Checking the diode bridge with a multimeter: video with instructions

In many devices operating from a 220 V network, a diode bridge is installed. This is a device consisting of four (for a single-phase network) or six (for a three-phase) semiconductor silicon diodes. It is needed to convert AC to DC. An alternating current is supplied to its input, and a pulsating voltage of constant sign is obtained at the output. These circuit elements often fail, pulling the fuse behind them. Let's see how the diode bridge is checked for serviceability in different ways.

What you need to know about diode bridges

To begin with, we will consider what are and what is inside the diode bridge. These elements of the circuit are found in two versions:

1. From discrete (separate) diodes. Usually they are soldered on the board and connected by tracks in the correct circuit.
2. Diode assemblies. Assemblies can be represented as single-phase bridges for rectifying both half-periods of an alternating voltages and assemblies of two diodes connected in a circuit by a common cathode or anode and other options inclusion.
   

In any case, the rectifier single-phase diode bridge consists of four semiconductor diodes connected in series-parallel manner. An alternating voltage is applied to two points at which the anode and cathode are connected (opposite poles of the diodes). Constant voltage is removed from the points of connection of the poles of the same name: plus from the cathodes, minus from the anodes.

In the diagram, the AC voltage connection point is designated by the symbols AC or "~", and the DC voltage outputs are "+" and "-". Sketch this diagram for yourself, it will be useful to us when checking.

If you imagine a real diode bridge and combine it with this circuit, you get something like:

The location of the diode bridge on the board and precautions

Diode bridges are installed in both pulse and transformer power supplies. It is worth noting that in the pulse units, which are now used in all household appliances, the bridge is installed at the 220V input. At its output, the voltage reaches 310V - this is the peak voltage of the network. In transformer power supplies, they are installed in the secondary winding circuit, usually with a reduced voltage.

If the device does not work and you find a blown fuse, do not rush to turn on the device after replacing it. First, if there are problems on the board, the fuse will blow again. Such a power supply must be turned on through a light bulb.

To do this, take a cartridge and screw a 40-100 W incandescent lamp into it and connect it to the break in the phase wire to connect to the network. If you are going to often repair power supplies, you can make an extension cord with a socket installed in the break in the power cable to connect the lamp, this will help save you time.

If the board has short circuit - when connected to the network, a high current will flow through it, a fuse or a track on the board, or a wire, will blow, or the machine will knock out. But if we inserted a light bulb into the gap, the resistance of the spiral of which will limit the current, it will light up at full intensity, preserving the integrity of all of the above.

If there is no short circuit or the unit is working properly, either a slight glow of the lamp or its complete absence is permissible.

The simplest and rough check

We need an indicator screwdriver. It costs a penny and should be in every household's toolbox. You just need to first touch the 220V input of the rectifier, if the phase wire lights up indicator, then the voltage is present, if not, the problem is clearly not in the diode bridge and you need check the cable. If there is a voltage at the input, we check the voltage at the positive output of the rectifier, it can reach 310 V at this point, the indicator will show it to you. If the indicator is off, the diode bridge is open.

Unfortunately, we cannot learn anything else with the help of an indicator screwdriver. About, how to use the indicator screwdriver, you can find out from our article.

Dialing a diode bridge with a multimeter

Any part on the board can be soldered out for checking or ringing without desoldering. However, the verification accuracy in this case decreases, since perhaps, the lack of contact with the tracks of the board, with visible "normal" soldering, the influence of other elements of the circuit. This also applies to the diode bridge, you do not need to unsolder it, but it is better and more convenient to solder it for testing. The bridge, assembled from individual diodes, is quite convenient to check on the board as well.

Almost every modern multimeter has a diode test mode, usually it is combined with a sound continuity of the circuit.

This mode displays the voltage drop in millivolts between the probes. If the red probe is connected to the anode of the diode and the black one to the cathode, this connection is called forward or conductive. In this case, the voltage drop across the PN junction of the silicon diode is in the range of 500-750 mV, which you can see in the picture. By the way, it shows a check in the resistance measurement mode, this is also possible, but there is a special diode test mode, the results will, in principle, be similar.

If you swap the probes - red for the cathode and black for the anode, the screen will display either one or a value of more than 1000 (about 1500). Such measurements indicate that the diode is working, if in one of the directions of the measurements are different, then the diode is faulty. For example, a continuity has been triggered - the diode is broken, high values ​​in both directions (as in the reverse connection) - the diode is cut off.

Important! Schottky diodes have a lower voltage drop, of the order of 300 mV.

There is also an express test of the diode bridge with a multimeter. The procedure is as follows:

1. We put the probes at the input of the diode bridge (~ or AC), if the dial is triggered, it is broken.
2. We put the red probe on "-", and the red one on "+" - a value of about 1000 is displayed on the screen, we change the probes in places - on the screen 1 or 0L, or another high value - the diode bridge is working properly. The logic behind such a check is that the diodes are connected in series in two branches, pay attention to the circuit, and they conduct current. If the plus of the power supply is applied to - (the point of connection of the anodes), and the minus of the power supply to "+" (the point of connection of the cathodes), this happens during a continuity. If one of the diodes is open, current may flow in the other branch and you may make erroneous measurements. But if one of the diodes is broken, the voltage drop across one diode will be displayed on the screen.

The video below clearly shows how to check the diode bridge with a multimeter:

Complete diode bridge check

You can also check the diode bridge with a multimeter according to the following instructions:

1. We set the red probe to “-”, and with the black one in turn we touch the terminals to which the alternating voltage “~” is connected, in both cases there should be about 500 on the screen of the device.
2. We put the black probe on "-", with red we touch the terminals "~ or AC", there is one on the multimeter screen, which means that the diodes do not run in the opposite direction. The first half of the diode bridge is OK.
3. The black probe is on "+", and with the red one we touch the AC voltage inputs, the results should be as in step 1.
4. We change the probes in places, repeat the measurements, the results should be as in point 2.

The same can be done with a "tseshka" (a universal Soviet-made measuring device). How to check a diode bridge with a pointer multimeter is described in the video:

By the way, the check can be performed without a tester at all - with a battery and control light (or LED). When the diode is turned on correctly, current will flow through the light bulb and it will light up.

In conclusion, I would like to note that diode bridges are installed everywhere: in a charger, a welding machine, on inverter, power supplies, etc. Thanks to the described technique, you can check the diodes for performance in home conditions.

It will be helpful to read:

• How to use a multimeter
• How to check a capacitor at home
• Checking the functionality of the resistor
• How to solder radio components from the board