Katı Hal Rölesi / Solid State Relay

Unlike electro-mechanical relays (EMR), which use coils, magnetic fields, springs and mechanical contacts to operate and switch a source, solid state relay or SSR does not have moving parts, but instead uses the electrical and optical properties of solid state semiconductors.

Like a normal electro-mechanical relay, SSRs provide complete electrical insulation between input and output contacts with output that acts as a conventional electrical switch, as it is very high when not conductive (on), has almost infinite resistance and very low resistance.

Although the solid state relay and electro-mechanical relay are fundamentally similar in that the low voltage inputs are electrically isolated from the output that switches and controls a load, the electro-mechanical relays have a limited contact life, can take up a lot of space and have slower switch speeds, especially large power relays and contactors. Solid state relays have no such limitations.

Solid state relay

Therefore, the main advantages of solid state relays compared to traditional electro-mechanical relays are that they do not have moving parts to wear out and therefore do not have contact splash problems, they can be placed in both "ON" and "OFF" position much faster than mechanical relays. The fixture can move, as well as zero voltage on and zero current shutdown, which eliminates electrical noise and transitions.

Solid state relays can be purchased in standard ready-made packages ranging from just a few volts or amps to hundreds of volts and amperage output switching capacity. However, very high current grade (150A plus) solid state relays are still too expensive to buy due to power semiconductors and heat-absorbing requirements, and therefore, cheaper electro-mechanical contactors are still used.

Similar to an electro-mechanical relay, a small input voltage, typically 3 to 32 volt DC, can be used to control a very large output voltage or current. For example, 240V, 10Amps. This makes them ideal for the microcontroller, PIC and Arduino interface, since it can be used as a low current, 5 volt signal of a microcontroller or logic gate to control a certain circuit load, and this is achieved by the use of opto. Insulators.

Solid State Relay Entrance

One of the main components of the solid state relay (SSR) is an opto-isolator (also called optocoupler), which includes one (or more) infra-red light emitting diode or LED light source and a light-sensitive device inside. One case. The opto-isolator insulates the entrance from the outlet.

The LED light source connects to the input drive section of the SSR and provides optical connectivity with an adjacent light-sensitive transistor, darlington pair or triage through a gap. When a current passes through the LED, it lights up and focuses its light on a photo-transistor/photo-triage throughout the gap.

Thus, the output of an opto-connected SSR is usually energized by a low voltage signal and made "ON". Since the only connection between the input and output is a beam of light, high voltage insulation (usually several thousand volts) is provided through this built-in opto-insulation.

The opto-isolator not only provides a higher degree of input/output isolation, but can also transmit DC and low frequency signals. In addition, the LED and light sensitive device can be completely separate from each other and can be optically combined via an optical fiber.

The input circuit of an SSR can consist of only a single current limiting resistance serially with the LED of the opto-isolator, or a more complex circuit with straightening, current regulation, reverse polarity protection, filtration, etc.

In order to transmit or make a sold relay "ON", a voltage higher than its minimum value (usually 3 volt DC) must be applied to the input terminals (equivalent to the electro-mechanical relay coil). This DC signal can be derived from a mechanical switch, a logic gate or a microcontroller, as shown.

Solid State Relay DC Input Circuit

Solid state relay
Solid State Relay DC Input Circuit

When using mechanical contacts, switches, push buttons, other relay contacts, etc. as an activation signal, the feed voltage used may be equal to the minimum input voltage value of the SSR, while solid state devices such as transistors, doors and micro are used – controllers, switching devices must have a minimum supply voltage of one or two volts above the SSR's opening voltage to take into account the internal voltage drop.

However, in addition to using a sinking or welding DC voltage to transmit the solid state relay, we can also use a sinusoidal waveform by adding a bridge rectifier for full wave straightening and a filter circuit to the DC input. As shown.

Solid State Relay AC Input Circuit

Solid state relay
Solid State Relay AC Input Circuit

Bridge rectifiers convert a sinusoidal voltage into full-wave directional pulses that are twice the input frequency. The problem here is that these voltage pulses start and end at zero volts; this means that the entry threshold of the SSR will fall below the minimum opening voltage requirements, causing the output to "turn on" and "close" with each half cycle.

To overcome this irregular ignition of the output, we can correct the corrected fluctuations by using a softening capacitor (C1) at the bridge rectifier outlet. The charging and discharge effect of the capacitor will increase the DC component of the pointed signal above the maximum opening voltage of the solid state relay input. Even if the continuously changing sinusoidal voltage waveform is then used, the input of the SSR is a constant DC voltage.

