Checking LEDs with a multimeter for operability

LEDs are used to transmit signals from remote controls, equipment, surveillance cameras, flashlights and lamps. They turn on in the forward direction, after the appearance of a positive voltage between the cathode and anode. Therefore, in case of breakdowns, you can check the LED with a multimeter, establish the cause of the malfunction and eliminate it.

Verification steps

Diodes operate at low DC voltage. It is generated by blocks to which it is problematic to connect. But part of the design of the LED is a semiconductor junction, due to which the current is passed in a given direction. If the current is sufficient, the lamp lights up.

Using a multimeter, it is easy to determine the health of the element. To do this, the device is set to dial mode, after which:

1. The probes are thrown onto the section of the semiconductor that needs to be checked.
2. A red probe with a positive charge is connected to the LED anode.
3. A black probe with a negative charge is thrown onto the cathode.
4. The indicator of the voltage drop after the p-n transition should be displayed on the device screen.
5. The polarity of the connection is reversed. In the absence of a voltage drop, the diode is operational.

If the multimeter does not have “Ringing” mode, switch it to 1 ohm with the switch.

Test procedure for LED strip

The LED strip is problematically checked by a multimert, since it does not light up. Weak light occurs when tested in Hfe mode. Testing is also complicated by burnouts not of the diodes themselves, but of the contact tracks or current-carrying sections. To find out about a malfunction:

1. Find conditional identical segments of 3 LEDs along the border of the contacts and the transverse strip.
2. Touch the probes to each area in turn, applying current to the power contacts.
3. Call the power supply unit - it fails due to a load drop.

Resistance testing will provide a complete picture of the integrity of the LEDs.

Features of checking LED bulbs

With the help of a multimeter, you can ring out colored, standard and super-bright diodes.

Standard bulb with E27 base

A similar lamp is used for household chandeliers or lamps. To check a working or non-working LED, you will need:

1. Remove the diffuser from the light bulb using a plastic bank card placed between the element and the body.
2. Carefully move the plastic card over the gluing. A strong seam can be warmed up with a construction hairdryer.
3. Open the board.
4. Touch the elements with the probes and wait until they glow with a dull glow.

If the diodes are off, the bulb is broken.

Superbright diodes

Blue, yellow or white LEDs are usually equipped with a garland. The test is carried out without probes using transistor sockets according to the following algorithm:

1. Determine the pinout of the SMD.
2. Find 8 sockets at the bottom of the device - 4 left for PNP transistors and 4 right ones for NPN transistors.
3. Place the probes by inserting the anode into hole E and the cathode into hole C.
4. Open the PNP element by applying a positive charge to the emitter E. A healthy LED will be on.
5. Reverse the polarity of the NPN transistors. The anode is placed on C, the cathode on E.

Transistor sockets are convenient for testing diodes with long, no-solder contacts.

How to check an LED spotlight

The LED test is carried out after determining the type of the element. The lanterns are installed:

• a board with small SMDs, which are checked for continuity by analogy with a standard light bulb;
• a large yellow cell powered by 10-30 V.

The voltage of a large element is a lot for the tester, therefore, only the driver can determine the operability of the element. It must match the performance of the diode.

Nuances of testing infrared diodes

The infrared LED emits invisible radiation, so it is important to monitor the readings on the multimeter display. The probes are installed by applying a plus to the anode and a minus to the cathode. Touching the probes to the working IR diode, you can see the number 1000 on the screen. When the polarity is reversed, 1 is displayed.

To accurately check the infrared diode with the transistor sockets, a smartphone camera or digital device is used. The IR LED will need to be placed in the transistor jacks and the camera pointed at it. Serviceability is evidenced by a luminous blurred spot on the gadget display.

Soldering the parallel red LED light will visually reflect the efficiency of the diode. If at the moment of flickering a signal is applied to the element, it should be replaced. If the backlight does not work, the remote control is faulty.

LED bridge test

The diode bridge is an assembly of 4 elements. They are connected so that the AC voltage is applied to two of the 4 terminals, turns into a DC voltage and is removed from the other 2 terminals. Zener diodes equalize the voltage in a narrow range.

