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Energy-saving relays

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April 23, 2019
Energy-saving relays
 
Michael A. Shustov (Russia), Andrey M. Shustov (Germany)
 
Energy-saving relays considered in this article are two-terminal devices. They can be connected in any order in series with the load and with a chain of normally closed push-button. When you push any of these buttons, the relay turns on the light source or other load for a specified period of time. After expiration of this time period the load will be turned off. By using of such relays, a noticeable saving of electrical energy is possible.
 
The device described in this article is much simpler and cheaper than its analogues.
 
In earlier publications [1, 2] we proposed a multi-button control of light sources. This device allows you to turn on the light, if you push on any of normally closed push-buttons (connected in series). When you push it again, the light will be turned off.
 
When using the device described in this article, you only need to push the button once. For example, when entering a room or corridor. After that, the electric light is turned on for a specified period of time. After expiration of this time period the light will be turned off automatically without pushing any button. One of these buttons can be installed on the door, automatically releasing when entering the room. In this case, there is no need to push the button.
 
Figure 1 shows a ciruit of one of such devices, powered by a 12 V DC-Source (battery, mains power supply). When the device is turned on, the storage capacitor C1 (the backup power source) is charged via the normally closed push-buttons SB1–SBn, via the lamp LA1 and via the diode D1. The light-emitting diode LED1 indicates the device's ready status, shows the location of the relay and partially illuminates the room. The transistor T1 is closed, no current flows through it.
 
When you push any of the buttons SB1-SBn, the current doesn’t flow through the diode D1. The base of the transistor T1 is connected to the common bus via the chain R1 and LED1. The transistor opens and therefore the capacitor C1 discharges to the coil of the relay RE1A.
 
The relay contacts (RE1B) connect the load LA1 (for example, a light bulb) to the power supply, while simultaneously shorting the power supply path of the energy saving relay.
 
The discharge time of the capacitor C1 to the coil of the relay (the turn-on time of the light) can be determined by an approximate formula:  For relays EMR151A12, RRE1A = 1050 Ohm, for G6DS-1AH 12DC, RRE1A = 1200 Ohm. Thus, for the first relay (EMR151A12), the turn-on time at the capacitance of C1 68000 μF will be about 40 seconds, at 33000 μF - about 20 seconds and at 10000 μF - 7 seconds.
 
The nominal value of the resistor R1 can be calculated as follows:  thereby ULED1 and ILED1 – voltage and current required to illuminate the LED. In case when the device is supplied by a battery power source, the value of this current must be reduced to a minimum in order to save energy resources.
 
After the capacitor C1 is discharged to the coil of the relay, the relay RE1A is turned-off. As a result, the light source is de-energized and the relay is returned to its original state.
 

Figure 1. Energy-saving direct-current relay.
 
The second energy-saving relay (Figure 2) uses a MOSFET transistor T2 2N7075 or 2N7085 as a switching element. In contrast to the previous relay, the switch-on time duration can be smoothly adjusted by the potentiometer R3 in the range from 0 to 50 seconds (1 kOhm of the resistance R3 corresponds approximately to 1 second). The advantage of this relay is the lower capacitance of the capacitor which defines the time constant, the controllability of the switch-on time duration, the increased load current determined by the properties of the switching transistor T2.
 

Figure 2. Energy-saving direct-current relay with transistor as a switch
 
The third energy-saving relay (Figure 3) is working at alternating-current with 230 V and 50 Hz and uses the thyristor VS1 2N6073 as a switching element. If the capacitance of the capacitor C3 (defines the time constant) is equal to 3300 μF, the switch-on time duration is 9 seconds; at 10000 μF – 27 seconds correspondingly. The difference of this device is, that after switch-off the light, in order to switch-on the relay again, the relay needs several seconds to re-charge the storage capacitor C3.
 
If there is no need for LED backlighting, the light-emitting diode LED1 (Figure 1–3) can be removed from the circuit. Thereby the resistor R1 with a nominal value of 2–20 kΩ must not be removed from the circuit.
Energy-saving relays can be manufactured in a compact, sealed housing with two terminals (for the DC-relay, the polarity of the terminals should be indicated).
 

Figure 3. Energy-saving alternating-current relay with thyristor switch
 
Literature
  1. Shustov M.A. Multi-switch Lights Control. For corridors and hallways // Elektor. – 2014. – V. 40 (450). – № 6. – P. 74–75.
  2. Shustov M.A., Shustov A.M. Electronic Circuits for All. – London: Elektor International Media BV, 2017. – 397 P.
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