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Q - Why Is it Essential to connect a Diode in reverse polarity in parallel to a DC operated electric lock?

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A - The diode shorts out the high reverse voltage which develops on a lock's coil (electromagnet) when the DC (Direct Current) power is turned off. In practical terms: For "Fail Secure" locks the power is applied during the unlocking time and then turned off, while for "Fail Safe" Locks (such as Magnetic Locks) it is applied for locking and turned off for unlocking. The reverse voltage could reach up to 10 times the nominal voltage. For a 12VDC lock it could reach 120VDC. The reverse voltage is powerful enough to destroy the controlling device relay contacts, erase programmable data of flash memory (commonly used in keypads and intercoms) or in worst case scenario destroy the intercom, keypad or any other control device. A diode in parallel to the lock coil will short out the high reverse voltage and clamp it down to about 0.7VDC which is a diode typical ON voltage. The Figure below shows typical keypad controlling a 12VDC Electric Door Strike or Magnetic Door Lock. The Diode must located as close as possible to the lock and connected in reverse polarity, Cathode (side with White band) to the + (Positive, RED wire connected to the N.O. Keyapd output in this example) and the other side to the - Negative (Black wire connected to the -12VDC terminal of the Power Supply)    


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Background

Many manufacturers of Access Control equipment such as Access Control keypads, Intercoms, Direct Voltage operated Electric Door  Strikes and Magnetic Door Locks enclose with the equipment accessories a small electronic device called diode. In essence the diode is a semiconductor device which is main characteristics is that it conducts  direct current only in one direction from the Positive Terminal (Anode) to the Negative Terminal (Cathode) while block the current flowing in the opposite way. In simplified terms it is like a close switch (very low resistance) in one direction and an open switch in the opposite direction (very high resistance) .

Although all of the relevant installation manuals enclosed with the equipment clearly  shows this diode wired in the system at a close physical location  to the DC lock, Anode connected to the negative terminal of the lock and the Cathode to the positive most of the installers DIY and professionals skip using this diode. The most common reply we hear when questioning why the diode was not used " It works without it...). Yes they correct at the time of the installation but may be wrong either within a very short time, a very long time or if they are luck never wrong.  We will use this opportunity to explain the cause and the destructive effect if diode is not used.

Reverse Voltage phenomena Explanation (See Image on the right)

Direct Voltage  (DC) electric locks (electric strikes and magnetic door locks) typically operates on 12VDc or 24VDC are essentially an electromagnet constructed of many copper wire windings wired next to each other in many layers.

As the current flows through the coil a self induced magnetic field is thus pulling the lock mechanism. When the current in the coil is turned "OFF", a large back emf (electromotive force) voltage is produced as the magnetic flux collapses within the coil (transformer theory). This induced reverse voltage value may be very high in comparison to the switching voltage, and may damage relay contacts, semiconductor device such as a transistor, microcontroller or at best erase data previously programmed into to the keypad.

Solution

Connect in parallel to the lock coil a diode in reverse polarity to short out the "kickback voltage, thus circumventing the damage. The diode reverse block voltage must be at least twice the peak of the reverse voltage that could be around 10 time the nominal voltage of the electric lock.

Example

A typical 12VDC Electric Door Strike draws I=250mA. Its coil inductance is assumed to be L= 150mH. The reverse voltage will develop across the coil when the keypad/intercoom relay turns off. The shorter the time it takes the keypad/intercom relay to turn off, the larger is the reverse voltage developing across the door strike coil. Typical relay turnoff time is 1-2msec. Assuming for the example shown in the highlighted area a turn off time of 1msec, the voltage that develops across the electric lock coil will reach 37Vdc. When the relay closes again, this voltage will dischargh to the keypad/intercom relays, thus potentialy damaging the relay contacts, destroying the electronics circuit, or at best earsing the data programmed into the keypad/intercom memory.   

 

 

Vr =  - L di/dt

Vr = - 150*10-3* 250*10-3/ 1*10-3=37Vdc

 

L- Electric Lock inductance

di - Current change from steady to 0

dt - Relay switch off time.