How to check a diode bridge of a generator using a multimeter: methods and stages of diagnostics

Any car includes an electromechanical device called a generator. In case of problems with it, car owners purchase a new device. At the same time, it is possible to repair the device yourself, which will save money.

An important part of the generator is its rectifier bridge. To carry out repairs or diagnostics, you will need to remove the generator from the car. It is easier to check the diode bridge using a multimeter.

The principle of operation of the generator and diode

A generator is a device that converts mechanical energy supplied from the engine into electrical energy. In cases of its malfunctions, the following problems occur:

• headlights become dim;
• the battery charge signal lights up continuously;
• an extraneous sound is heard in the area where the generator is located;
• the analog arrow of the voltmeter goes into the red zone.

Since the generator is directly connected to the charge circuit, the primary signs of a rectifier malfunction will be an overcharge or complete lack of battery charge. If such problems appear, then generator needs to be checked.

The generator outputs a sinusoidal signal, but the devices located in the car require a constant voltage. To generate a constant signal, a rectifier unit is built into the generator. It consists of six power diodes.

A diode is a semiconductor element with a pair of leads, electrodes. The principle of its operation is based on the ability to pass current in one direction. If the positive polarity of the signal is applied to p area of ​​the device, and negative - by n, then such an inclusion is called direct. If the polarity is reversed, then this is reverse inclusion.

Direct connect resistance p-n transition is negligible and does not affect the passage of electric current, and when it is turned back on, it practically does not pass it.

Diode bridge connection method

The diodes are connected in three groups, each of which includes two elements. The opposite leads of the diodes of each group are connected to each other at one point, and the remaining leads are connected to the generator terminal: one to the positive, and the other to the negative. All switching is carried out in the direct connection of devices. Thus, three diodes are connected to each terminal of the generator. Each group is switched through an additional diode connected between two opposite terminals of the rectifier diodes, with a common bus, connected to the control relay.

Depending on the type of heat dissipation, the rectifier unit is of two types:

• lamellar;
• ribbed.

The plate rectifier unit is a set of plates with diodes installed in them by crimping. A finned heat sink is made like finned radiators with special holes in the fins for inserting diodes into them, which are most often round.

The rectifier itself is reliable, but due to the influence of external factors, it can be damaged. The causes of malfunctions may be contact with moisture or other conductive substance, polarity reversal when connecting the battery terminals, short circuit in the car's electrical network.

Rectifier diagnostic steps

To test the diode bridge using a multimeter, remove the generator and pull the rectifier out. Before proceeding with disassembly, it is possible to make sure that the rectifier is working properly, without disassembling the generator.

Light bulb pre-check

Before checking the diode bridge with a multimeter, it is necessary to perform preliminary diagnostics with a 12 V light bulb. The check is carried out using an incandescent bulb as an indicator light. Connecting it to different terminals of the battery, conclusions are drawn about the device's performance. When starting the test, you need to make sure that the rectifier plate has a tight contact with the car body. One contact of the light bulb is connected to the generator body, and the other to the body of the rectifier mounting bolt and the starter connection points.

If the light does not light up and does not blink when connected, then the device is working properly. If one terminal of the light bulb is connected to the positive terminal of the storage battery, and the other to the negative terminal of the generator or the starter connection points, the light will glow brightly. The rectifier unit is working.

When deviations are detected when checking with a light bulb, you need to proceed to diagnosing the diode bridge using a multimeter. To identify a faulty semiconductor element, the generator will have to be removed.

Disassembling the device

To determine which semiconductor is faulty, the diode bridge of the generator is checked with a multimeter. To remove the generator, you will need to disconnect all terminals from the battery. Using wrenches, remove obstructing parts of the machine. For example, a wheel, protective elements, air conditioning compressor, etc. Dismantle the transmission belt by loosening the roller. Unscrew and pull out the output signal wires, remove the generator.

Disassembling the generator begins by removing the nuts from the positive terminal and removing the plug. Having twisted the fastening nuts, the top cover is pulled out and access to the regulator with a diode bridge is opened. Using a 60 W soldering iron, the contacts are unsoldered from the rectifier, and it is removed with protection elements.

Multimeter dialing method

To ring the rectifier, you need to use a device with a continuity function or with the ability to measure resistance. The multimeter is used both digital and pointer type. Each semiconductor is individually checked for a reliable measurement..

To check a diode using a digital multimeter, set the tester's operating mode to the diode continuity position. This mode is indicated on the panel as a diode icon or o))). The work uses two test leads: red and black. One end of the first wire (red) is connected to the multimeter connector marked with V / R, and the other end of the other wire (black) is connected to the tester ground.

With the probe of the red wire we touch the anode of the diode, and the black wire - the cathode. The anode on the diode is graphically depicted with a vertical bar. The tester will display a value corresponding to the forward voltage drop; this value can range from 400 to 600. If you swap the probes, then the device will show infinity. This measurement behavior indicates that the diode is operational. In this case, if the continuity shows a short circuit in both directions, the diode is faulty.

If there is no continuity mode, the transitions are measured by measuring the resistance. For this, the meter is transferred to the position of measuring the resistance, the range is set to 20 kOhm. When a workable diode is turned on in the forward direction (plus to the anode, minus to the cathode), the tester will show a resistance of up to 1 kOhm, in the opposite direction, the multimeter will show a gap.

Similarly, measurements are carried out with an arrow tester. The multimeter is calibrated before starting the test. To do this, the probes are closed on each other, and the tuning knob on the device is used to set the position of the arrow to zero. When the terminals of the diode are touched, the arrow shows the resistance of the junction with a direct connection, and an open circuit with a reverse connection. If the arrow is set to zero, then the diode needs to be replaced.

Instead of a faulty diode, a new one is installed in its place, with the same marking or the same parameters as it. The main parameter of a diode is the maximum current that a semiconductor can pass through itself without damage. p-n transition. The value of the current value for car diodes is in the range of 35-120 A.

After installing a new diode, a control measurement is carried out. To do this, one probe of the multimeter is installed on the positive terminal, and the other on the negative. If the rectifier unit is working properly, the device will show resistance for any polarity of connecting the probes. If a short circuit or an open circuit is detected, this means that not all defective diodes have been replaced or assembly errors were made.

The whole structure is assembled in reverse order, the generator is installed in place and a test run is performed for a short period of time.