Questions

Date:2016-2-18

1、Questions list
※ What is solid state relay? What are the advantages and disadvantages?
※ Applications of solid relay?
※ Classifications of solid state relay?
※ Suitable power frequency for solid state relay and other power module?

※ Difference between Zero Cross and Random turn-on for solid state relays?

※ Difference on use of zero-cross turn on type and random turn on type?

※ Difference between normal model triac output SSR and enhanced type SCR anti-paralleled output SSR?

※ How to determine whether the solid state relay has been damaged?

※ Is heat sink necessary for solid state relays?

※ Solid state relay heat and selection of heat sink?

※ Points on the installation of heat sink on solid-state relay  

※ RC Snubber and off state leakage current

※ Solid-state relay voltage grade selection and over-voltage protection

※ Selection of current grade and over current protection of SSR

2、Answer for questions

※ What is solid state relay? What are the advantages and disadvantages?
      Solid-state relay (ab. SSR) is a contactless switching device with relay characteristics, that use semiconductor instead
of traditional electric contact as switching device. Single-phrase solid state relay is an active device with four terminals,
two of them are input control terminals, the other two are output terminals, opto-isolation between input and output.
When control voltage signal at input side reach a certain current value, the output end shift from off-state to on-state.
Compared with traditional electromagnetic relay, solid state relay is more reliable, with long service life, no noise,
no spark, fast switching speed, insulation resistance, vibration resistance, compatible with TTL, DTL and HTL logic circuit. With a small control signal, SSR can directly drive large current electronic switch. Major deficiency: on-state voltage drop(need corresponding cooling measures), off-state leakage current, AC/DC have different applications, less contact sets,
besides index of over current, over voltage, voltage rising rate, current rising rate and etc is bad.


※ Applications of solid relay?
     Solid state relay currently has been widely used in air conditioning, water heater, refrigerator, automatic washing machine, instrumentation, medical equipment, photocopiers; Computer peripheral interface devices, electric furnace heating control system, CNC machinery, remote control system, electric actuators, industrial automation device; Signal light, flasher, light control system; Automatic fire control, security systems, etc. In addition, SSR is frequently used in chemical industry, coal mine or other occasions that need explosion-proof, moisture-proof, anticorrosion.


※ Classifications of solid state relay?
     AC solid state relay can be divided in many ways. Depending on switch methods, there are zero-cross turn on type
and random turn on type; depending on output switch components, there are triac output type(normal type) and SCR
anti-parallel type(enhanced type); depending on installation methods, there are PCB welding type, panel screw lock type
and rail type(with relay socket or cooling fan); in additional, at input terminal there are constant current source type,
wide range input (DC3-32 v), and series resistance current limiting type, etc.

 

※ Suitable power frequency for solid state relay and other power module?
     Our SSR product is suitable for the power frequency at 50 Hz or 60 Hz, but not for low frequency or high order harmonic occasions. For example, output of frequency converter have multiple sets of load that need to be switched respectively, using SSR as switch at higher harmonic occasion may cause unreliable shut-off and explosion of SSR internal RC
absorption circuit by overheat.

 

※ Difference between Zero Cross and Random turn-on for solid state relays?

     There are two switching modes for Solid state relays: 1) Zero Cross turn on 2) Random turn-on.

    Once effective input single is activated, the Random turn-on SSR breakover immediately, but the Zero Cross model only after the load voltage cross zero area. When the input single is cancelled, both Zero Cross and Random SSR turn off after current is smaller than holding currents.

Although the Zero Cross turn on will delayed at longest 1/2 circle, they reduce instant impact and RFI on the load. They are the best choice for low electronic noise interference applications.

   The major features for Random turn-on Solid state relays are their quick response and can be used together with Phase-shifted trigger modules to adjust AC voltage.

 

※ Difference on use of zero-cross turn on type and random turn on type?

     Zero-cross turn on SSR is used as “ on-off” switch ( From the function of “on-off” switch, it is equal to ordinary relay or contactor). Solid state relay we referred most is zero-cross turn. (Zero-cross turn on can only switch but not voltage regulating)

Random turn on SSR is mainly used for chopper control (But the control signal of SSR must be square signal that synchronize with power grid and rising edge could be charged within 0~180 degree, single voltage signal or 0-5V analog signal can’t make voltage regulation. In the view of voltage regulation function, Random turn on SSR is totally different from common relay or contactor).

   One point need to be emphasized on is that internal output contacts of all types of voltage regulator module or solid state relay are SCR, and to realize purposes of voltage regulation via changing the lead angle of SCR. So the output voltage waveforms are "missing corner" of sine wave (Sine wave is different from the output of Auto former regulator). So there will be a little higher harmonics, a certain noise and the grid has some "pollution" (Domestic and foreign products are the same, which is the principle decided by the chopper).

 

※ Difference between normal model triac output SSR and enhanced type SCR anti-paralleled output SSR?

    On occasion of inductive load, when SSR from on to off, due to the current and voltage phase imbalance, there will be a big large voltage rise rate dv/dt to load on two ends of TRIAC. If the value exceeds TRIAC commutating dv/dt index (typical value of 10V / μs) will lead to delay shutdown, or even failure.

