choosing rectifier diodes - Page 1
Forum section: electronics / beginners

author: oliver602
author_rank: Regular Contributor
timestamp: October 23, 2011, 08:23:04 pm
content: I am trying to choose diodes for a bridge rectifier. I wanted to pick a more general purpose diode instead of one of those 4 pin rectifiers. Something like the 1N400X or 1N540X series.My question is, What are the trade offs when selecting higher current and higher reverse voltage diodes than necessary?

author: amspire
author_rank: Super Contributor
timestamp: October 23, 2011, 10:12:55 pm
content: For a mains rectifier operating at 50Hz or 60Hz, there are no major consequences selecting over rated devices. Diodes with a higher current rating will run cooler then lower current diodes anyway which is a good thing.If you were talking diodes for a switching regulator running at 20kHz plus, then the diode selection is very important as switching losses are a major factor.Richard

author: oliver602
author_rank: Regular Contributor
timestamp: October 26, 2011, 05:46:02 pm
content: Thanks Richard.You'd pick the 1000V max reverse voltage if they were cheaper than the 50V version? For a 50Hz rectifier.

author: amspire
author_rank: Super Contributor
timestamp: October 26, 2011, 09:19:17 pm
content: If the 1000v diode can handle the current, then yes. It I'd commonly done.Back when some of the traditional diodes first came out in the 60s and 70s, they really struggled to make the higher voltages, do it suited them to sell lower voltage version at a much lower price. If you look at the data sheets for the 1n400X range, they all have the same forward voltage drop, so that means they were always trying to make the 1000V 1N4007, but it worked maybe 1% of the time. All the ones that didn't make the grade became 50v to 800v diodes. Now they can probably make 1N4007s with their eyes shut.But just double check the spec for the forward voltage drop at the current you are using. If the 50v diode is less, it will run cooler.Richard

author: tecman
author_rank: Frequent Contributor
timestamp: October 26, 2011, 10:44:04 pm
content: Quote from: amspire on October 23, 2011, 10:12:55 pmThe voltage drop in a diode is the same without respect to the size or current rating. Therefore the power dissipated will generally be the same. Larger, higher current rating diodes may appear to run cooler, but this is due primarily to slightly better dissipation of larger leads and diode bodies.paul

author: amspire
author_rank: Super Contributor
timestamp: October 27, 2011, 12:44:25 am
content: Quote from: tecman on October 26, 2011, 10:44:04 pmNot quite true. Forward voltage for standard P-N junction diodes can be in a range of about .6 v to just over 1v dependent on the diode doping, die size and internal resistance.Schottky diodes can be half the forward voltage of the P-N junction diodes.But you are right in the higher power diodes often are packaged in bigger cases to dissipate more heat along with much thicker leads to conduct heat away to the PCB copper tracks.It is only a quick of history that makes the 1N400X diode range all have the same forward voltage and current specs. It is just the fact that when the range was released, the process control was so bad that many 1000V diodes batches were only good for 50V (hence the 1N4001). It would appear they had trouble controlling the depletion layer width. That doesn't happen any more.If you look at any newly released ranges of diodes, they are optimized for the rated voltage so the low voltage diode will have lower forward voltage and will handle more current then the high voltage diode in the same range. This is particularly noticeable in the Schottky diode ranges.Richard