In India, we have a large power distribution system with heavy distribution losses and variations in industrial/ domestic load. This results in voltage variations that may damage electrical/ electronic appliances like light, fan, television, mixer-grinder, air-conditioner, heater, water pump, toaster, etc. Here, we describe how to make a solid state voltage stabiliser that does not use electro-mechanical relays and is suitable for most purposes. Key features of the solid-state voltage stabiliser are:
- Wide range of voltage variation from 120 V to 280 V
- Only two settings are required low voltage and high voltage
- Stabilised output of 220V
- Compact size
- Silent operation and no relay chattering sound
- Bar graph LED voltage indicator
- Low/high voltage indicator and cut-off protection
The block diagram of a solid state voltage stabiliser is shown in Fig.1.
The circuit diagram comprises following four sections:
1. Analogue voltage to digital step changer
2. Isolated solid-state power relay
3. Control power supply unit
4. Mains transformer
Analogue voltage to digital step changer
The circuit diagram of a solid state voltage stabiliser is shown in Fig.2. The heart of the stabiliser is IC1 (LM3914) bar display driver. It is used as LED type bar graph voltmeter with lower voltage and upper voltage settings through presets VR1 and VR2. IC1 senses mains voltage. The difference between the lower voltage and upper voltage is divided into 10 steps. every LED indicates one step or one voltage level and is lit depending on the level of voltage received.
All the 10 outputs of IC1 that are used to lit the LEDs are also fed as inputs to dual decoder/demultiplexer CD4556. CD4556 is used for converting analogue voltage to digital steps to ensure that, at a given time, only one tapping of mains transformer gets input supply voltage from mains. In all conditions only one step can be active as per analogue input voltage.
Assume the first condition when the mains voltage is less than the lower set value. All the output pins (1, 18, 17, 16, 15, 14, 13, 12, 11, 10) of IC1 will be high. IC3(A) will be disabled and no step will be selected (means low volt 16, 15, 14, 13, 12, 11, 10) of IC1 will be high. IC3(A) will be disabled and no step will be selected (means low voltage cut-off).
As the mains voltage increases to more than the lower set value, LED1 of the bar graph voltmeter glows as pin1 of IC1 is low and all other outputs pins are high. In this condition IC2(A) is enabled because input E (pin 1) is low. As inputs A0 and A1 of IC2(A) are high, out put Q3 goes low. This is step 1 of step charger.
When voltage increases, input A0 of IC2(A) goes low and its output Q2 also goes low. This is Step 2 of step changer.
Both these outputs are combined with 1N4148 diodes and given to cathode pin of internal LED of IC7 (MOC3011). As internal LED of IC7 glows, TRIAC1 conducts and provides AC mains to tapping ‘A’ of mains transformer X2.
When voltage increases further, both inputs A0 and A1 of IC2(A) go low, while both of its outputs go high , and TRIAC1 goes off. Input A1 and output Q2 of IC2(A) generate enable input E for IC2(B) with the help of set and reset input pins (S and R) of flip-flop IC5(A) (CD4013). Pin 1 of IC5(A) provides low signal to enable input E of IC2(B) and output Q3 of IC2(B) goes low. This is Step 3 of step changer. Similarly, other conditions work in the same manner (see Table).
The number of tappings for transformer X2 and the number of solid-stat relays to be used depend on the voltage range to be covered. If the minimum voltage can drop to 100 volts and the maximum could rise to 300 volts, we need to cover 200 volts deviation. This can be managed either through ten tappings with 20V difference or just five tappings with 40V difference between each.
Isolated solid state power relay
Isolated solid-state power relay comprises an opto-isolatortriac driver MOC3011, bridge rectifier (5A) and triac BT136. The opto-isolator triac driver MOC3011 is used for controlling the steps and connecting AC mains power supply to correct tapping of mains transformer X2 via solid-state relay. The capacity of solid-state relay depends on both the components traic and bridge rectifier. Here triac BT136 and 5A bridge rectifier are used for 1kW load. Triac BT139 with 10A bridge rectifier can be used for a solid state relay of more than 1 kVA and less than 3 kVA. You can use up to 3 kVA solid-state voltage stabiliser with 3 kVA transformer.