Question E6F08:
Why are optoisolators often used in conjunction with solid-state circuits that control 120 VAC circuits?  

Choices:  
A: Optoisolators provide a low-impedance link between a control circuit and a power circuit  
B: Optoisolators provide impedance matching between the control circuit and power circuit  
C: Optoisolators provide an electrical isolation between a control circuit and the circuit being switched  
D: Optoisolators eliminate the effects of reflected light in the control circuit    

Correct Answer is  C: Optoisolators provide an electrical isolation between a control circuit and the circuit being switched  

Correct answer (C): Optoisolators provide electrical isolation between a low-voltage control circuit and the high-voltage (120 VAC) circuit being switched. The LED and photodetector are optically coupled but electrically separate, preventing shock hazards, ground loops, and damage to sensitive solid-state logic from mains transients and noise.

Why the other choices are incorrect:
- A: “Low-impedance link” is the opposite of the goal. Optoisolators deliberately avoid a direct conductive (low-impedance) path; they provide galvanic isolation.
- B: They are not used for impedance matching. Unlike transformers or matching networks, their key spec is current transfer ratio, not impedance transformation.
- D: “Reflected light” is not relevant. Optoisolators are enclosed; the purpose is electrical isolation, not optical noise suppression.

Useful formulas and rules of thumb:
- Current transfer ratio (CTR):  
  $$CTR = \frac{I_C}{I_F}$$  
  Use CTR to ensure the output device can be driven by the input LED current.
- LED drive resistor:  
  $$I_F = \frac{V_{CC} - V_F}{R} \;\;\Rightarrow\;\; R = \frac{V_{CC} - V_F}{I_F}$$
- AC mains peak voltage (for understanding insulation needs):  
  $$V_{\text{peak}} = \sqrt{2}\,V_{\text{RMS}} \approx 1.414 \times 120 \text{ V} \approx 170 \text{ V}$$
- Typical isolation ratings are on the order of kilovolts (e.g., $2.5$–$5$ kVrms); always check the datasheet.
- For switching 120 VAC, opto-triac drivers (e.g., zero-cross types) are commonly used to reduce EMI and provide safe isolation.