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!
