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     Solenoid operated valves are a common component in modern hydraulics (pumps and motors included) which means that tests of solenoid operated valves make part of any hydraulic workshop daily routine. It is about these tests that I would like to talk about in this post. Testing a solenoid operated valve seems to be an easy task - I mean - how difficult can it get? Just feed the rated voltage and see if the valve functions, right? Well, yes, but... Through my practice I've seen more than one perfectly functioning solenoid end up inoperable and be replaced with a new one only because the person who tested the solenoid followed the above mentioned "straightforward solenoid testing manual".

    I bet some of you already guessed that I am referring to DC solenoids that have polarity, meaning that there's only one correct way of connecting them to a DC source. As a rule such solenoids have asymmetric connectors that should guarantee correct connection, but due to the fact that in most real life workshop tests a couple of stripped wires (the most universal connector of them all!) will be used instead of a connector, the test of the solenoid valve can easily become a 50 percent chance gamble...

    A lot of mechanics are convinced that a solenoid is nothing else but a coil of wire - hence no polarity. This indeed is a definition of a solenoid, and there's no arguing about it, but the hydraulic valve switching elements, commonly called solenoids, are not always simple coils.

    For certain applications a solenoid valve manufacturer can opt for incorporating a so called suppressor device inside a DC solenoid. When a solenoid is de-energized, the collapsing magnetic field causes the voltage across the solenoid to rise substantially ( a 24 V solenoid, for example, can easily produce a 700 volt spike when switched off) , which can damage or reduce the life of switching elements (relays or solid state switches). This voltage spike is often called the inductive kickback, and is the reason why you can get a painful electric shock when playing around low voltage DC solenoid circuits with bare hands.

    There are several ways to snub the spike, and one of the ways is application of a diode across the coil, reverse biased against the current supply. This diode is often called a flywheel diode, and it provides the path for the current in the coil when the solenoid is switched off - thus eliminating completely the voltage surge. One of the drawbacks of this arrangement is the increased valve switch-off time, but when the switching speed is not an issue - this is a cheap and reliable solution to protect the circuitry against the spike. If you want to know more about coil transient voltage suppression - you can easily find more detailed information online.

   When such a solenoid is connected incorrectly to a DC source, the suppressor diode becomes forward biased (read - a short) with all the current the power source can provide by-passing the coil through it!

    The following course of events depends upon the reaction time of the "individual in charge". If the DC supply is equipped with an ampere-meter, and the technician notices that the coil is drawing an astonishing 15 amperes instead of expected 0.5, he might be lucky to turn off the current just in time to save the poor suppressor. If not, there's a good chance that the diode will melt down to a low-impedance short, rendering a perfectly good solenoid useless... If the power source stays on for a couple of extra minutes, there's a chance the diode will eventually melt open (not the worst outcome, since the valve will become operational again, though unprotected against the kickback) or a small fire will start, with smoke and everything. In any way, the solenoid will never be the same as before!

   It is always a good practice to read the current when testing an unfamiliar solenoid valve, a still better practice is to use a DC source that has an adjustable current limiter (I personally like using those) - in this case even when you accidentally forward bias the suppressor diode of an unknown solenoid, the current will be limited to a set value - so no fire and no explosion!

   Nothing saddens me more than a good working component spoiled by amateur overhauling or, which is even worse, incompetent testing. Solenoids damaged by reverse polarity are a good example of such an atrocity, IH readers will never be held  responsible for!