Building power supplies - safety standards, etc

[hammondlewis]

I'm interested in building my own power supplies for my projects. Linear at the moment, but maybe switchmode in the future, and I'm finding it quite difficult to find information on safety practices, etc. There are lots of schematics available, but more often than not they stop just shy of showing the transformer, fuses, etc. Can anybody recommend a text that goes into detail about selecting fuses, where to place them, where to use crimp connectors rather than soldering, earthing, etc? Is it hard to find this information because of different standards in different countries? I'd also like to know more about protection on the secondary side: over-volt & over-current protection. slow-start, protection diodes, etc. If anybody can recommend a good resource that'd be fantastic.

[Bored@Work]

Is it hard to find this information because of different standards in different countries?

That is one reason. Other reasons are that the standards are typically not free, and are in fact rather expensive for a hobbyist. And standards build on each other. You quickly end up having to buy a bunch of them just to understand one. And then you move to the next standard and find that you again need to buy more.

And once you have them all you need to interpret them correctly. Which is an art in itself. Standards come from existing practice, and often don't give you all the details, because those in the know didn't find it necessary to specify "that" particular "trivial" detail "everyone" knows. Filling in the holes, explaining the details, etc. can take a lot of effort itself. One reason professionals hate to explain the details to layman.

Other reasons are that professionals like to get payed, like to be part of a "secret society" (electricians tend to be very guilty of that) sometimes protected by law, or want to avoid to become liable if someone manages to kill himself after receiving instructions.

Back to standards. Depending on the standard, some standards also don't tell you how to do it or what materials to use. They just tell you the end result you must achieve and contain or reference measurement setups to verify if you managed to do something standard compliant.

Of course that is not all. Which standards apply also depends on the indented use of the product. One and the same thing can be covered by rather different standards. And on top of that there are, typically vague, laws defining what standards or standard categories might apply.

It is a mess. There is no simple answer to the question how to do it right. An easier way to get some information, but it might be the wrong one, instead of looking at standards is to look at training material for industrial electricians, technicians, and the like.

[hammondlewis]

It is a mess. There is no simple answer to the question how to do it right.

Thanks, I was afraid it'd be something like this, which is disheartening. I'm not really trying to bring a product to market or anything, rather I'd just like to be confident that I'm not going to kill myself. I've watched videos on Youtube from the likes of  glasslinger https://www.youtube.com/user/glasslinger and photonicinduction https://www.youtube.com/results?search_query=photonicinduction, who do crazy stuff with high voltage and I think they must be very confident that they're handling things safely, and I can't help but wonder where you can get that kind of knowledge. It certainly doesn't seem readily available on the net. Not that I have any intention of dealing with 50,000 volts any time soon.

[AndyC_772]

It comes from working in the industry. Typically this knowledge - as well as a lot of other 'tips & tricks' - is passed from senior to junior engineers, under supervision, and where the student's work will be reviewed later. The loop is closed, and that's a big difference compared to how you might learn from the internet.

For example, I could probably explain to you how to get a mains powered product through IEC 60950, which is the safety standard that applies to a lot of fairly ordinary industrial electronics. I could write about creepage and clearance, fuses, earthing, isolation barriers and so on - and even if I got all the details right, and you followed my guidelines to the letter, you could still end up with a product which was deemed unsafe by a test lab. Maybe I missed something out, or maybe you were just unlucky and got a jobsworthy box-ticker testing your product.

That's not to say even a failed product is actually "unsafe". Safety isn't black and white, and arguably the standard of safety needed to ensure a product won't kill anybody when it's mass produced by the million and sold to complete idiots is higher than the standard needed for a one-off piece of kit being used correctly by its designer.

I'd like to say "insulate live conductors, isolate it from anything you might touch, earth the box and install a fuse", but that much is fairly obvious. It may - and I mean 'may' - be all you need to do for your purposes. Or you may need reinforced isolation barriers, ultra-low earth leakage, separate enclosures, fire proofing... nobody can really tell you, I'm afraid.

[Rerouter]

Can anybody recommend a text that goes into detail about selecting fuses, where to place them, where to use crimp connectors rather than soldering, earthing, etc?

 I'd also like to know more about protection on the secondary side: over-volt & over-current protection. slow-start, protection diodes, etc.

Listing books it not something i can help with, i'm well read technically but i havent touched a book in years (know the terms and know the solution kind of thing)
but i will try and help explain some bits so you know where to start looking

Selecting fuses can be a very big or a trivial matter depending on what your doing, when a fuse is in circuit it has to not blow at the rated current, and usually not until its exceeded by a very large margin (200%+) when it does blow it has the handle the power output from the link burning out (HRC fuses are on the high end, some glass fuses can explode) and once it burns out it has to separate the conductors enough to maintain the breakdown voltage,

There is fast blow, slow blow, spring loaded high voltage (actually relying on a spring to pull apart the contacts) and many other flavours, mainly to cater for all the ways you could want it to protect your circuit, e.g. if you use a zener to protect your circuit from over/reverse voltage you wold want it to blow very fast as if its sitting there for half a minute before blowing passing X watts, it may fuse open circuit leaving your circuit unprotected... things like that,

Next up, crimp connectors over soldering, you have to consider that an ideal joint is both mechanically and electrically strong, when you get into larger wire sizes it gets harder to ensure a reliable mechanical joint with just solder, and with motion from vibrations and impacts over time can become brittle, thus at-least in aus, they dont like wires being joined by solder unless its also mechanically supported, (which in some cases can seem redundant)

thus the crimp connectors come in, when well sized and done, short of soaking in a strong oxidizer should not fail within a few decades, and should not leave any of the conductor easily accessible to the outside world

