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

Dec. 1, 2007
Occasionally when installing access controls, things will not go the way you thought they ought to. Some call it a “bug” or a “ghost in the machine,” but your equipment is misbehaving. There are a number of possible reasons for this, and you need to determine exactly what breed of bug or ghost you've got.

Occasionally when installing access controls, things will not go the way you thought they ought to. Some call it a “bug” or a “ghost in the machine,” but your equipment is misbehaving. There are a number of possible reasons for this, and you need to determine exactly what breed of bug or ghost you've got.

An accurate digital volt meter (DVM) is an essential tool for those involved in electronic access control.


I prefer to clearly differentiate between power supply, which in my vernacular provides DC (Direct Voltage) and may also provide battery backup, and transformers, which step down a line voltage input to a low voltage output, usually 12 or 24 volts. Transformers can come in metal enclosures, or with wire pigtails, or in a wall-wart module. You should always use UL approved transformers and power supplies, and if the device is not supplied with a line cord, call in a licensed electrician to hook it up.

Power supplies are usually in metal enclosures, but are also available as separate modular components such as the regulator, rectifier and transformer, so that an advanced technician can create a custom setup for a particular application.

Depending on a number of considerations, it is typically preferred to use a separate power supply for your access control module and a separate power supply for your locking device.

Electronic access control problems can be divided into three basic categories:

1. Not enough power (to either the controller or the locking device, or both)

2. Excessive voltage drop in wiring

3. Wrong kind of power

You'll run into the “not enough power” problem when you're trying to operate too much equipment off a single power source, and the result is fluctuations in voltage level as the system is operated. It may be that your line voltage receptacle is not at proper level or you might not have selected the right power supply for the application.

When you overload a power supply, it can heat up: Obviously this is dangerous and must be avoided. Eventually excessive heating will result in a component failure in the power supply, a blown fuse or breaker, or a fire.

Overloading a power supply can also cause the power quality of the supply to suffer, which in turn can have a crippling effect on your system operation.

Excessive voltage drop occurs when the combination of the gauge and length of wire between the lock power supply and the electric locking device, and the power requirements of the electric locking device result in a drop in voltage at the locking device which results in the lock not operating correctly because it isn't getting adequate energy.

One typical problem will be when a valid credential is entered into the card reader or the REX (Request To Exit or also referred to as remote unlock) is activated, the door lock does not unlock for the time you programmed. Instead it will instead only momentarily activate, and then immediately relock.

TEST: When you disconnect the door lock and use a credential or the REX, the relay activates and times out properly. Remove the power and lock from the controller and place an ohmmeter on the contacts on the access controller you are using for switching the electric lock. The meter should also track as the relay changes state, and indicate pretty close to zero ohms in one position and infinity in the other. It depends on whether you're working with a failsafe or failsecure type locking device.

Electromagnetic locks are an example of a FAILSAFE locking device because if power is interrupted to the electromagnetic lock, it unlocks. A Von Duprin exit device with electric latch retraction is an example of a FAILSECURE locking device, because it has to be powered to unlock it, and if power is removed, it locks is a secure mode.

When you connect the lock directly to your lock power supply, the lock works properly; that is to say, it locks and unlocks when power is applied and removed. Assuming you did not bring the power supply over to the door or the lock over to the power supply for your test, and the power supply is in its permanent location, as is the door lock, then you are not dealing with voltage drop.

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.