause health problems such as insomnia, nightmares, poor breathing, etc. Poor sleep causes the couple’s relationship to be affected, easy to quarrel and quarrel.

Third: Choose by lamp size

A note for you, when choosing a bedroom light, pay attention to the size of the lamp, so that it matches the bedroom interior and the room area. For large bedrooms, you should choose a chandelier to let the light spread throughout the room. For bedrooms with a small area, choose tree lights or wall lights.

Fourth: Suitable for the target audience

For each age, it will be suitable for each type of lamp with different uses. For young children, parents should choose bedroom lights with lots of fun textures and colors to keep them out of reach of children to make them happy before going to sleep.

For young singles, they should choose bedroom lights with delicate and minimalist motifs but bring elegance and youthfulness to the bedroom space.

For the elderly, it is necessary to choose easy-to-use night lights with moderate brightness so as not to dazzle or irritate eyes.

*** Read more: Top 9 Best Ceiling Lights For Bedroom [Expert Picks]

2, Synthesis of popular healthy night lights today

Choosing a decorative light for the bedroom is a matter of concern for many families in the design of the bedroom space because the night light not only provides illumination but is also a decorative item for the bedroom. Friend. But it is very difficult to choose a bedroom lamp that ensures the following factors: decoration, lighting and good health. The following is a collection of healthy night light samples that you cannot ignore.

2.1 Wall-mounted night light

Wall-mounted night lights are used quite commonly because of their space-saving advantages because they are installed on the wall, taking advantage of the space on the head of the bed. This type of lamp is often used to decorate small bedrooms. Wall lights come in a variety of designs and shapes. If you decide to choose a wall-mounted night light, you should choose a lamp with a small, moderate size that is suitable for the bedroom area and the brightness of the lamp is softer because the lamp is often hung at the head of the bed.

Wall lamp models are often used to decorate bedrooms with Japanese, Korean, classical, Asian, etc.

2.2 Led Night Lights

The led night light is one of the lamps with a simple design but brings a comfortable and pleasant feeling when entering the room. Currently, decorative LED lights are designed with many different designs, designs, and colors of light to bring suitability for many spaces, as well as meet customer satisfaction. Here are 5 types of LED lights for bedrooms that are most popular today.

+ LED ceiling lights:

Led drop light is a type of LED interior light with a base that is attached to the ceiling combined with a drop cable and a light bulb. LED ceiling light bulbs are available in many different designs and colors that are extremely delicate and beautiful. This is a kind of interior decoration light suitable for modern spaces such as living room, bedroom, kitchen, coffee shop, bar…

*** Read more: https://www.decoist.com/2014-03-21/bedroom-lighting-ideas/?chrome=1

+ Decorative LED chandelier:

In addition to the LED ceiling light, the LED chandelier is also an ideal suggestion for you. Perhaps you think that LED spotlights will be more suitable for the living room, but if the bedroom design has locations where the space is designed deep inside, this type of light will be suitable for you. At this time, decorative chandeliers are the best choice when choosing lights for the bedroom.

+ LED decorative string lights:

Decorative led string lights with many types and designs are suitable for interior decoration and outdoor decoration such as garden, street, cafe space… Led string lights when used in combination with many types of decorative lights Other furniture such as led ceiling lights, led recessed lights, etc. make the interior space more wonderful. Led string lights have many different colors, choose for your bedroom the led string light system with soft colors, moderate brightness to help you easily fall asleep deeper and better.

Please use a fixed color for the entire led string light system, limit the use of many colors to decorate the bedroom. Although beautiful, it is not suitable for a quiet space that needs relaxation.

+ LED wall lights

LED string lights for bedroom decoration are great, but you should not use this type for the main lighting of the room.

2.3 Chandeliers

Decorative chandeliers with sophisticated and luxurious designs are often pickier about users because they are suitable for large bedroom spaces and luxurious bedroom design styles. Therefore, if you love this lamp model, you should choose a product that is suitable for the size, interior space as well as design style of the lamp itself to avoid causing inconsistency in the whole. Bedroom.

Decorative chandeliers with sophisticated and luxurious designs are often pickier about users because they are suitable for large bedroom spaces and luxurious bedroom design styles. Therefore, if you love this lamp model, you should choose a product that is suitable for the size, interior space as well as design style of the lamp itself to avoid causing inconsistency in the whole. Bedroom.

*** Read more: Top 7 Best handheld spotlight for boating Expert Picks [2022]

2.4 Bedroom table lamp

When it comes to healthy night lights, the bedroom table lamp is an indispensable choice. With a beautiful, luxurious design and an affordable price, the bedroom table lamp is the choice of many families today. Simple to use, you just need to put it on the bedside table or nightstand to both make a night light and read a book before sleeping. The current table lamp can adjust the light intensity very conveniently, the lampshade with many different designs and materials can be used to decorate and beautify the bedroom space to become more vivid.

3. Some serious mistakes when choosing bedroom lights

– Do not pay attention to color and light when choosing lights for the bedroom:

Light is an extremely important factor and you need to pay attention to it first because this is the factor that determines whether your sleep is good or not. Therefore, when choosing a bedroom decoration lamp, you should choose lights with light, soft colors that do not cause glare to get a deeper sleep to help the body stay healthy and relax well, avoid choosing night lights. too many colors.

– Choosing the wrong lamp size:

Depending on the space of each bedroom, you can choose the right type of lamp, the basic rule is that the small room should not be too big. If your bedroom space is too small, you should not choose large night lights but should prioritize choosing table lamps placed on the head of the bed to optimize the space as much as possible.

If your bedroom space is spacious, the chandelier will help to add elegance and sophistication, but when choosing a bedroom chandelier, you should use copper chandeliers or low-color chandeliers to ensure privacy. your bedroom. And depending on the object of use, you should choose the bedroom light more carefully.

– Choosing the wrong place to place the lights:

Choosing the wrong lamp position will increase the psychological pressure on people lying down and easily cause health-related problems such as frequent insomnia, poor breathing, or even nightmares. Therefore, you should not place the night light above the sleeping path or the ceiling, because it adversely affects health and creates a feeling of insecurity.

– Select lights without adjustment mode:

Choosing a night light without an adjustment mode makes it difficult for users to adjust the light and color. Therefore, when choosing a lamp for the bedroom, you should choose a night light that has a color adjustment mode for convenience for users and when changing the color, the light power of the lamp will make the room become brighter. more fresh. At the same time, different colors will help your mood become more comfortable and have a deeper sleep.