Insane Hydraulics

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Something You Need to Know Before Testing a Solenoid Valve

In this post I want to talk about one thing you should be aware of before connecting a power source to an unfamiliar solenoid valve in order to test it.

Some of you may say - Dude, isn't testing a solenoid valve just a matter of feeding the rated voltage and seeing if it functions? Well, yes, it is - quite often... but not always. Trust me - I have seen more than one perfectly functioning solenoid end up in scrap because the person who tested it followed the "just connect it" technique.

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, these solenoids have asymmetric connectors that supposedly prevent incorrect connection, but due to the fact that in real life a couple of stripped wires will be used if the correct plug is not available, such a test will have a 50 percent chance of rendering the coil inoperable if you don't know what you're doing.

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

In certain applications a valve manufacturer can opt to incorporate the so called suppressor device directly inside a DC solenoid coil. When a coil is de-energized, the collapsing magnetic field causes the voltage across it to rise substantially (a "normal" 24 V coil, for example, can easily produce a 500 volt spike when switched off) , which can damage or reduce the life of switching elements. 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 them lies in application of a diode across the coil, reverse-biased against the supply. This diode is often called a flywheel diode, and it provides the path for the current when the solenoid is switched off - thus eliminating completely the voltage surge. One of the drawbacks of this arrangement is the slightly 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 voltage spikes.

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

What can happen to a coil protected by a fly-back diode after it has been connected incorrectly? If the shop DC supply is equipped with an ampere-meter, and the technician notices that the coil is drawing an astonishing 15 amperes instead of the expected 0.5, he might be lucky to turn the current off just in time to save the poor suppressor. If not, the diode will melt into 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 burn out and open the short (not the worst outcome, since the valve will become operational again, though unprotected against the kickback). Although there's an even bigger chance that the coil will split open and become either a firestarter or a smoke generator.

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 prefer 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 explosion and no burned out suppressor diodes!

Nothing saddens me more than a good working component spoiled by incompetent testing. Solenoids damaged by reverse polarity are a good example of such an atrocity (the readers of this blog will never be responsible for)!