Inductor commutating circuits
A popular use of diodes is for the
mitigation of inductive "kickback:" the pulses of high voltage produced
when direct current through an inductor is interrupted. Take for example
this simple circuit:
When the pushbutton switch is actuated,
current goes through the inductor, producing a magnetic field around it.
When the switch is de-actuated, its contacts open, interrupting current
through the inductor, and causing the magnetic field to rapidly
collapse. Because the voltage induced in a coil of wire is directly
proportional to the rate of change over time of magnetic flux
(Faraday's Law: e = NdΦ/dt), this rapid collapse of magnetism around the
coil produces a high voltage "spike."
If the inductor in question is an
electromagnet coil, such as might be seen in a solenoid or relay
(constructed for the purpose of creating a physical force via its
magnetic field when energized), the effect of inductive "kickback"
serves no useful purpose at all. In fact, it is quite detrimental to the
switch, as it will cause excessive arcing at the contacts, greatly
reducing their service life. There are several practical methods of
mitigating the high voltage transient created when the switch is opened,
but none so simple as the so-called commutating diode:
In this circuit, the diode is placed in
parallel with the coil, in such a way that it will be reverse-biased
when DC voltage is applied to the coil through the switch. Thus, when
the coil is energized, the diode conducts no current:
However, when the switch is opened, the
coil's inductance responds to the decrease in current by inducing a
voltage of reverse polarity, in an effort to maintain current at the
same magnitude and in the same direction. This sudden reversal of
voltage polarity across the coil forward-biases the diode, and the diode
provides a current path for the inductor's current, so that its stored
energy is dissipated slowly rather than suddenly:
As a result, the voltage induced in the
coil by its collapsing magnetic field is quite low: merely the forward
voltage drop of the diode, rather than hundreds of volts as before.
Thus, the switch contacts experience a voltage drop equal to the battery
voltage plus about 0.7 volts (if the diode is silicon) during this
discharge time.
In electronics parlance, commutation
refers to the reversal of voltage polarity or current direction. Thus,
the purpose of a commutating diode is to act whenever voltage
reverses polarity, in this case, the voltage induced by the inductor
coil when current through it is interrupted by the switch. A less formal
term for a commutating diode is snubber, because it "snubs" or
"squelches" the inductive kickback.
A noteworthy disadvantage of this method
is the extra time it imparts to the coil's discharge. Because the
induced voltage is clamped to a very low value, its rate of magnetic
flux change over time is comparatively slow. Remember that Faraday's Law
describes the magnetic flux rate-of-change (dΦ/dt) as being proportional
to the induced, instantaneous voltage (e or v). If the
instantaneous voltage is limited to some low figure, then the rate of
change of magnetic flux over time will likewise be limited to a low
(slow) figure.
If an electromagnet coil is "snubbed"
with a commutating diode, the magnetic field will dissipate at a
relatively slow rate compared to the original scenario (no diode) where
the field disappeared almost instantly upon switch release. The amount
of time in question will most likely be less than one second, but it
will be measurably slower than without a commutating diode in place.
This may be an intolerable consequence if the coil is used to actuate an
electromechanical relay, because the relay will possess a natural "time
delay" upon coil de-energization, and an unwanted delay of even a
fraction of a second may wreak havoc in some circuits.
Unfortunately, there is no way to
eliminate the high-voltage transient of inductive kickback and
maintain fast de-magnetization of the coil: Faraday's Law will not be
violated. However, if slow de-magnetization is unacceptable, a
compromise may be struck between transient voltage and time by allowing
the coil's voltage to rise to some higher level (but not so high as
without a commutating diode in place). The following schematic shows how
this may be done:
A resistor placed in series with the
commutating diode allows the coil's induced voltage to rise to a level
greater than the diode's forward voltage drop, thus hastening the
process of de-magnetization. This, of course, will place the switch
contacts under greater stress, and so the resistor must be sized to
limit that transient voltage at an acceptable maximum level.
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