But SCR is single polarity working condition and only influenced by static voltage voltage rise rate dv/dt ((typ 100V / μs).With regard to dv/dt, enhanced SSR that is consisted of two anti-paralleled SCR has been greatly improved than SSR with one TRIAC. Therefore, it is more appropriate to choose enhanced SSR for inductive or capacitive loads.

 How to determine whether the solid state relay has been damaged?

    Under conditions of off position, AC resistance at two ends of solid state relay is close to zero measured by the omh band of the multimeter. Then it indicates the internal SCR of solid state relay is failure. Besides of this, the quality of solid state relay must be tested through circuit with load. The correct methods are like following, when the input current is zero, voltage displayed on the voltmeter are voltage of this power grid, the lamp does not light (lamp power to be more than 25W); when the input current reaches a certain value after the bright lights, voltmeter measured the voltage drop of SSR (below 3V).

 

※ Is heat sink necessary for solid state relays?

    Yes, except load current of solid state relay is smaller than 5A and heatsink intergrated SSR.

 

 Solid state relay heat and selection of heat sink?

     Heat value of solid state relay or modules are related to actual current of load drive, but have little to own current size.The role of heat sink is to dissipate heat produced by solid state relays or modules. The heat dissipation effect is not only related with the size of the heat sink, but also with the ambient temperature (season), ventilation (natural cooling or forced cooling and air volume) and the installation density and other factors are related.

Reference standard of heat dissipation effect: To ensure temperature not exceed 85 degrees between base plate of solid state relay or modules and contact surface of heat sink even under worst conditions.

 

※ Points on the installation of heat sink on solid-state relay  

    There must be consistent thin coating of Heat-Conducting Silicone Grease between the mounting surface of the solid state relay and heat sink. 

 

※ RC Snubber and off state leakage current

     RC snubber is to absorb surge voltage and improve static dV / dt index,but the SSR internal RC snubber produce off state leakage current. Generally, leakage current of SSR between 2A~5A have not influence on load raged larger than 10W (Motor); 10A leakage current doesn’t have any effect 50W more load. In addition, on practical inductive load application, RC snubber can be paralled to two output ends of SSR.

 If load power is small for some users (middle relay, contactor’s coil and electromagnetic Micro power motor), our company can customize SSR with leakage current smaller than 1mA to avoid wrong action of SSR due to charging and discharging of RC snunner.

 

※ Solid-state relay voltage grade selection and over-voltage protection

   If Peak voltage load on two AC ends of solid state relay exceed highest Peak voltage that can afford, internal SSR components will be breakdown and SSR will be damaged. Select appropriate voltage grade and parallel piezoresistor can protect SSR well. If AC load is 220VAC resistance load, please choose 240VAC SSR.

a) If AC load is 220VAC resistance load, please choose 240VAC SSR

b) If AC load is 220VAC inductive load or 380VAC resistance load, please select 380VAC SSR

c) If AC load is 380VAC inductive load, please select 480 VAC SSR; for other special, high reliable demands please select 480VAC SSR, like motor reversing application.

Over voltage protection: Besides internal RC snubber, Metal oxside varistor( MOV) could be paralled to protect the SSR. Area of MOV decides absorption rate; thickness decides protection voltage value. Generally, 240V SSR can parallel with 470V MOV, 380V series of SSR can parallel 750 MOV; 480V series of SSR can parallel 1100V MOV.

Important notice: If MOV value is too small, MOV is easy to burn down and leads to short circuit ( SSR isn’t damaged);if MOV value is too bigg, SSR couldn’t be protected. So if need to adopt MOV to protect our SSR,please refer to recommended value above.

 

※ Selection of current grade and over current protection of SSR

    Over current is the one of main reasons to cause permanent damage of output SCR of internal SSR .High speed fuse and air breaker are main protection methods of over current protection Fuses could also be applied to small capacity SSR.

  Surge current is produced by many loads at the moment of connection. Because of less heat dissipation, surge current could also be one of main reasons to damage output SCR of internal SSR like over current. Therefore,to guarantee a certain current balance is extremely important when selection solid state relays.

a) For resistance load, current grade of SSR shall be 2 more times of load rated current.

b) For AC motor load, current grade of SSR shall be 6~7 more times of motor rated current.

c) If load is alternating electromagnet,intermediate relay and inductance coil, current grade of SSR shall be 4 times of load rated current; for transformer load,needs 5 times of rated current; for special inductive and capacitive load, shall based on actual experience and enlarge current allowance.

Notices: Due to the over current burning speed of internal output SCR of SSR is within the same orders of magnitude of fusing speed of high speed fuses during load short circuit, the high speed fuses can’t protect solid state relays perfectly. The principles to select fuses current grade is a few larger then MAX load current. Current grade of SSR is as bigger as possible; the high speed fuses can protect SSR reliably. Because compensating capacitor and motor have big surge current, it is better to adopt air breaker.

 

Air breaker is divided into Low and Fast. Low air breaker is used for load of motor and power capacitor that with big surge current. Fast air breaker is applied to resistance and other inductive load. Protection speed of air breaker is lower than high speed fuses. Thus it can’t 100% protect SSR either.

TypeInfo: Technical

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