Voltage reduction resistance, the values of R1 and softening capacitor C1 are selected to match the feed voltage, 120 volt AC or 240 volt AC and the input impedance of the solid state relay. But he would do something around 40kΩ and 10uF.

Then, when this bridge rectifier and softening capacitor circuit are added, a standard DC solid state relay can be controlled using an AC or non-polarized DC welding. Of course, manufacturers already produce and sell AC ground floor state relays (usually 90 to 280 volts AC).

Solid State Relay Output

The output switching capabilities of the solid state relay can be AC or DC, similar to input voltage requirements. The output circuit of most standard solid state relays is configured to perform only one type of switching action, which is equivalent to the normally open, unipolar, single-shot (SPST-NO) operation of an electro-mechanical relay.

The solid state switching device, which is widely used for most DC SSRs, are power transistors, Darlingtons and MOSFETs, whereas the switching device for an AC SSR is either a triage or back-to-back thyristors. Thyristors are preferred due to their high voltage and current capabilities. A single thyristor can also be used, as shown in a bridge rectifier circuit.

Solid State Relay Output Circuit

Solid state relay

The most common application of solid state relays is the switching of an AC load, whether it is to control AC power for on/off switching, light dimming, engine speed control or other applications where power control is required, these AC loads can be easily controlled by low current DC voltage using solid state relay, which provides long life and high switching speeds.

One of the biggest advantages of solid state relays over an electromechanical relay is the ability to "SHUT DOWN" AC loads at zero load current point, thereby completely eliminating arc, electrical noise and contact splash associated with conventional mechanical relays and inductive loads. .

This is because ac switched solid state relays use SCRs and TRIAC's as output switching devices that continue to transmit after the input signal is removed, until they fall below the AC current threshold flowing from the device or below the holding current value. Then the output of an SSR can never be TURNED OFF in the middle of a sine wave peak.

Zero current shutdown is a great advantage to use solid state relay as it reduces electrical noise and the back emk associated with switching inductive loads seen as arcs by the contacts of an electro-mechanical relay. Consider the output waveform diagram below of a typical AC solid state relay.

Solid State Relay Output WaveForm

Solid state relay

Without any input signal applied, the load current does not pass through the SSR, as it is effectively OFF (open circuit) and the output terminals see the full AC supply voltage. With the application of a DC input signal, due to the zero voltage switching properties of the SSR, the output is only opened when the waveform passes, regardless of which part of the cycle passes through the positive or negative, sinusoidal waveform. ground zero.

As the supply voltage increases in a positive or negative direction, it reaches the minimum required to fully turn the output thyristors or triage into the ON position (usually less than about 15 volts). Voltage drop in the output terminals of the SSR, status voltage drop of switching devices, VT (usually less than 2 volts). Thus, high sudden currents associated with reactive or lamp loads are greatly reduced.

When the DC input voltage signal is removed, the output does not suddenly turn off as it is triggered by transmission once, the thyristor or triage used as a switching device remains ON for the rest of the half-cycle until it falls under the devices holding load currents. current becomes OFF at this point. This greatly reduces the high dv/dt back emfs associated with replacing inductive loads in the middle of a sinus wave.

So the main advantages of the AC solid state relay over the electro-mechanical relay are the zero transition function, which turns on the SSR when the AC load voltage is close to zero volts, thereby suppressing any high sudden currents as the load current will always start. Natural zero current closure from a point close to 0V and tristor or triage. Therefore, there is a maximum possible shutdown delay of one half cycle (between the removal of the input signal and the removal of the load current).

Phase Dimming Solid State Relay

Solid state relays can perform direct pass-through switching of a load, while performing much more complex functions through digital logic circuits, microprocessors, and memory. Another excellent application of solid state relay is lamp dimming applications, whether at home or for a show or concert.

Solid state relays with non-zero (instant-on) switching open immediately after the implementation of the input control signal, unlike the zero-pass SSR, which waits until the next zero transition point of the AC sinus wave. This random ignition switching is used in resistant applications such as lamp dimming and applications that require energy to be energized for only a small part of the AC cycle.

Random Switching Output WaveForm

Solid state relay

This allows phase control of the load waveform, while the main problem of randomly opened SSRs is that the first load fluctuation current at the moment the relay is opened can be high due to SSR switching power when there is a supply voltage. close to peak value (90o). When the input signal is removed, it stops transmitting when the load current falls below the tristor or triage-holding current, as shown. Explicit on-OFF replacement action for a DC SSR takes place instantly.