You can call the LED-bridge like this:

1. Find which pin to connect the multimeter to by making a conditional numbering.
2. Ring the first diode by placing the probes on pins 1 and 2.
3. Test the second LED by connecting the probes to pins 2 and 3.
4. Measure the parameters of the third diode by connecting the probes to pins 1 and 4.
5. Determine the serviceability of the fourth element by throwing the probes on terminals 4 and 3.
6. View readings on the scoreboard.

The voltage stability is checked in the maximum range mode - 220 V. It is increased gradually and stopped until the current flows through the circuit.

The black probe will need to be thrown to the anode, the red one to the cathode, and then connect the anode to the current limiting resistor, and the cathode to the power source.

The specifics of the dialing mode

A multimeter is a versatile tester that tests light emitting diodes and other elements. During operation, the device emits a squeak, or ringing, therefore the mode is called ringing.

The operation of the multimeter in the dial mode has several features:

• the switch is put to test the diodes, the probes are thrown onto the contacts;
• the polarity of the leads is determined, but if it could not be detected, the LED light source will not fail;
• with the correct connection of the probes to the contacts and the correct polarity, the working diode will light up;
• during the dialing process, no current with a high value is supplied, therefore, the backlight of the diodes is visible only in a darkened room;
• in case of difficulties with dimming the lighting, they look at the instrument panel - the indicator of the working SMD is different from 1;
• Powerful LEDs without soldering are tested after inserting the adapters.

Before starting work in the ringing mode, determine the anode and cathode of the light source under test.

Checking LEDs without unsoldering

You can check the LED lamp without desoldering its diode elements. You will need an adapter, which is made independently from paper clips, separate wire cores, pieces of sewing needles, twisted pair of wiring. The selected product is soldered to the gauge probes. A PCB gasket is made between the adapter parts, and then the whole structure is wrapped with insulating tape.

Multimeter probes with an adapter are connected to the contacts of the light-emitting diode or to the PNP pads. Testing is performed sequentially for each element.

Checking the performance of light emitting diodes in a flashlight

The standard lantern test is a clear example of work for which you do not need to desolder the elements. To find out if LED sources are working, you need:

1. Disassemble the flashlight, remove the board with LEDs from it.
2. Without removing the solder, place the probes on the pins of the PNP connector, observing the polarity.
3. Put the switch on the dial.
4. Look at the scoreboard and at the backlight.
5. Determine if the circuit is working properly by checking its resistance. A resistance indicator equal to zero, when connected in parallel, indicates a malfunction of one LED.

Test each diode separately.

Materials at hand for verification

In addition to a multimeter, a luminaire, lantern or LED spotlight can be checked:

• Battery. A CR2032 battery is suitable for the computer motherboard. Its voltage of 3 V is sufficient for all types of diodes.
• A 4.5 and 9 V battery together with a ballast. It will give a voltage drop to a safe value. 750 Ohm is supplied to the "Krona", from 150 to 200 Ohm - to 4.5 V products.
• Battery from radio call or remote control. The LED strip is tested with a 12V element. Its contacts are thrown onto the poles, after which there are points with burned out LEDs. Connectors are tested in the same way.
• A special led tester based on AA batteries with parallel connection.
• An old charger, from which the plug on the phone is removed and the contact is protected. The red wire will be a plus and will go to the anode. Black is used as a minus and is connected to the cathode. If there is enough voltage, the LED lights up.

UV diode testing is complicated by its high voltage sensitivity. A nominal value of no more than 3.4-4 V is supplied to it.

Self-made probe

It is problematic to ring a small LED with a standard probe, so you can make it yourself for comfortable use with a multimeter. Several elements are used for this.

Sewing needle

You will need:

• cases from black and red handles for handles;
• plugs and cable;
• steel sewing needles 35-45 mm in length and 0.8-1 mm in diameter;
• cutting copper wire (a pair - 250-300 mm long and a pair - 120-150 mm in length);
• rosin or alcohol rosin.

