Q&A
capacitor across the contacts, the
cap acts like a short circuit when the
contacts open. At this point, the cap
begins to charge using the formula t =
RC, where R can be calculated from R
= E/I, E being the voltage across the
points and I being the current flowing
at the time of the break.
If we can balance the charging
time of the capacitor with the time it
takes for the contacts to open wide
enough that the voltage can’t jump
the gap, the arc is suppressed.
Typically, this is 0.1 mS for a frame
relay of the type you describe.
Plugging these values into our equations, we get R = 90 volts/1.5 amps =
60 ohms. Calculating for capacitance, we get C = t/R = 0.0001/60 =
1.67 µF.
If you’re trying to figure out the
mathematics for an AC voltage suppressor, forget it. A sine wave is self-extinguishing because it crosses
zero. That is, two times every cycle,
the voltage across the contacts is
zero as the voltage reverses itself
from positive to negative. It’s very
hard to maintain an arc under these
conditions — as many a welder will
attest to.
Relay
Coil
Relay
Coil
Relay
Coil
MOV
Figure 2
EMF Suppression
same happens across the
relay contact when connected to an inductive — like a
motor described in the question “Relay Contact Life” —
where this creates contact
arcing. This is energy that
must be dissipated somehow.
A diode placed across
the inductor (or contacts) will
conduct when the magnetic
field collapses and will effectively
short circuit the voltage. The decision
whether to use a diode or not
depends on the voltages and currents
involved. A coil with higher inductance, like a frame relay, will store
more energy — both voltage and current — than will a small coil, like a
reed relay. As a rule of
thumb, a diode should
always be included — but
other factors have to be
considered, not the least of
which is space. If the relay
is small, space is limited,
and Vcc is low, I often omit
the diode.
Also, placing a diode
across the inductor causes
current to flow longer and
lengthens the hold-in time
of the relay. This is unac-
ceptable for some applications. By
inserting a zener diode in series with
the rectifier, hold-time is greatly
reduced. This is because the diodes
can’t conduct until the reverse voltage
exceeds the zener voltage, plus
0.7 volts.
In some circuits, the diode(s) can
EMF Suppression
1uF
1k
Audio In
20W Power
Amp
+12V
+ LM1876
-12V
20k
1k
+
22uF
SPKR
Figure 3
QHow come you sometimes put a
diode backward across the relay
contacts and other times you don’t? I
have read that the diode is a must.
What’s the real story?
Mike Edwards
Los Angeles, CA
AI think you’re talking about putting the diode across the relay
coil, not the contacts, but they are
both one and the same. When the
current flow through an inductor — or
coil — is interrupted, a high voltage in
the reverse direction is generated by
the collapsing magnetic field. This
voltage can be as high as 10 times
the Vcc source voltage.
If the coil is connected to a transistor or other semiconductor, the
back EMF (as it’s called) can be high
enough to destroy the device. The
APRIL 2005
Circle #77 on the Reader Service Card.
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