Question E6F07:
What is a solid-state relay?  

Choices:  
A: A relay that uses transistors to drive the relay coil  
B: A device that uses semiconductors to implement the functions of an electromechanical relay  
C: A mechanical relay that latches in the on or off state each time it is pulsed  
D: A semiconductor switch that uses a monostable multivibrator circuit    

Correct Answer is  B: A device that uses semiconductors to implement the functions of an electromechanical relay  

Correct answer: B

1) Why B is correct
- A solid-state relay (SSR) performs the same switching function as an electromechanical relay, but with no moving parts. It uses semiconductors—typically an optocoupler for input isolation and power devices like triacs/SCRs (AC) or MOSFETs (DC)—to turn a load on or off.

2) Why the other choices are incorrect
- A: “Transistors to drive the relay coil” still describes a conventional electromechanical relay (with a coil and contacts). That’s not a solid-state relay.
- C: A “mechanical relay that latches” is a latching relay (bistable), and it is still mechanical, not solid-state.
- D: A “monostable multivibrator” is a one-shot timing circuit, not a definition of a relay. An SSR may include control electronics, but it is defined by semiconductor switching replacing mechanical contacts.

3) Useful rules of thumb and quick formulas
- DC SSR (MOSFET-based) conduction loss:  
  $$P_{\text{loss}} \approx I^2 R_{DS(on)}$$
- AC SSR (triac/SCR-based) conduction loss:  
  $$P_{\text{loss}} \approx V_{ON} \times I \quad (\text{with } V_{ON} \sim 1\text{–}2\ \text{V})$$
- Thermal rise estimate:  
  $$\Delta T \approx P_{\text{loss}} \times \theta \quad (\text{sum of thermal resistances})$$
- Input drive (opto LED) power:  
  $$P_{in} \approx V_F \times I_F$$
- Practical notes:
  - AC SSRs often include zero-cross turn-on to reduce EMI; not suitable for phase-angle dimming.
  - SSRs have off-state leakage current; very small loads may not fully turn “off” without a bleeder.
  - Triac-based SSRs require a minimum hold current to stay on.