My first power supply. AC-DC converter - Page 1
Forum section: electronics / beginners

author: spacedementia87
author_rank: Contributor
timestamp: August 01, 2016, 02:40:49 pm
content: Hello everyone,This is my first post here. I used to do a lot of electronics stuff back at school and I remember making an AC -DC converter but was obviously very guided.It has now been years since school and I am out of practise but have been looking into building a power supply as a project.I have found various projects on here that require an unregulated DC input and I have found many online tutorials for transformers + rectifiers that have specific parts with no education factor. I don't want to follow a specific tutorial but learn to understand it to be able to build to my own specs.I have come up with the following:Specifications:Provide 30v @ 2A unregulated (voltage regulator coming later)Mains information (UK)v230V +10% -6% @ 50Hz(216.2 - 253v)Transformer:30v +2v for losses in rectifier = 32v32/1.4 = 22.8So use 24v transformer24*2.5 = 60VA24V 100VA transformer should be sufficient.Rectifier:Current at least double so >5AVoltage >50V4 x BYV29-500 should easily meet these specifications in a TO-220 case.Max repetitive current of 100A should easily cope with inrush current.Attach a 13K/W heat sink + 2.5 junction to base + 2 base to HS + 20? margin = 18K/WWorst case 1.25V drop at 3A = 3.75WTmax = 18*3.75 + 25 = 92.5 (<150 so ok)Smoothing Cap:Max pk-pk = 4VC = 2.5/(100*4)C = 6250uF (closest sensible value 6800uF)Vmax = (24*1.4*1.1) = 37V6800uF 50V cap should be sufficient.How does this look? Am I missing anything?EDIT: this is intended to be unregulated and will be fed into a regulator that I have already built.Sent from my ONE A2003 using Tapatalk

author: Simon
author_rank: Unknown
timestamp: August 01, 2016, 03:02:14 pm
content: I am a little bit confused as to what type of power supply you are trying to achieve. Is this just a transformer rectifier and smoothing capacitor? As you seem to already understand this will be wildly unregulated and could be anywhere between the RMS output voltage of the transformer minus the rectification drop or that voltage multiplied by the square root of 2. This voltage will vary depending on how much load you put on circuit. If you are using a 24 V RMS output transformer then your supply technically is going to be 24 V minus the losses in the rectifier. You can use Schottky diodes which will have a lower voltage drop produce less heat. If you require an exact voltage you will need a regulator either a linear regulator or a switch mode if you want to increase efficiency.Switch mode regulators are not too difficult these days if you find a good quality controller chip with a comprehensive datasheet which will even show you how to lie the parts out and calculate the inductors and capacitors required.

author: spacedementia87
author_rank: Contributor
timestamp: August 01, 2016, 03:15:12 pm
content: Quote from: Simon on August 01, 2016, 03:02:14 pmHello, thanks for the response.Yes this is just intended as an unregulated ~30V supply.I have build a regulator circuit already that accepts 30V and outputs 1.5 - 25V.The only transformer I ha e though outputs 12V DC. This means I looked out on a large amount of the output possible!So I thought I would build my own!Sent from my ONE A2003 using Tapatalk

author: spacedementia87
author_rank: Contributor
timestamp: August 02, 2016, 05:38:59 pm
content: Shall I take silence as a sign of warning or that it looks ok?Sent from my ONE A2003 using Tapatalk

author: MosherIV
author_rank: Super Contributor
timestamp: August 02, 2016, 06:09:02 pm
content: QuoteThe only advice I would give is be careful!You are dealing with mains voltage on one side of the transformerYou specified 100VA - whoa, that is a BIG psu for a first timer. Be careful.Make sure you fuse the transformer or you could burn out the windings.How are you going to test it?