How can I prevent reverse voltage in coils?

Counter electromotive voltage generated when the coil voltage of a relay is interrupted causes overvoltage to various elements in the circuit, resulting in malfunctions and breakdowns. Thus countermeasures to suppress counter electromotive voltage are necessary. Diodes are generally used for DC coils, but for some high-capacity relays a combination of a Zener diode as well as a diode is recommended because the contact open/close speed is reduced and the electrical durability is greatly affected.

 
 

image What happens if I use only diodes?


Diodes have a small forward voltage drop, resulting in slower power dissipation speed and longer coil current duration. Since the magnetic force of the coil depends on the coil current, a long coil current decay time slows the speed at which the relay contacts open and close, inducing premature welding failures of the contacts.
 
 

Circuit diagram for diode

Waveform for diode

 
 
When a Zener diode is connected in series with a diode as shown in the below figure, the large voltage drop of the Zener diode shortens the decay time of the coil current, which increases the opening speed of the relay contacts and prevents failure due to premature welding of the contacts.
 
 

Circuit diagram for diode & Zener diode

Waveform for diode & Zener diode

 
 
If the Zener voltage is too low, it is less effective in increasing the contact opening speed. For this reason, select the optimum Zener voltage for each model.
 
 
image What is the optimum Zener diode voltage?


When a Zener diode is connected in series with a diode as shown in the above figure, the large voltage drop of the Zener diode shortens the decay time of the coil current, which increases the opening speed of the relay contacts and prevents failure due to premature welding of the contacts. On the other hand, if the Zener voltage is too low, the contact opening speed will slow down. And if the voltage of the Zener diode is too high, it may adversely affect the peripheral elements. For this reason, the optimum Zener voltage is set for each model.
 
 

Model Recommended Zener Voltage
* Rated coil voltage ratio
G9KA x2 to 3
G7EB x3
G9KB x3
G7L-X x1 to 2
G2RG-X x3
G5PZ-X x1 to 3

 
 

When used with a Zener diode, the diode should be above the Zener voltage and the forward current should be above the rated current of the coil.
 
 
Please refer to the table on the left for the recommended Zener voltage for each model.

 
 


 
 
image Can’t I use varistors for countermeasures against back EMF?


Instead of a combination of a diode and a Zener diode, a varistor can be used. In this case, too, the coil current waveform is similar to that of the combination.

Circuit diagram for diode

Waveform for diode

 
 
The recommended varistor voltage is basically the same as the recommended Zener voltage. If the varistor voltage is smaller than the maximum value of the power supply voltage, however, there is a possibility that voltage will not be correctly applied to the coil. Be sure to select a voltage greater than the maximum power supply voltage.
 
  
 


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Easy-to-understand explanations of the unknowns when using high-capacity power relays


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