As explained in “02 How can coil power consumption be reduced?”, power consumption can be reduced by using a holding voltage for the coil. In addition, some relays must be used at holding voltage to maintain their rated characteristics. Please check the datasheet of each model for the specified holding voltage value of high capacitance power relays before use.
There are several recommended methods for holding voltage circuits. Below are some of the introductions.
The CR method is an electric circuit consisting of a capacitor and resistor, and is the simplest configuration to achieve a holding voltage. A capacitor and a resistor are connected in parallel to form a circuit. The voltage applied to the coil varies depending on the state of charge of the capacitor.
Determine the resistance value R so that the coil voltage falls within the holding voltage specified for each model. By selecting appropriate values for capacitor and resistor, the holding voltage duration and stability can be adjusted. While long-term use can be expected since there are no physical wear parts, care must be taken to avoid failures due to capacitor deterioration.
Reference diagram of holding voltage circuit
(1) Apply a voltage higher than the rated voltage to the coil.
(2) After the relay is turned on by the current flowing in C (capacitor), the current is suppressed by R (resistor).
* C (capacitor) should be set so that a current greater than the rated current flows for 40 ms or more, and R (resistor) should be set so that a current greater than 50% of the rated current flows.
Apply the rated coil voltage of 100% or more and within the range of 40 ms to 3 s.
Easier circuit design thanks to a simple circuit structure.
The voltage is divided by the ratio of R (resistance) to the resistance of the relay coil, eliminating the need for sequence control as in the switch method
The presence of resistor reduces power-saving effect. To apply the rated voltage to the relay coil for 100 ms, a voltage higher than 100% of the coil’s rated voltage must be applied to the circuit.
Increased capacitance of capacitor. The circuit voltage is divided according to the state of charge of the capacitor. Since 100% of the coil rated voltage must continue to be applied to the relay coil for 100 ms, a capacitor capacitance must be selected that will complete the charge in a sufficiently longer time than 100 ms.
A switch can be used instead of a capacitor to hold the charge. There are two types of switching.
The first switch method is a simple electrical circuit configuration consisting of a switch and a resistor. A switch and a resistor are connected in parallel to form a circuit. With the switch is ON, the rated voltage is applied to the relay coil by applying voltage to the circuit. When the switch is turned OFF after 100 ms, the voltage is divided by the resistance of R (resistance) and the relay coil, and a holding voltage is applied to the relay coil.
Reference diagram of holding voltage circuit with switch
(1) By turning the switch ON, the rated voltage is applied to the coil.
(2) When the switch is opened, the current is suppressed by R (resistance).
* Set the R (resistance) value so that a current of at least 50% of the rated current flows when the switch is off.
Apply the rated coil voltage of 100% or more and within the range of 100 ms to 3s.
- Easier circuit design due to simpler structure than CR method
- The presence of resistor halves power-saving effect
The second switch method requires two applied voltages (power supplies) and therefore requires two switches in its electrical circuit configuration. Since more than 100% of the rated voltage must be applied to the relay coil for more than 100 ms, both the coil drive switch and the voltage switching switch must be turned ON when the power is turned on. The higher supply voltage (applied voltage A) is applied to the relay coil. When the voltage switching switch is turned OFF after 100 ms, only the power supply with 50% coil rated voltage (applied voltage B) is connected to the circuit, and the holding voltage (50%) is applied to the relay coil.
Reference diagram of holding voltage circuit with switch
(1) Turn on the voltage switching switch and turn on the coil drive switch.
(2) When the relay is turned ON, turn off the voltage switching switch.
Apply the rated coil voltage of 100% or more and within the range of 100 ms to 3 s.
- No resistance maximizes power saving effect
- Increased equipment size and cost due to the need for two power supplies
- Complicated design on the power supply side
PWM (Pulse Width Modulation) control is a method of controlling power by repeatedly turning it on and off using semiconductors. Fast ON/OFF switching maintains constant voltage with less power.
The more time the voltage is off, the less power is consumed. By turning more voltage signals on and off per second, it is possible to apply a constant average voltage while increasing the OFF time. This ON/OFF ratio is called duty ratio.
We do not recommend the use of PWM control circuits because of the power loss caused by the Zener diode and the difficulty in significantly reducing the duty ratio in general PWM control circuits. Mount a switch in parallel with the Zener diode and bypass it during PWM control. By turning off the applied voltage after the switch is turned off first, the relay is then normally turned off by the Zener diode + diode.
Typical PWM circuit + Zener diode
* Due to the Zener diode, the PWM circuit may not operate as expected.
Recommended PWM circuit
The following are examples of coil current values at various duty ratios for the G9KA power relay type. A typical PWM circuit & Zener diode combination requires a duty ratio of 86% or more to keep the coil current to maintain the relay on. This causes the relay to generate more heat than in the recommended PWM circuit holding state because of the increased power consumption. The effect of power savings is also reduced.
On the other hand, the recommended PWM circuit can meet the criteria for coil current to maintain with a duty ratio of 45% or higher.
Example of G9KA
Typical PWM circuit + Zener diode
* Due to the Zener diode, the PWM circuit may not operate as expected.
Recommended PWM circuit
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