The manufacturing process is carried out in stages:

1. The wire is cut and tinned with solder.
2. The needles are tinned with solder so that 8-10 mm remain to the sharp parts.
3. Next to the ears of the needles, conductors of 0.3-05 mm in diameter are attached, and then they are wound in turns to the tinned area.
4. The winding is covered with solder.

   

5. The tinned cable folds in half around the screwdriver. Free areas are fastened to each other in a pigtail. The resulting loop is bent at an angle.
6. The conductors are attached to the needles with a soldering iron.
7. Plaque is removed from all joints with alcohol.
8. A thread is wound in the center of the needles until bulges appear. They will need to be covered with Moment glue and inserted into the tips of the handle bodies, fixing them as evenly as possible.
9. After the glue has dried, epoxy is poured into the cavities, which hardens for 24 hours.
10. The ends of the probes are tinned and soldered to the plugs.
11. The problem areas are placed in a heat-shrinkable tube to protect the shell from friction.
12. Flexible conductors are made of 1 m long red and black copper wires.
13. The tips with needles are connected to the flexible conductors with a soldering iron. Pieces of handles are held together.

The optimum wire cross-section is 1.3 mm2.

Plug

You will need:

• Soviet plug from electrical appliances with brass pins;
• old probes from a multimeter;
• plastic tube;
• wire with thick copper conductors;
• banana plugs.

Progress:

1. Removing the pins from the fork by unscrewing the top bolt.
2. Removing the base from old probes - the pins can be removed with pliers.
3. File the bent part of the pins and grind them so that they fit with effort into a piece of plastic pipe.
4. Separation and stripping of the acoustic wire.
5. Tinning of the ends of the cable and the ends of the pins at the soldering points.
6. Insert a wire into the base of the old multimeter probes and solder a brass plug to it.
7. Pulling the cable back and fixing the area of ​​its entry into the tube with heat shrink.

The other end of the wire is threaded into the connector. For firm fixation, the cable will need to be clamped with a bolt.

Hairpin from laser CD drive

You will need:

• steel hairpin with sharp points;
• heat shrinkable tubes of different sizes;
• two markers (black and red);
• a tube the size of the pin;
• copper wires, designed to operate in a network with a voltage of 300 V.

The procedure for making a probe:

1. The hairpin is cut into 2 parts. The sawn-off edges are covered with flux.
2. The ends of the wires are protected by 5 mm and tinned.
3. Tinned wires are attached to the sawn-off sections - one for each.
4. Thermal tubes are put on and seated on the structure.
5. The handles of the probes are made from felt-tip pens - it is enough to cut off 5-7 cm from the beginning.
6. Studs with soldered wires are inserted into the marker pieces so that the tips protrude from the marker.
7. Elements are fixed with epoxy.
8. After drying, the handle is installed in a colored heat-shrink tube.
9. The plugs are made from pieces of brass pipe from the antenna 3 cm long.
10. The brass tube is inserted into the connector, the plastic is fitted under it.
11. The rest of the ends are soldered onto brass tubes and wrapped with electrical tape so that they fit the diameter of the plastic ones.
12. Pieces of heat pipes 4 cm long are put on the plugs and sit down.

Epoxy resin can be replaced with Secunda glue with a pinch of baking soda.

Tips & Tricks

In the process of diagnosing LED devices, the following factors must be taken into account:

• if the voltage rating is at the limit, and the luminous flux did not appear, you can briefly increase the current;
• when high power is supplied, the LED source heats up;
• the normal heating temperature of the diode is from 70 to 75 degrees (you cannot burn your palm when touched);
• using a battery, you can additionally set the resistance of the diode connection;
• when the polarity is reversed, even a working element will not have a backlight;
• the optimal material for a homemade probe is nickel-plated needles, which are easy and quick to solder;
• A working IR LED glows when the radiation is directed to the sensitive area.

Checking LED light sources with the ability to work with a multimeter is easy. The user needs to prepare conditions for testing - choose polarity, design probes or adapters, make special contacts.