 # Thumbs Up for Ohm!

Recently, I had the opportunity to attend an educational advisory meeting that was attended by many leading technical people in the security industry. When the question came up as to what is the most needed skill for today’s alarm technician, the overall unanimous comment was, “A better and more complete understanding of technical fundamentals.”

While technicians are often absorbed with having the correct tools in their tool belt and keeping up with the latest high-tech trends, they often overlook having other important tools such as good comprehensive knowledge of basic trade technology rules and standards. After all, what good is it having a volt meter if you can only read a voltage and not understand the relationship of that voltage to finding a problem within the whole electrical alarm circuit?

This month, I have decided to revisit (see the November 2002 “Tech Talk”) what is probably the cornerstone rule of electrical circuit analysis: Ohm’s Law. Along with this, we will look at a lesser-known, yet very important rule called Kirchoff’s Law. The mastering of these laws is critical to becoming a superior security technician.

Technicians should emphasize not just reviewing or even memorizing these rules, but truly understanding how they relate and support daily security industry applications. Now, what does your thumb and Ohm’s Law have in common? Stay tuned to find out.

There’s No Place Like Ohm
First, lets talk a bit about the Ohm’s law formula and the mathematical topic algebra. Algebra, as many of you fondly remember from high school, is the rule of equality and working with an unknown value. If faced with the problem of 4 + X = 8, what does X equal? What did you do to get the answer? You subtracted 4 from both sides of the equation equally and found out that X equals 4. You treated both sides of the equation equally, so nothing really changed.

Ohm’s Law deals with an equation of three unknown values in which two of the values are known and you must find the remaining single unknown value. The basic Ohm’s law formula is voltage (E) across a device equals current (I) through the device multiplied by the resistance (R) of the device. The formula for this law is E = I X R or E = IR.

So if I take a voltmeter and read the voltage drop across a device and I also know the resistance of that device, I could then calculate what the current would be going through that device in the alarm circuit. (HINT: Make sure to have a digital voltmeter (DVM) with a high impedance of 10 megaohms or higher, so the meter does not affect the load being measured. Some inexpensive meters may have low-impedance inputs.)

To find the current, one needs to divide both sides of the equation by R. You don’t even need to know the value of R since you are dividing both sides of the equation by the same R value. The algebraic rule simply states that whatever you do to one side of the equation you must do to the other equally. We now have the formula I = E/R. Just plug in your values for E and R, divide E by R as the formula dictates, and you now have the current through your device.

Rule of Thumb
OK, now we need the assistance of your thumb. This technique is an easy alternative to working with Ohm’s Law. Place your thumb on top of any one of the three Ohm’s Law unknowns (I, E or R) in the triangle-shaped diagram (see illustration). You will now see the two remaining known values and note their mathematical relationship.

An example would be if I were looking for resistance of an unknown electrical component (see photo). If I place my finger over the R, the diagram would indicate I should divide the voltage by the current. Remember that known values of an electrical component can either come from a physical measurement or by referencing the component’s written specifications. There is very little math with this technique. You can download your own copy of this diagram from SSI online.

Kirchoff Also Laid Down the Law
Another important, but lesser known rule for alarm circuit analysis is the Kirchoff voltage and currents laws. The voltage law simply states that the total of voltage drops across all the devices in a series circuit must equal the voltage of the power source to that series circuit. (HINT: Don’t forget that the circuit’s cable can drop substantial voltage in a circuit if the wire diameter is small and the current is large. Also, be aware that highly resistive, faulty equipment cable terminations can drop a lot of voltage as well).

Also keep in mind that devices connected in parallel will all have the same voltage as the power source across them.

Kirchoff’s current law is pretty much the opposite of the voltage law. It states that the current draw through all devices in a series electrical circuit will be the same and is dependent on the overall resistance (again, don’t forget the overall resistance of the circuit wire) of the circuit. This is proven by Ohm’s Law for current, which is I = E/R, where E is the source voltage and R is the total resistance of the circuit.

When devices are connected in parallel and connected through a common point, this law states that the current leading into a connection junction must be equal to the total of the electrical current in the circuit branches leaving the same junction.

Applying Rules in the Field
Enough talk about theory. Let’s take a look at a real-world example of applying these laws. This application example comes from Patrick Riley, senior service technician with Ingersoll Rand (IR) Security Technologies. The question: Is there any way I can use a 12V relay on a 24V access control circuit? The answer is to set up what is known as a basic voltage divider circuit.

Knowing the rules of Kirchoff’s voltage law, we can deduce that if we can somehow drop 12V in the circuit, it will allow the remaining 12V to control the 12V relay. We will use a resistor to drop this voltage. If we select a resistor that is the same resistance value as the relay coil, then we will drop half of the source voltage of 24V or 12V across the resistor and the other 12V across the relay coil.

Looking in our truck, we find an ELK-912 relay module. The relay specs tell us the relay is a 12V relay and the relay coil draws 30 milliamps. To set up a voltage divider, we need to know the resistance. Putting our thumb on R in the diagram, we see voltage divided by current, or 12/0.03. The relay’s resistance is 400 ohms.

Another quick calculation to make is the power rating of the resistor you select. Using the power formula P = I E, you should select a 1/4W or 1/2W 400-ohm resistor. Actually, either will work. Place the resistor in series with the relay, and you now have a 12V relay working off of a 24V source.

For the complete version of t his story, see the August issue of Security Sales & Integration magazine.