Tech Tips

Diode Testing

At times, diodes are used as an electric “check valve”, allowing electric current to flow in one direction and not the other. They are also used as a filter for “spike” voltage. Diodes are commonly used on Hydraulic Valve Coils, in Joystick Controllers, in Control Boxes, and more. 


For those of you that have an electrical background, we commonly use the conventional theory (voltage flowing from positive to negative) and not the electron theory (voltage flowing from negative to positive).

 Diode Diagram



 The drawing above is a typical diode showing both the anode and cathode with the arrow showing the direction of flow from positive to negative. The silver band on the end of the diode shows the direction of flow. These are commonly used so that multiple items/components can be activated using only 1 input signal.


Diodes can be easily tested using a multi-meter that has a diode tester on the control panel or by using an ohm meter. Connecting the positive end to the anode and negative to the cathode, the diode tester will usually beep in one direction and will not beep when leads are reversed. On an ohm meter, you will have very little to no resistance in one direction and very high or an open (OL) when leads are reversed.


Diode failures can be a “short”, conducting in both directions; an “open”, not conducting in any direction. Diodes can also breakdown under a load. This is not easy to detect as they will usually only let low amounts of voltage through the circuit. Typically, this is a “back feed” situation.




Diodes as Filters


Diodes are often used as filters for spike voltages. Spike voltages are created when a magnetic field collapses around a metal object. The easiest example of this is when a solenoid, solenoid valve or an unfiltered alarm or strobe light shuts off. Much like an ignition coil on a car, this produces very high voltages, but very low amperage. (Typically a solenoid valve will produce 1000 to 1500 volts when power is removed). This flow will most always be in a negative to positive direction. If the diode is installed in the “opposite direction” it will dissipate the voltage as it tries to go through the diode.


Replacing a diode or another part affected by a defective diode on your boom or scissor lift? Shop C-Tech, your source for new boom and scissor lift parts!  We offer Free Technical Service by contacting us at 1-877-755-7311, email us, or chat with us on our website.  Our friendly and knowledgeable staff is standing by to help!