Security Sales July 2021 - 68

SMARTS & PARTS
Tech Talk by Bob Dolph
Double the Voltage,
Double the Fun
It is crazy the number of ways
I have seen techs try to splice
multiple wires together. Now
you have a neat and reliable
method with the Splice Connector
from WAGO. Not only
do you get quick and easy
connections, but it makes
your install neat and tidy.
TOOL OF THE MONTH
▶ ONE OF THE BEST practices goals of every
technician should be not just acting on a particular
technology, but understanding that technology.
Th is is why this month I have decided
to take a closer look at why it may be advantageous
to use a 24-volt system over the typical
12-volt system.
You've heard the old gum commercial saying,
" double your fl avor, double your fun? " Well, in
security circuits, if you double the voltage from
12V to 24V, you truly could double your fun.
But the question for inquisitive techs is, why?
Let's fi nd out.
First, let me give an example. Someone has
Bob Dolph has served
in various technical
management and advisory
positions in the security
industry for 30+ years. To
share tips and installation
questions, email Bob at
bdolph.ssi@gmail.com. Access
content from his 15+ years as
Tech Talk columnist by going
online at securitysales.com/
author/bdolph.
68
specifi ed and you have installed an electrifi
ed door strike. You were provided single pair
22-gauge cable, a 12V power supply and a 12V
door strike. Once installed, you fi nd the new
strike is not working. You replace it and that one
does not work either. What is wrong? What can
you do to fi x the problem?
To start, we need to take a close look at a couple
of old-time rules. One is Ohm's Law and the
other is Kirchhoff 's Voltage Law. Understanding
these theories is critical for any tech who wants
to truly understand basic circuit theory. As you
will see, these laws are applied to almost every
aspect of system design, and especially in troubleshooting
practices. For best practices you
must know them inside and out. If you need to
bone up on these laws a good tutorial source is
Khan Academy.
Let's get back to our problem with the door
strike. It appears that we have only two options.
One is if you have an extra free pair in the cable
you could double them up to off er less resistance
in this long cable run. Don't forget, according
to Kirchhoff , the cable has resistance
and may be dropping enough voltage so the
door strike cannot work properly. Nice try, but
in this case that is not an option. Another is to
pull new and heavier gauge cable. However, this
was a very long and diffi cult cable run to install,
so it is not an option.
Security Sales & Integration JULY 2021
bdolph.ssi@gmail.com
Don't forget the title of this column, " Double
the Voltage, Double the Fun. " Many devices
today have the option of operating under
both 12V and 24V. Taking a closer look at the
power supply and door strike you notice that
both have a 12/24V switch. No, it cannot be
that simple! By switching both the door strike
and power supply to the 24V option, the door
strike works fi ne.
Now, what happened? It is time to bring in
Ohm's Law: E(voltage)=I(current) X R(resistance).
Remember from your algebra days? If you
raise the value on one side of an equation, you
must raise it on the other proportionally to keep
the equation balanced. Well, we can agree that
for the most part the resistance of this circuit (cable
and strike) remain constant, then doubling
the voltage (E) allows us to double the current (I)
on the other side of the equation/circuit. Now,
by doing this the door strike will have enough
current to operate even if the cable gauge did not
increase. It did not need to.
Hopefully, now you have gained an extra
appreciation for Ohm and Kirchhoff . Simple
things like a bad termination can cause a high
resistance point in your alarm circuit. Th is will
cause a high voltage drop and steal available
voltage meant for other devices in the circuit. To
troubleshoot this, you would need to take your
digital multimeter and check for this high voltage
drop. Don't assume it is a bad device in the
circuit causing the problem, it could easily be
bad workmanship.
I picked on electrifi ed door hardware for
my example because they are usually more
demanding of power. However, you may also
run into 24VAC (alternating current) devices.
Many falsely think that the AC feature helps
with our problem above, but I am sorry to say
it does not.
Th e use of AC is usually due to low-cost strike
solenoids and motors, less relay contacts erosion,
and better arc suppression. Lower cost AC solenoids
work on impedance (AC's version of resistance)
while DC solenoids work on the resistance
of the coil wire. Th ere is also less electromagnetic
interference. Don't forget with you DC solenoids
to protect devices when the coil fi eld collapses by
using a back EMF diode.
Th ese days most fi re alarm systems are 24V.
Camera systems often off er the option of 24V
as well.
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Security Sales July 2021

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