In this column, Kristen answers questions about all aspects of electronics, including
computer hardware, software, circuits, electronic theory, troubleshooting, and anything
else of interest to the hobbyist. Feel free to participate with your questions, comments, or
suggestions. Send all questions and comments to: Q&A@nutsvolts.com.
; WITH KRISTEN A. McINTYRE
Relay Remote Control
QI have a Struthers-Dunn (Magnecraft) 120V AC relay that I would like to manage with a remote control. I see remotes at Home Depot (Defiant indoor/outdoor). What kind of snubber circuit
would I need to use to not blow up the remote with the
kickback from the relay? Or, do these remotes have a
zero-crossing circuit that would eliminate the need for a
snubber? Or, maybe there’s a simpler way to remotely turn
on and off a 240V water pump in a fountain.
AIt’s hard to say exactly what the control circuit in the Home Depot remote unit looks like without a schematic. I’m going to guess that it’s like many so-called solid-state relays and
uses back-to-back thyristors, commonly called a triac. The
Defiant controller that I found online is rated at 13 amps.
It would be important to check the coil VA rating for the
Struthers-Dunn relay to be sure that the controller is fine
with that much current.
Let me explain how to calculate that, but first let’s look
at some principles to understand.
Since the relay coil is very much like a transformer
primary, it will appear to be inductive. The relay contacts
are not in motion once they’ve settled into place. So, from
a physics perspective, there is no work being done. Work
is a specific concept in physics, and in the mechanical case
it’s defined as F • d, or force times (actually, a vector dot
Work is equivalent to energy, by the work-energy
principle. Power is energy per unit time. There shouldn’t
be any power dissipated because no work is being done.
You would think that the power — and thus the current —
would be zero, but not exactly. If you look at it over each
AC cycle, the net power is zero, but energy moves from
the supply into the magnetic field and back again. It’s that
peak current when it’s moving back and forth that has to
It’s worth mentioning that wire isn’t perfect, and so
there will be power dissipated by the wire as heat. In the
AC relay case, however, the dominant thing is the energy
moving back and forth.
This isn’t really power because work is not being done;
thus, the unit used is VA, as opposed to watts. VA is a
voltage and amperage product, and represents a notion
of imaginary or reactive power. It’s called that because of
the complex number representation of the voltage-current
product. It’s typically a root mean squared, or RMS value.
This means that the peak is the RMS value multiplied by
√ 2; roughly 1.414.
If the relay is rated for 120V, dividing the VA rating of
the coil by 120 will give you the RMS current. The peak
current is roughly that value multiplied by 1.414. If the
controller can handle that much current, it should be fine.
See Figure 1 for the equation.
As to the inductive kick, the thyristors should be
alright, but again I can’t be sure without knowing the
device. Often, these things are used to control motors
which have very similar inductive characteristics to a relay
coil. Most thyristors/triacs are very tough devices with
high voltage ratings. It looks like those Defiant devices are
pretty cheap. So, if it dies, you won’t be out too much
• Relay Remote Control
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Q & A
; FIGURE 1. Equation for AC relay peak current.
8 December 2017