When the thermostat is open the voltage across C1 will charge to a gate firing

When the thermostat is open the voltage across c1

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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.
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Silicon Controlled Rectifier (SCR) The following formulas can be used in determining the average voltage, average current, rms voltage, and rms current using the firing delay angle and the conduction angle of SCRs and other thyristors. current rms d i T 1 dt i T 1 Irms current Average d i T 1 dt i T 1 Iave voltage rms d v T 1 dt v T 1 Vrms voltage Average d v T 1 dt v T 1 Vave 2 1 2 t2 t1 2 2 1 t2 t1 2 1 2 t2 t1 2 2 1 t2 t1
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Silicon Controlled Rectifier (SCR) eous) (instantan current i eous) (instantan voltage v angle time t load the to applied current or voltage of angle of end 2 load the to applied current or voltage of angle starting 1 load the to applied current or voltage of time end t2 load the to applied current or voltage of time starting t1 age input volt of period T : where
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Silicon Controlled Rectifier (SCR) t V AK t t t Firing Delay Angle = 60 0 Conduction Angle = 120 0 (180 0 -60 0 ) t Supply voltage t Supply voltage V AK V RL V RL Firing Delay Angle = 120 0 Conduction Angle = 60 0 60 0 180 0 120 0 180 0 (180 0 -120 0 ) SCR is on SCR is off SCR is off SCR is on SCR is off SCR is off t1 or 1 t2 or 2 0  
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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: current rms d cos2 - 1 2 1 2 Ip d sin Ip 2 1 d i T 1 Irms voltage rms d cos2 - 1 2 1 2 Vp d sin Vp 2 1 d v T 1 Vrms 2 period T angle delay firing θ F load at current Average θ F cos 1 2 Ip Iave R load at the current peak R Vp Ip signal input of ge peak volta Vp load at voltage Average θ F cos 1 2 Vp Vave θ F cos - cos 2 Vp - cos - 2 Vp d sin 2 Vp d sin Vp 2 1 d v T 1 Vave θ F 2 θ F 2 2 θ F 2 θ F 2 θ F 2 2 θ F 2 L L θ F θ F θ F θ F
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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: period T half positive the during angle delay firing θ F R load at the current peak R Vp Ip signal input of ge peak volta Vp : where load at current Average θ F cos 1 Ip Iave load at voltage Average θ F cos 1 Vp Vave θ F cos - cos Vp - cos - Vp d sin Vp d sin Vp 1 d v T 1 Vave L L θ F θ F θ F θ F
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