Transformer Safety and power supply design for the beginner - Page 1
Forum section: electronics / beginners

author: thomst77
author_rank: Contributor
timestamp: July 21, 2013, 03:22:43 am
content: So I have never had any kind of electronics schooling, But I have been learning a lot through google, youtube and the internet in general. I want to build a lab power supply. Obviously messing with mains is dangerous stuff. And I am taking this very seriously. So I thought I would ask the pros here the Dos and Donts. Obviously you don't want to touch any traces when powered up. Stand back and plug it in and be ready to disconnect it in a hurry if need be. Be careful of the caps even when unplugged. Wear rubber soled shoes. So what else should I watch for?I am confident I could build a basic power supply, But I would like to incorporate a Volt and Amp meter. And I would like to be able to control the amperage out just like a real lab power supply. So I am also interested in finding some design info for this type of supply. I am looking to build maybe around a 1.2 to around 15v output and I don't think I would need more than 2amps out. I would greatly appreciate any input.ThanksTom

author: Paul Price
author_rank: Super Contributor
timestamp: July 21, 2013, 03:39:46 am
content: Try these ideas:Just look at the many postings about this same subject in the last two months, there have been many posts like yours in the last two months, many on this very same subject.Ok, you probably got the idea. Just look here back a coupla days or weeks or maybe even a few months for this on the Beginner Forum for some.Keyword to look for: Power Supplyhttps://www.eevblog.com/forum/projects/diy-power-supply-schematics/msg248251/#msg248251

author: IonizedGears
author_rank: Regular Contributor
timestamp: July 21, 2013, 03:43:56 am
content: Quote from: Paul Price on July 21, 2013, 03:39:46 amJust go into the subforum you want(beginners in this case) then search what you are looking for with the searchbar in the top right corner.

author: Rerouter
author_rank: Super Contributor
timestamp: July 21, 2013, 03:49:37 am
content: as far as mains safety goes, have a fuse and a switch on the mains side,use a iec socket for power, and have bleeder resistors across all your caps so they discharge within a minute, and if your enclosure is metal and your country provides earth pins at sockets, ground the case, you don't necessarily have to ground your supply, just the casing,

author: IanB
author_rank: Super Contributor
timestamp: July 21, 2013, 04:20:42 am
content: Quote from: thomst77 on July 21, 2013, 03:22:43 amPossibly.QuoteAsk yourself, why not?QuoteAsk yourself, why?QuoteAsk yourself, why?QuoteThe idea of a step down transformer is that it isolates and separates the dangerous mains voltage on the primary side from the safe low voltages on the secondary side.On the mains side there should be a switch and a fuse, and all the connections should be shrink wrapped or wrapped in electrical tape. You should have no chance of touching any bare conductors.On the secondary side, there will be no high voltages on PCB traces or capacitors. Although you won't go touching these needlessly, there should be no reason to fear for your safety.In order to be truly safe, you should understand how your system works, and understand why you should or should not do something.If you try to follow rules blindly, without understanding, you will still be in danger.

author: c4757p
author_rank: Super Contributor
timestamp: July 21, 2013, 04:27:41 am
content: Quote from: thomst77 on July 21, 2013, 03:22:43 amShoes? Indoors?I think my feet would go on strike!Seriously, rubber-soled shoes is excessive. The point of a mains transformer is to isolate, soisolate. Make all the primary-side connections, seal everything up well, and test for continuity to make sure there's nothing resembling a connection between the mains input and the secondary side.Then take off the damn shoes and power 'er up. Perhaps stand back and look away, if you've got big caps and you're worried they'll blow if hooked up wrong.

author: thomst77
author_rank: Contributor
timestamp: July 21, 2013, 05:17:49 am
content: Perhaps this would make a good subject for a video on Daves youtube vlog.

author: Rerouter
author_rank: Super Contributor
timestamp: July 21, 2013, 05:24:55 am
content: probably not, the liability of suggesting to anyone to make connections to mains is not something most would want to touch with a 10 foot pole,it also comes down to each and every countries wiring codes, with aus being one of the more stringent, but drastically different to many others,so as you seem a smart enough person to pose the questions of what to beware, us on the forum are happy to assist you, but as far as telling people what is safe and what is not for mains, ....lets just say common sense is as rare as hens teeth on YouTube,

author: vk6zgo
author_rank: Super Contributor
timestamp: July 21, 2013, 06:19:47 am
content: Every newbie wants to design their own,but with no previous knowledge,I would warn against that!Instead,I would suggest finding a real project in a magazine on making a transformer type power supply,& build it as described.I would shy away from some of the projects on the Internet,as they may not be designed for your mains supply.The best ones are those in magazines like Silicon Chip,Practical Electronics international(or whatever they call it now),etc.The ARRL Handbook would be good,too,although if you are in a 220/230V country,you need to take note of the different mains voltage.These sources have proper schematics,descriptions,(usually) layout diagrams,& photos of the unit.

author: thomst77
author_rank: Contributor
timestamp: July 21, 2013, 07:31:39 am
content: Putting aside the Transformer Safety part of my question. I will be more specific. I am in the US so 120vac is the source. Lets say I were to build a power supply based around an LM317. Lets say around 24 volt 2A Transformer to bridge rectifier to filter cap to Voltage regulator. Where would I connect a voltage meter? Just across the outputs? And where would I connect the Amp meter? And how can I adjust the current from 0-2 amps. Or where can I find some detailed info on this? I feel I can easily build the supply. I just don't know how to limit the current or connect the V/A meters?Thank YouTom

author: Zero999
author_rank: Super Contributor
timestamp: July 21, 2013, 08:45:33 am
content: You can't adjust both the current and voltage simultaneously. On a bench power supply, you set the maximum output current and voltage but which one applies depends on the load, in accordance with Ohm's law. If you set the output voltage and current to 10V and 1A respectively, the voltage limit will apply for loads with a resistance of over 10Ohm and the current limit will apply to loads under 10Ohm.

author: Rerouter
author_rank: Super Contributor
timestamp: July 21, 2013, 08:55:59 am
content: as for the meters, the voltmeter will be on the output, and the ammeter after the regulator but before the feedback network, so it just looks like a trace to the regulator,as for limiting the current you can do similar to Dave's power-supply approach, by raising the voltage on the feedback pin, likely an op amp/ comparator (powered off unregulated supply) via a diode, and a second op amp to measure the current (differential measurement), to this point you can measure across your ammeter shunt and save yourself the trouble of 2 shunts,if you want to simplify it down to 1 op amp, move your ammeter to the negative return, and measure the voltage across it, just make the negative of your feedback and the negative of your output capacitance on the external side of the shunt, so it doesn't cause any voltage drop issues,

author: thomst77
author_rank: Contributor
timestamp: July 21, 2013, 11:32:28 am
content: Ok Thank You all for your input. As I am a beginner. I am going to build a basic supply. Maybe a 24v 1or 2 amp transformer to lm317. Can anyone recommend a good digital voltmeter to mount on the front panel?Tom

author: ampdoctor
author_rank: Frequent Contributor
timestamp: July 21, 2013, 01:49:13 pm
content: there used to be a book published and distributed through radio shack called how to build your own power supplies or something along those lines. It provided basic ac and dc theory without it getting too complicated, and provided several rudimentary power supplies, perf board layouts, and design commentary. The supplies are a bit dated but they will work just fine without being overly complex and is a good step off book for somebody trying to conceptually understand what's going on. Right up there with the old Forrest Mims books from the days when the radio shack name actually meant something.