When the thermostat is open, the voltage across C1 will charge to a gate-firing potential through each pulse of the rectified signal, turning the SCR on. When SCR is on, current flows through the heater.The charging time is determined by the values of C1 and R1.Arrows indicate the flow of current during the positive half of ac signal when the SCR is triggered on. When the thermostat is closed, C1 will not charge to a gate firing potential, and SCR will be off. No current flows through the heater.
Silicon Controlled Rectifier (SCR)•The following formulas can be used in determining the average voltage, average current, rms voltage, and rmscurrentusing the firing delay angle and the conduction angle of SCRs and other thyristors.currentrmsdiT1dtiT1IrmscurrentAveragediT1dtiT1IavevoltagermsdvT1dtvT1VrmsvoltageAveragedvT1dtvT1Vave212t2t1221t2t1212t2t1221t2t1
Silicon Controlled Rectifier (SCR)eous)(instantancurrent ieous)(instantanvoltagevangletimetloadthetoappliedcurrent or voltageofangleofend2loadthetoappliedcurrent or voltageofanglestarting1loadthetoappliedcurrent or voltageoftimeendt2loadthetoappliedcurrent or voltageoftimestartingt1ageinput voltofperiodT:where
Silicon Controlled Rectifier (SCR)tVAKtttFiring Delay Angle = 600Conduction Angle = 1200 (1800-600)tSupply voltagetSupply voltageVAKVRLVRLFiring Delay Angle = 1200Conduction Angle = 600600180012001800(1800-1200)SCR is onSCR is offSCR is offSCR is onSCR is offSCR is offt1 or 1t2 or 20
Silicon Controlled Rectifier (SCR)•When the input signal is a sine wave and the SCR is supplying power to the load during a portion of the positive half of the input signal, and the SCR is on from firing delay angle F up to radians, the following formulas can be used:currentrmsdcos2-1212IpdsinIp21diT1Irmsvoltagermsdcos2-1212VpdsinVp21dvT1Vrms2periodTangledelay firingθFloadat current AverageθFcos12IpIaveRloadat thecurrent peak RVpIpsignalinput ofgepeak voltaVploadat voltageAverageθFcos12VpVaveθFcos-cos2Vp-cos-2Vpdsin 2Vpdsin Vp21dvT1VaveθF2θF22θF2θF2θF22θF2LLθFθFθFθF
Silicon Controlled Rectifier (SCR)•When the load voltage is a full wave which is rectified (load has a certain voltage during positive and negative half of input signal), the following formulas can be used:periodThalfpositivetheduringangledelay firingθFRloadat thecurrent peak RVpIpsignalinput ofgepeak voltaVp:whereloadat current AverageθFcos1IpIaveloadat voltageAverageθFcos1VpVaveθFcos-cosVp-cos-Vpdsin Vpdsin Vp1dvT1VaveLLθFθFθFθF