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In Reply to: Re: Spec'ing Diodes posted by Chris O on February 16, 2006 at 04:37:46:
With all due respect, you were just about correct in everything you said. However, I should mention that when a transformer is rated at 350-0-350VAC, or 700VAC CT, or if you actually measure that voltage on your DMM, the voltage you are measuring is actually the RMS voltage. For a full wave power supply with a capacitor input filter, you need to use the peak voltage to calculate the PIV for your power supply diode. You can find that by multiplying the 350V times the square root of 2 or 1.414, which will give you 495V. It is that 495V that you must double to calculate the diode's PIV, which is 990V.Now it is not good engineering, but it has been my experience that whenever one is so close to the rated voltage, 990V to the 1000V PIV of a 1N4007 diode, for instance, that you should assume that everything will go wrong. For instance, you should assume that the power transformer voltage will vary 10% in the worst possible way, at the worst possible time. Consequently, you should series connect two 1N4007 diodes, four total for both ends of the power transformer secondary, because 20 years from now the power supply will still be working, if you do.
Current in high voltage applications is at once, simpler but trickier. For instance, a 5U4 rectifier tube can conduct 1000mA of current. That's one whole amp of current as an absolure maximum. That same lowly 1N4007 diode can conduct that same one amp of current all day, and with a voltage drop of less than one volt. In addition, the 1N4007 is capable of as much as 30 amps of surge current in order to charge up filter capacitors. With the 5U4, the one amp is all you get.
One other thing which you mentioned that concerned me. While it is true that it is a good idea to have more than sufficient current capability for diodes...having a 3 amp, 600V PIV diode for instance, an extra current capability does not compensate for an insufficient PIV voltage capability. I did this when I first started rebuilding amps. I thought, 'Well, the 600v PIV is almost enough voltage, but with that extra current capability of 3 amps, I should be in the clear.' It doesn't work like that. High current diodes just blow up with a louder 'Snap'.
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- Re: Spec'ing Diodes - corerosin 10:02:53 02/16/06 (0)
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