Suitable diode for bridge rectifier - Page 1
Forum section: electronics / beginners

author: pope
author_rank: Frequent Contributor
timestamp: November 22, 2024, 09:48:16 pm
content: Hi everyone,for a 1A transformer (2x25v 50VA or 2x30V 60VA, etc...) what diode would be more suitable? 1n4007 or something with higher current rating like the 1n5408(3A)?The load won't be anywhere near 1A but my understanding is that you have to choose the diode for the transformer, not the load.So, maybe the 1n4007 is marginally "OK" but better go with the 1n5408?thanksp.

author: Benta
author_rank: Super Contributor
timestamp: November 22, 2024, 09:55:59 pm
content: Depends.If your load is purely resistive, 1N400x is OK, but marginal.If you have storage caps after the bridge, you'll see extremely large currents when turning on the transformer, and significant current peaks during operation.1N540x would be minimum, a 6 A type better.

author: pope
author_rank: Frequent Contributor
timestamp: November 22, 2024, 10:02:48 pm
content: This would be a typical LM317/LM337 power supply so not purely resistive load I guess.Now, the interesting part is that for many years I've been using 1n4007 with 1A transformers and I've never run into any problems? Pure luck?I think the 1n4007 can withstand large peak currents (up to 30A if I'm not mistaken) and the 1A rating is the average current rating.

author: Benta
author_rank: Super Contributor
timestamp: November 22, 2024, 10:23:23 pm
content: The 30 A is for a single half-cycle only. You've been lucky, but they're rugged little things.

author: floobydust
author_rank: Super Contributor
timestamp: November 23, 2024, 01:11:10 am
content: I find smaller VA power transformers can't dish out the high current peaks. A 1A transformer cannot make 30A peaks. The windings' DC resistance, magnetic circuit, diode resistance all contributes to an impedance that protects the diodes, so 1N400x works OK.'That said... I have a cheap chinese power supply with mains input. It's using 1N4007 and 15uF primary cap and plugging that into 120VAC power there can be a pretty big loud spark. Nothing to protect against the inrush really. I'm surprised it still works.

author: pope
author_rank: Frequent Contributor
timestamp: November 23, 2024, 10:56:43 am
content: So, you think something like this is a wiser choice?https://www.digikey.com/en/products/detail/onsemi/GBU6M/1055744Quite frankly, I'm a bit surprised. As I mentioned, I have used many (and by many, I mean tens) of supplies on an almost daily basis, and I have never had a single issue with 1A transformers and 1N4007 diodes.That said, if using a higher-rated diode (e.g., ≥3A) is recommended, I will go with this from now on.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 23, 2024, 11:57:18 am
content: The 1N4007 should be OK, they are rated for 1 A average. Keep in mind that the 1 A transformer is good for about 0,66 A of DC after rectifier and filter caps. Ideally each diode would not see more than 0.4 A on average without overloading the transformer.How much the diodes can withstand also depends on the thermal design - all 4 close together with little cooling could be an issue.The relevant point is the load and not so much the transformer.

author: MrAl
author_rank: Super Contributor
timestamp: November 23, 2024, 12:22:22 pm
content: Quote from: pope on November 22, 2024, 09:48:16 pmHello there,In a bridge rectifier, there are four diodes, but only two of them conduct on any given half cycle. That means that two conduct on one half cycle and the other two conduct on the next half cycle, then back to the first two for the third half cycle. That means each diode sees current for only half of the time which means lower power dissipation in each diode. It may be a bit more than half, but it's still not the full load current.This means that each diode would not get as hot as if it had the full load current all the time.Thus, for a 1amp continuous output current from an old style wall wart, the diodes only see around 1/2 amp on average.Now the peak will be higher, and how high that is depends on the transformer. Old style wall warts have natural leakage inductance not only on purpose but because of the way they have to be wound to meet the isolation Class requirements. This reduces the peak current.So you see it becomes a little more complicated to answer the question "Will 1 amp diodes be OK for a 1 am transformer", but generally speaking it would work based on the 1/2 cycle conduction times. If you are not sure, just measure the current or see if the diodes get very hot during operation.There is a another side to this story though...For Hobbie application I always like to overshoot the actual requirements by 100 percent. That's because it's often just a one-off project or maybe 5 pieces only. In a production environment, we like to keep costs down to a minimum, but for a Hobbie project we have a lot more flexibility; a lot more room for a little more cost. That means if I expected a 1 amp current I might go for a 2 amp diode. Not knowing anything else about the transformer, that would be a good idea even though they only conduct for 1/2 of the cycle. Since 3 amp diodes are not that expensive, I might go with that just because I'm only making one piece not 100000 pieces. This is something to consider because the overall cost is going to increase by a low percentage of the total cost if you use higher rated diodes. You then have a little bit more piece of mind.I will reiterate though that since each diode only conducts for 1/2 of the time, 1 amp diodes are going to be fine in most cases, but for an extra piece of mind you can go higher on the diode rating as long as you have room for the larger diodes and if you use a PC board you have to have more space and bigger holes for the diode leads.Also remember that you can always do some simple measurements and life testing. New wall warts made for a production run go through an overnight test running at full load. That's roughly 12 hours or more running at full load. This is always a must for any kind of new power supply.

author: BillyO
author_rank: Super Contributor
timestamp: November 23, 2024, 06:29:14 pm
content: The 1N4007 is just fine for you application.

author: pope
author_rank: Frequent Contributor
timestamp: November 25, 2024, 05:10:36 pm
content: Thank you all for your replies.I might go with 3A bridge rectifier unless it is absolutely unnecessary and the 1n4007 would be just fine.BillyO, can you elaborate please?Also, can you recommend some ways to do some real measurements? As far as I'm concerned I need a current probe which I don't have. Also, I don't have any DMM that can log data. So, probably I won't be able to do any testing, right?

author: BillyO
author_rank: Super Contributor
timestamp: November 25, 2024, 05:48:43 pm
content: Quote from: pope on November 25, 2024, 05:10:36 pmIt has already been mentioned, but let me summarize:Those little transformers have a fairly high resistance. One I have here measures 4Ω total. The electrolytic filter capacitors will also have some resistance, as will the connecting wires. Your peak "inrush" is likely to be maybe 10A.Add to that what Mr. AL said. Any given rectifier only conducts for half a cycle. So, no worries for normal operationI you are using 2000 uF capacitors, they will charge through 2 ohms in about 0.004 seconds, which is less than 1/4 a cycle. So, under 10A for 0.004 seconds. Well within the specs for the 1N4007.

author: MarkF
author_rank: Super Contributor
timestamp: November 25, 2024, 07:36:47 pm
content: The 1N540x series diodes are absolutely huge.If you want a little more current than the 1N400x series,go with a 1N539x series (@1.5A) which is the same physical size as a 1N400x.