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Rectifying Diodes
19 questions By Tony R. Kuphaldt
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Question 16 of 19
What could you do if you had an application for a rectifying diode that required a forward current rating of 2.5 amps, but you only had model 1N4001 diodes available to use? How could you use multiple 1N4001 rectifying diodes to handle this much current?
Reveal answerUse three 1N4001 diodes connected in parallel, like this:
Follow-up question: while this solution should work (in theory), in practice one or more of the diodes will fail prematurely due to overheating. The fix for this problem is to connect “swamping” resistors in series with the diodes like this:
Explain why these resistors are necessary to ensure long diode life in this application.
Notes:The answer to this question should not be much of a challenge to your students, although the follow-up question is a bit challenging. Ask your students what purpose the swamping resistors serve. What do we know about current through the diodes if one or more of them will fail due to overheating without the swamping resistors?
What value of resistor would your students recommend for this application? What factors influence their decision regarding the resistance value?
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Question 17 of 19
Suppose you were building a simple half-wave rectifier circuit for a 480 volt AC source. The diode needs to withstand the full (peak) voltage of this AC source every other half-cycle of the waveform, or else it will fail. The bad news is, the only diodes you have available for building this rectifier circuit are model 1N4002 diodes.
Describe how you could use multiple 1N4002 rectifying diodes to handle this much reverse voltage.
Reveal answerUse seven 1N4002 diodes connected in series, like this:
Follow-up question: while this solution should work (in theory), in practice one or more of the diodes will fail prematurely due to overvoltage. The fix for this problem is to connect “divider” resistors in parallel with the diodes like this:
Explain why these resistors are necessary to ensure long diode life in this application.
Notes:The answer to this question should not be much of a challenge to your students, although the follow-up question is a bit challenging. Ask your students what purpose the divider resistors serve. What do we know about voltage dropped across the diodes if one or more of them will fail without the divider resistors in place?
What value of resistor would your students recommend for this application? What factors influence their decision regarding the resistance value?
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Question 18 of 19
What diode performance parameter establishes the limit for maximum frequency of AC which it may rectify? If you were to examine a diode datasheet, what parameter (or parameters) would be the most important in answering this question?
Reveal answerReverse Recovery Time (trr) is a very important parameter limiting maximum rectification frequency. Junction capacitance (Cj) is another.
Notes:Ask your students to describe what the “ideal” trr and Cj values would be for a diode with unlimited rectification bandwidth.
