So I'm getting ready to build a power supply, and it calls for a bridge rectifier. However, I can't find any information about HOW rectifiers are rated, except for a note that (and I don't have it in front of me) reverse voltage should be three times normal to protect against spikes…or some such.
My input voltage is 110VAC, with a transformer converting it to 18VDC/1A.
Is the rectifier voltage the MAXIMUM it can take? or something else?
What about amperage? Same deal? or, in my instance, do I need to have a 1A rectifier?
I have a choice between a 35V/10A, 50V/2A and 60V/4A rectifier, and I'm leaning toward the 60V/4A, guided ONLY by the fact that it's the only one that meets the 18V*3 note that I read.
Will I encounter a blue smoke demon? or is this a good way of thinking about it? How do I interpret rectifier ratings against a schematic when there's no specification beyond "bridge rectifier" in the parts list?
Crappy camera phone image. If it's illegible let me know, I can upload a scan in a couple hours.
Best Answer
I guess the \$\times\$3 rule of thumb refers to the transformer's secondary voltage, the AC voltage before rectifying. If your output is 18 V DC then the transformer will be a 14 V type. That 14 V is the RMS value, peak value is \$\sqrt{2}\$ times higher, or 20 V. Subtract 2 diode drops and you arrive at 18 V.
So applying the \$\times\$3 rule to the 14 V already gives you 100 % headroom, applying it to the 18 V gives you almost 200 % headroom, which is overly cautious IMO. Since 20 V is the highest voltage to expect all three will do fine.
The current is a different story.
If the smoothing capacitor is charged the input voltage will be too low for the greatest part of the cycle to make the diode conduct. Only for a short period you'll have a current that recharges the capacitor. So you can expect a large current peak there, several times the average of 1 A. A 1A diode like the 1N4001 can handle this current, though it's a good idea to have some headroom here as well. For a 1 A output a 2 A diode is a good idea, so also for current all three rectifiers will do.