The solid state relay is ideal for a wide range of ON/OFF switching applications as it does not have moving parts or contacts unlike an electro-mechanical relay (EMR). Since they use semiconductor switching elements such as thyristors, triagers and transistors, there are many different commercial types that can be selected for AC and DC output switching as well as AC and DC input control signals.

But using a good combination of opto-isolator and a triage, we can make our own cheap and simple solid state relay to control an AC load such as heater, lamp or solenoid. Since an opto-isolator only needs a small amount of input/control power to operate, the control signal can be from any MICROcontroller such as a PIC, Arduino, Raspberry PI or similar.

Solid State Relay Example

Let's say that we want a microcontroller with a 120V AC, only a +5 volt digital output port signal to control a 600 watt heating element. For this we can use the MOC 3020 opto-triage isolator, but the built-in triage 120V AC can only exceed a maximum current of 1 Amp (ITSM) at the top of the feed, so an additional switching triage must also be used.

First, let's consider the input features of the MOC 3020 opto-isolator (other opto-triagecs are available). The Opto-isolators datasheet tells us that the decrease in the forward voltage (VF) of the input light emitting diode is 1.2 volts and the maximum forward current (IF) is 50mA.

The LED needs about 10mA to shine quite brightly up to its maximum value of 50mA. However, the microcontroller's digital output port can only provide a maximum of 30mA. Then the value of the required current is somewhere between 10 and 30 milli amps. Then:

Solid state relay

Thus, a series current limiting resistance with a value between 126 and 380Ω can be used. Since the digital output port always switches to +5 volts and to reduce power loss through the opto-coupling LED, we will choose a preferred resistance value of 240Ω. This gives an LED forward current lower than 16mA. In this example, it will make any preferred resistance value between 150Ω and 330Ω.

Heating element load is 600 watts resistant. Using a 120V AC source gives us a load current of 5 amps (I = P/V). Since we want to control this load current in both half cycles (all 4 quarters) of the AC waveform, we will need a mains switching triage.

The BTA06 is a 6 amp (IT(RMS)) 600 volt triage suitable for general purpose ON/OFF switching of AC loads, but any similar 6 to 8 amp grade triage will do the trick. In addition, this switching triage requires only a 50mA gate driver to initiate transmission; this is much lower than the maximum value of the MOC 3020 opto-isolator of 1 amp.

Consider that the output triage of the opto-isolator is on at the peak value (90o) of the 120VRMS AC supply voltage. The value of this peak voltage is: 120 x 1.414 = 170Vpk. If the opto-triage maximum current (ITSM) is 1 amp peak, the minimum required serial resistance value is 170/1 = 170Ω or 180Ω to the nearest preferred value. This 180Ω value will protect the opto-coupling output triage, as well as the BTA06 triage door on the 120VAC feed.

If the triage of the opto-isolator switches to the ON position at zero pass value (0o) of the 120VRMS AC supply voltage, the minimum voltage required to ensure the required 50mA gate drive current that forces the switching triage to transmission will be: 180Ω x 50mA. = 9.0 volts. Then, when the sinusoidal Gate-to-MT1 voltage is greater than 9 volts, the triage passes to the transmission.

Therefore, the minimum voltage required after the zero transition point of the AC waveform will be 9 volt peaks, since the power loss in this series door resistance is very small, so the nominal resistance of 180Ω, 0.5 watts can be safely used. Consider the circuit below.

AC Solid State Relay Circuit

Solid state relay

This type of optocuplor configuration forms the basis of a very simple solid state relay application that can be used to control the load fed from any AC network, such as lamps and motors. Here we used MOC 3020, a random switching isotope. The MOC 3041 opto-triage isotope isotope has the same characteristics, but with built-in zero pass detection, it allows the load to get full power without heavy sudden currents when replacing inductive loads.

Diode D1 prevents damage due to inverted input voltage, while 56 ohm resistance (R3) eliminates incorrect triggering by shunning any di/dt current when the triage is OFF. It also connects the door terminal to the MT1 and ensures that the triage is completely closed.

If the pulse width is modulated and used with the PWM input signal, the ON/OFF switching frequency for the AC load must be set to a maximum of less than 10Hz, otherwise the output switching of this solid state relay circuit may not be able to catch up.