Solving Cases of Missing Volts

Therefore, by using Ohm’s Law, we can figure the voltage drop of the wire (Vw) and the voltage drop of the device (Vd) by E = IR where I is Ic and R is the resistance of the device. (Note: Not sure what the operating current of a device is when specs say it is 12V @ 500ma? Then apply Ohm’s Law of R = E/I or in this case 24 ohms.)

So overall we will be looking at Kirchoff’s law for this simple circuit. Therefore, the Vs = Vw + Vd. I have some bits of data that will help me. The resistance of the wire is fairly constant and will be rated in ohms/length and the resistance of the device is either given or easily calculated as shown in note above.

So now using E = IR, I can find the voltage drop for each device. If the current is high enough and the wire run long enough, the wire can drop so much voltage that the device will not have enough voltage to operate properly. The wire’s high resistance has stolen enough voltage; remember, Vs = Vw + Vd or VD = VS-Vw.

How can I resolve this problem? One way would be to rewire with a heavier gauge wire, which would provide lower resistance and circuit voltage drop. Another way would be to move the power supply closer to the door device and reducing the length of wire and its resistance.

This is the reason you see many door access power supplies found close to the door. The often high current surges needed for these devices will create a large voltage drop on the cable so the distance of the cable needs to be short. Also, remember that a poor electrical connection in your circuit can cause high resistance and, therefore, a large voltage drop as well. So make sure you have good connections in your alarm circuit.

There you have it. Knowing how voltage drops in cabling affect the performance of various devices will make for a smarter technician and a better performing system.

Tech Talk Tool Tip


Manufacturers are constantly listening to the feedback from techs in the field. Altronix realizes that all devices are universally neither 12V nor 24V, and has come up with the VR3T-VR4T converter devices.

These products are handy when you are faced with having to apply a 12VDC device in an existing 24VDC circuit. I like simple devices like this as they can save you time and money without having to adapt on the fly to both technologies.
Thanks, Altronix!

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