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DC Relays-Technical Guidelinesnew-1.Eps DC Output Solid State Relays Introduction Relay Connections Inductive Load Considerations DC output solid state relays are Because the input and output When a DC load is inductive, special being used more and more widely. terminals of dc solid state relays are precautions have to be taken to One of the major reasons for the use electrically isolated by up to 3,750 protect the solid state relay at turn of solid state relays for switching dc Vrms the relative positions of the off. Energy is stored in the magnetic is the fact that as there are no input signal and load connections flux created by the current flowing moving parts there is no contact are irrelevant. As shown in Fig 1, the through the inductive load. When arcing or wear out mechanism. input can be supplied either from a the solid state relay is turned off the However, there are some source or sink configuration and the collapse of themagnetic flux creates precautions which have to be taken load can be connected to either the an electro-magnetic force with a when using solid state relays with relay positive or negative output polarity that tries to maintain the inductive dc loads. terminals. The solid state relay can pre-existing current flow.This is be used as a level shifter because shown schematically in Fig 2. no electrical relationship is required between the control and power output sections of the relay. + - DC + 3 2 + DC SSR + DC DC Relay - LOAD - 4 1 - + I VR=IR + - + 3 2 + LOAD E + + - DC DC Relay DC - - - 4 1 - + - DC SSR + 3 2 + DC + DC DC Relay - LOAD + - - 4 1 - + VBR VR=IR + i - - + 3 2 + LOAD E + + - DC DC Relay DC - - VL=(VBR+ iR) -E - 4 1 - + Fig. 1 - Connecting Solid State Relays Fig. 2 Page 1 of 2 If no electrical path is provided for In some applications it is necessary to An alternate method to create the the inductive load current to flow, discharge the inductive current rapidly same inductor voltage is to the collapse of magnetic flux will (for example to open a solenoid as connect the zener or TVS diode generate a voltage high enough to quickly as possible). If rapid current across the output terminals of the break over any limiting voltage discharge is a requirement, then, the solid state relay, as shown in element in the circuit, one element discharge path must be designed to figure 3(c), in this case the clamp- is usually the solid state relay, require a high voltage to be generated. ing voltage can be 100 volts. shown schematically as VBR. In This follows from the fact E = Ldi/dt, However, as this voltage is in the case of a solid state relay, where E is the voltage generated by series with the 48 Vdc supply the either the output power semicon- the collapse of magnetic flux, L is the end result is the same as putting ductor device or one of the driving inductance of the load and di/dt is the the clamping component across semiconductors will be broken rate of change of current. Obviously for the load, a voltage of 52 volts has over into conduction. In many a fixed value of L the greater the value to be generated by the inductive cases this break over will perma- of E, then the greater the di/dt and the load. nently damage the semiconductor more rapidly the load current is resulting in a relay with a permanently reduced to zero. Increasing the shorted output. voltage which has to be generated to DC create afreewheel path can be accom- SSR It is fairly straightforward to provide a plished by changing the standard + - circulating path for the inductive diode shownin Fig 3(a) to a zener or + Z1 current in most circuits. This is usually TVS diode as shown in Fig 3(b). VR=IR + - accomplished by the addition of a VZ E - freewheel diode as shown in Fig A. It must be recognized that the voltage- - V =(iR+V )-E appearing across the solid state relay i L Z1 is the sum of the supply voltage and + the zener or TVS diode voltage. So for DC a system using a 48 Vdc supply and a Fig 3 (c) SSR 100 volt rated solid state relay, the voltage across the freewheel compo- + - nents cannot be greater than 52volts Conclusion + at maximum load current. + Inductive E D1 i Load DC output solid state relays give - DC - VSSR =E+VD1 SSR designers a great deal of flexibility in the relative connections of + control and load voltages. - + Fig A VR=IR + D1 - Care must be taken when a solid E VD1 i - state relay is used to control and - Unless the solid state relay is to be VL=iR+VD1 inductive load, to ensure that at turned on while current is still flowing in turn off, the voltage rating of the + the freewheel diode, the diode can be solid sate relay is not exceeded. a standard recovery type. If, however, This is normally accomplished by DC the use of a freewheel diode but in the solid state relay can be turned on VSSR=E+VD1+VZ1 SSR before the load current has been com- some circumstances it is neces- + sary to force the load to generate pletely discharged, then a fast recovery - + D1 a high voltage for rapid discharge diode must be used in the freewheel VR=IR VD1 of the load current. In these cases position. The use of a fast recovery + - i - diode reduces the instantaneous E Z1 a zener or TVS diode isused to - VZ1 increase the voltage necessary to inrush current amplitude and duration VL=iR+VD1+VZ1 when the relay is turned on into an provide the inductive load with a existing freewheel diode current. + freewheel path. Fig 3 (a – b) Page 2 of 2.
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