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Rectifying Diodes
19 questions By Tony R. Kuphaldt
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Question 10 of 19
Predict how all component voltages and currents in this circuit will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):
- Diode D1 fails open:
- Diode D2 fails open:
- Load resistor fails open:
- Transformer T1 primary winding fails open:
For each of these conditions, explain why the resulting effects will occur.
Reveal answer- Diode D1 fails open: Load resistor receives half-wave rectified power instead of full-wave, more voltage across D1.
- Diode D2 fails open: Load resistor receives half-wave rectified power instead of full-wave, more voltage across D2.
- Load resistor fails open: No current on secondary side of circuit, little current in primary side of circuit, no voltage drop across either D1 or D2.
- Transformer T1 primary winding fails open: No current or voltage anywhere on secondary side of circuit, no current in primary side of circuit.
Notes:The purpose of this question is to approach the domain of circuit troubleshooting from a perspective of knowing what the fault is, rather than only knowing what the symptoms are. Although this is not necessarily a realistic perspective, it helps students build the foundational knowledge necessary to diagnose a faulted circuit from empirical data. Questions such as this should be followed (eventually) by other questions asking students to identify likely faults based on measurements.
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Question 11 of 19
An important parameter for many semiconductor components is thermal resistance, usually specified in units of degrees Celsius per Watt. What does this rating mean, and how is it related to temperature?
Reveal answer“Thermal resistance” is a measure of the heat differential required for a semiconductor component to dissipate a given amount of power.
Notes:Discuss the nature of heat with your students: that a differential temperature (∆T) is required for transfer of heat through a medium such as a solid. Compare this phenomenon with differences of electrical potential (E) and electric current (I). How do we express an electrical conductor’s ability to carry a moving charge under the influence of a potential difference?
Ask your students what difference it makes whether a semiconductor component has a high or a low thermal resistance. What is ideal for a semiconductor device, a high thermal resistance or a low thermal resistance? Why?
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Question 12 of 19
Rectifying diodes, like many other types of semiconductor components, should be derated at elevated ambient temperatures. Datasheets often provide “derating curves” that prescribe the maximum current for a range of ambient temperatures.
Explain just what “derating” is, and why it is so important for semiconductor devices.
Reveal answer“Derating” means to downgrade the maximum power rating of a component, in a response to changes in other factors affecting the component’s operation.
Notes:Discuss with your students why temperature is such a critical factor in semiconductor component operation. What happens to a semiconductor junction when it is heated? What may happen if it is heated too much?
