Solving Cases of Missing Volts

I have noticed through the years how many technicians have accepted the use of power supplies without really understanding the electrical concept of the alarm circuit.

The manufacturers of both security power supplies and user devices have gone to great lengths to make these devices fit many physical variations in real-world applications. Understanding the basic electrical and physical principles about these circuits makes it easier for technicians to specify and troubleshoot devices.


Some technicians learn about voltage drop theory the hard way, otherwise known as trial and error. You connect a power supply for some electrified door hardware, go to power up the door latch and it does not work; you check your wiring and all looks correct. What could be the problem? It may not be the device but the cable gauge you selected or the distance from power supply to the device. But without understanding voltage drops this can remain a mystery.

Let us look at some of the basics that can answer these questions. Many techs in the security industry work with DC alarm circuits every day. It is the main power source for alarm control panels and sensors such as PIRs. But what is the problem with a device not having enough power to activate? This is where we will look at the mysteries of Ohm’s and Kirchoff’s laws of electricity.

Not Knowing Resistance Is Futile

Ohm’s Law is probably the cornerstone of low-voltage DC circuits. The formula for this law is E = IR or voltage equals current times resistance.

First, we must understand that every device in an electrical circuit has some resistance. That resistance is typically pretty steady (it can vary with temperature, but that does not apply here). In our basic understanding of algebraic equations – what happens to one side of an equation must equally happen to the other side of the equation to keep it balanced. So if the current going through a device goes up then the voltage potential of that device goes up as well.

So far you have noticed that we are not even talking about specific numbers but relationships of electrical properties in our low-voltage circuits. We can always plug the numbers in later once we truly understand the relationships. That is where the real secret lies. If you understand the relationships of electrical properties then you can take measurements and say “something is not right here,” and find the problem.

Keeping Current on Circuits

Kirchoff’s rules of voltage and current are the rules that provide the rest of the solution to our voltage drops. Kirchoff has two rather basic, but powerful rules.

The first is the voltage rule that simply states the source voltage (e.g. a power supply) must equal the sum of all the voltage drops in the electrical alarm circuit. What do I mean by voltage drop? Well that relates to the voltage potential I talked about in Ohm’s Law when current is flowing through a device such as the coil in an electrical door latch.

The second Kirchoff rule is the current rule, which simply states that the current flowing into a junction, or node, of other circuit branches must equal the sum of the total current values in all the outgoing branches in that node. Simply put, what goes in must equal what goes out. The voltage rule will apply here as most alarm circuits are powered in a simple closed loop.

That’s it. We now have all the rules needed for a technician to be a wiz at estimating, calculating and testing electrical alarm, fire, CCTV and access control low-voltage circuits. We can also calculate the reason why some devices seem to work in a circuit and others do not. Let’s put our newly acquired rules to the task.

Catching Voltage Drops

Let’s say we have a simple series alarm circuit with a power supply (Vs) at one end and a door maglock device at the other.

We first have to look at the entire circuit as we will be dealing with two loads, or resistances, on the circuit. Yes, I did say one physical device in this circuit, but we must always consider the resistance of the wire in our circuit as well as the resistance of the device. Also remember, the smaller the gauge and the greater the length (don’t forget the return path), the greater the resistance. 

We will have a certain amount of current (Ic) that the circuit (both wire and device) will be drawing from the power supply. Remember the current is constant through a simple series circuit with no branches.

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