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How to Check a Diode

Water is commonly used as a comparator when you’re learning the fundamentals of electricity. The problem: You can see water, but you can’t see electrons flowing through a wire. A volt/ohm (VOM) meter is required to confirm the presence of electricity in a circuit. Once you get accustomed to it, using a VOM can be as intuitive as looking for drips to find a leak in a pipe.

Depending on the design and purpose of an electrical circuit in a machine or vehicle, it’s common to find a diode. A diode is nothing more than a one-way check valve for electricity. It’s very similar in concept to a backflow protector used on the water feed to a sprayer tank. The diode causes current to pass only in one direction. If a diode fails, there will be no flow of electricity (valve stuck closed), or the current will flow both ways (valve stuck open). When a diode fails, the equipment or component may stop working, operate incorrectly, or damage other electrical components. 

Once you recognize the importance of a diode, you’ll be surprised how many are in use in areas you hadn’t thought of. One common application is in the wiring to the electromechanical clutch employed to engage the air conditioner (AC) compressor on most engines. 

The AC clutch is the same basic design as an electric PTO clutch. Because of the way an AC system works, the engagement of the compressor is cycled based on thermal load and the system’s operating pressures. 

Going down the field or the road, you may not be aware of this duty-cycle since the interior duct discharge temperature stays almost constant. 

Like any electromagnet, it is a coil (similar to the ignition coil on a gasoline engine). In this instance, when the AC is signaled to cycle off, the ground is opened to the clutch and an inherent collapsing of the magnetic field occurs. This will step up the voltage as an ignition coil would.

In most – if not all – designs, a diode is placed in the wires that go to the AC clutch at the connector. It keeps the amplified voltage, when the compressor cycles off, from going back into the wire, which could overload any electrical component in that circuit.

In many applications, the circuit may not work if the diode fails. Or there can be repeated failures of an electronic controller on the vehicle. It usually takes many exposures to the amplified voltage to hurt the part. In the case of an AC clutch, it may encompass an extended period due to seasonal use.

diode use in an alternator 

An alternator also uses a series of diodes internally to convert the alternating current into the direct current the machine works on.

Depending on the alternator design and which diode fails or becomes weak, the system may still charge (about 1 volt below specification and the charge light may glimmer dimly), feeding the circuit nonrectified AC current. This will cause a great deal of havoc with monitors, controllers, and the drive motors used on electric seed meters. 

Some circuits use a modified diode called a zener diode. This can be considered a check valve that, at 1 voltage input, will flow both ways. That value is identified as the avalanche voltage.

A diode is easy to check with a VOM. The circuit needs to be unpowered and the VOM set to ohms. Take the red meter lead and touch one end of the diode. Take the black lead and touch it to the other side of the diode. Take a reading. It should be continuity (0 ohms) or infinity (open). Now reverse the leads. If the diode is good, the reading will be opposite of the first. 

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