R and RC Triggering of SCR

R AND RC TRIGGERING CIRCUITS

AIM:

To design and setup R and RC triggering circuits for a half controlled rectifier for different firing angles and draw relevant wave forms.

EQUIPMENTS AND COMPONENTS REQUIRED:

THEORY

R Firing circuit:

Figure shows the most basic circuit. R₂ is the variable resistance. R_b is the stabilizing resistance. If R₂ is zero, gate current may flow from source through R₁, D and gate to cathode. This current should not exceed maximum permissible gate current I_gm. R₁ can therefore be found from the relation (V_m/R₁) ≤ I_gm. The function of R1 is to limit gate current to a safe value, as R₂ is varied. Resistance R_b should have such value that maximum voltage drop across it does not exceed maximum possible gate voltage V_g. This happens only when R₂ is zero. Under these condition . As resistances R₁, R₂ are large, gate trigger circuits draw a small current. Diode D allows the flow of current during the positive half cycle only i.e., gate voltage is half wave DC pulse. The amplitude of this DC pulse can be controlled by varying R2. The potentiometer setting R₂ determines the gate voltage amplitude. When R2 is large, current I is small and voltage across R_b is V_g = I x R_b. As V_g is less than V_gt SCR will not turn on. Trigger circuit consists of resistance only. Vg is therefore in phase with source voltage V_s. Adjust R₂ in such a way that V_g = V_gt at different firing angles depending upon instantaneous values of input voltage. When R₂ is large enough to such that V_g become equal to V_gt only at peak value of output voltage, corresponding firing angle will be 90⁰. The resistance triggering cannot give firing angle beyond 90⁰.

CIRCUIT DIAGRAM:

Design:

• When R₂ is minimum, I_g will be maximum. I_gmax = 100 mA
• Therefore  = 170 Ω. (Use 220Ωstd.)
• When R₂ is maximum, current will be minimum and voltage drop across R_b will be maximum and it should not exceed V_gm
• V_gmax = 2.5 V, V_m = 12√2 V
• When R₂ = Max, V_gt = 0.8 V, V_m = 12√2 V
• R₂ = 0.447 Ω (Use 1K pot std)

Sample Wave forms:

RC Firing circuit:

Fig.2 shows the RC half wave triggering circuit. By varying the value of R, firing angle can be controlled from 0⁰ to 180⁰. In the negative half cycle capacitor C charges through D₂ with lower plate positive to peak supply voltage V_m at ωt = 90⁰. During the period ωt = -90⁰ to ωt = 0⁰, V_c may fall from –V_m at ωt = 90⁰ to some lower value at ωt = 90⁰. As the SCR anode voltage passes through zero and becomes positive C begins to charge through variable resistance R from initial voltage. When the capacitor charges to positive voltage equal to V_gt, SCR is fused and after this capacitor holds to small positive voltage. D₁ is used to prevent breakdown of cathode to gate through D₂ during negative half cycle. The SCR will trigger when V_c= V_gt+ V_d­. At the instant of triggering V_C is assumed constant.

Sample Wave forms:

PROCEDURE:

Set up the circuit on breadboard. Turn on the SCR by adjusting triggering pulse. Note down the maximum and minimum firing angle. Observe the wave forms corresponding to each firing angle.

RESULTS:

The circuit is setup and the wave forms for various firing angles are observed and plotted.