Troubleshooting Power Supply and Electro Magnetic Field Issues in Electronic Access Control

Taking voltage measurements is a good way to verify that your power supply is matched to your lock. Many electric locks and power supplies are field-programmable so they can be set to either 12 or 24 volts. A mismatch between the power supply and the locking device is not a good thing.

If you've gotten to this point and you still have problems, consider the possibility that you are dealing with electrical noise/surge in the circuit which is reflecting into the electronic access control.

Generally speaking, it is not easy to measure these phenomena with portable test devices like DVMs , because noise and surges are so fast that they cannot be detected without lab instrumentation.

Two usual courses of action for the field engineer is to install surge suppressors or use an isolation relay.

Surge suppressors are like small electrical shock absorbers that clamp the spikes in the voltages to hopefully resolve the issue.

Isolation relays separate the electric lock and its power supply from the access control circuitry and its output relay. A relay is a set of isolated switch contacts and an electrical coil. There is no electrical connection between the coils and the contacts. When voltage is applied to the coil of the relay, the contacts switch. When the power is removed from the coil, the contacts switch back. Relays are available in a variety of coil ratings, and contact configurations and ratings. By using isolation relays, you also protect the on board output circuit on your access control against damage as well as noise.

An isolation relay can also be a solution for applications where voltage drop is an issue. For example, if your access controller is an excessive distance from the locking device, the problem can be overcome by relocating the lock power supply and isolation relay closer to the electric lock, and activating the relay coil from the controller. In reality, the distance between the power supply and the lock is not the real problem; it is the relative resistance between the wire and the load. The resistance of the wire is a function of its gauge, and in most practical applications, the larger the diameter of the wire the lower the resistance and consequently the lower the voltage drop will be.

Voltage drop calculators are readily available on-line. Check one out and try plugging in a few values for loads and voltages to see how voltage drop increases as wire resistance and load increases.

For electric locking devices which operate on AC (Alternating Current), use electronic devices referred to as Varistors , (a/k/a MOVs ) with a 30 volt rating. MOVs are relatively common, and are the active ingredient in many of the inexpensive surge suppressor type products sold at home centers. However the MOVs used for line voltage are a different rating than the ones used for low voltages. Also the spikes and aberrations possible on line voltage far exceed those likely to be experienced in a low voltage electric lock circuit.

While MOVs are effective for door locks, they are not adequate if you're serious about protecting equipment such as your access and video installations. A $10 power strip with built in surge protection is not a good investment.

For electric locking devices which operate on DC (Direct Current), use diodes. A diode is a semiconductor device which passes voltage in one direction but not the other. They are used in various configurations as rectifiers to convert AC to DC. They also effectively clamp reverse induced current caused when an electrical field switches (such as powering and unpowering an electric strike or electromagnetic locking device). Use a diode rated for 6 Amps, 100 PIV.

I am a strong believer in using isolation relays. They provide the noise isolation and added design flexibility to your installations. They also will extend the lifetime of your electronic access control circuit boards by relieving the heavy lifting off the little relay that is supplied with the product, and placing it on the isolation,(a/k/a/ slave relay) which can be easily and inexpensively replaced. A wide assortment of these relays are available to the installer, along with many function modules which can be indispensable for custom applications.