Earthing is a very big topic, from a safety perspective it needs to be the barrier a live conductor will hit before it makes contact with any exposed contacts or metal, and needs to be low enough resistance to blow your fuse or trip your circuit breaker,

Most of this i believe to be current and true, but look around, i am fallible,


Next up a much safer topic, Output modulation / protection,

Softstart, in older linear supplies this may have been a RC filter on the set-point or the supply for the potentiometer, so it gradually comes up to set point without any sudden jerks or spikes, (this ties in heavily to control loops, in general the slower the set point change the more stable it will react while changing its output) but be aware on the reverse that this can slow down it shutting off, if the set point has to ramp back down, (things get nasty if the control loop looses power before the rest of the supply)

Protection Diodes, Depending on the supply architecture it may be sensitive to negative voltages, or voltages that exceed its supply, in addition allowing supplies running in series to be turned on or off at different times, in most cases you will find a diode close to or exceeding the supplies rated current with anode to negative cathode to positive, this covers running in series and reverse voltages, while more rarer, you can find TVS diodes or similar to handle ESD spikes fed into the output wires, in most cases the output capacitance will handle it, some constant current supplies with tiny output capacitance makes it more of a concern,

Over Voltage Protection, sometimes called a crowbar circuit, essentially a circuit that when the voltage its set for is exceeded, will bring the output to 0 as fast as possible, sometimes at the expense of the supply itself, dead shorting the output, tripping it into constant current mode (or blowing input / output fuse) and ensuring the device being powered cannot be damages,

Over current protection, likely the flip side, when you exceed the current, open circuits the output, to prevent any more current flowing, remembering this is generally instantaneous protection not regulation, and a constant current mode is not the same thing, but as you look around, they can be made more like a circuit breaker where they allow X% over rating for Y Time, with a fancy curve,

The other thing is fail safe modes, the biggest failure modes in a linear supply are generally the input rectifier (those big filter caps slurp down Kilo-Amps on switch on without some form of limiting device, generally the transformer), and the pass element, from wattage abuse, over current or over voltage, you want to make sure that should something fail, its not going to explode or catch fire, or if the regulation stage looses power, that the output will fall to 0, (over current / over voltage protection can be made to remain active even when the supply is unplugged, it just comes down to the implementation)

[ConKbot]

Crimping vs soldering is a good starting point.  Crimping can make a better connection than a solder joint if you have a pin designed to go on your wire, you have crimpers made to go on your pin.  If you dont have all 3 items matched. Dont rely on the crimp, and solder it too.  A proper crimp does the same thing that soldering does,  transform your lug/wire into a solid piece of metal.  Just by compression instead of filling.
this is a cross sectioned crimped joint, see how all the strands are deformed slightly, making it one big piece.


Also, remember:

Don't expect this to crimp anything. The only thing a pair of strippers like this is good for is stripping wire, and cutting screws.   How many times have you crimped on a red/blue/yellow lug and have to be careful hooking or unhooking it because it will pull out if youre not?  Not acceptable. Ive done my own pull test on an amphenol plastic insulated diamond grip (PIDG) ring terminal with a 14 awg wire crimped into it by a pair of crimpers meant for the PIDG series. The ring started deforming and bending over, and tearing, and the wire stayed put. 


This is not a crimper.  This is a lug mutilator, and makes garbage connections, but the person putting lugs to battery cables with these doesn't have the tools to measure that properly and goes with anecdotal evidence that they "work fine." Crimping a 4+awg wire takes a metric-shit-ton™ of force, and while you may be able to get that with the anvil and good whack with a 5lb sledge, the repeatability is crap.  I wouldn't risk it on a connection that could be carrying hundreds of amps, and in an environment with vibration and moisture.  Id rather have a cheapie chinese made hydraulic crimper from ebay (which can be had for a bit more than the anvil type mutilators) though their dies can be a bit weird. 


Molex goes over crimp inspection here and here if you want to see what they say what a good crimp looks like.  I wont go so far as to say only use manufacturer tooling for crimping for personal use, and there was a suggested tool by a bunch of people on here that was pretty cheap, and they said it worked well for lugs.  I cant recall the model no or manufacturer though. 

The advantage of crimping over soldering is that, other than the crimp area, the strands arent solidified by solder, so the strain relief on the lug/pin works correctly.  If the solder wicks up under the insulation, then its rigid all the way up to where the solder stops, and stress builds up there and it breaks there.  That and you have to crimp when youre trying to attach to wire you cant solder to.  I.e. aluminum wire. Or if its something that gets hot enough to melt the solder, i.e. nichrome heater wire. 

[David Hess]

Next up, crimp connectors over soldering, you have to consider that an ideal joint is both mechanically and electrically strong, when you get into larger wire sizes it gets harder to ensure a reliable mechanical joint with just solder, and with motion from vibrations and impacts over time can become brittle, thus at-least in aus, they dont like wires being joined by solder unless its also mechanically supported, (which in some cases can seem redundant)

thus the crimp connectors come in, when well sized and done, short of soaking in a strong oxidizer should not fail within a few decades, and should not leave any of the conductor easily accessible to the outside world

If I have to use a crimped connection, I usually solder it as well.  For mechanically difficult soldered connections, I have sometimes used silver jeweler's solder.

Earthing is a very big topic, from a safety perspective it needs to be the barrier a live conductor will hit before it makes contact with any exposed contacts or metal, and needs to be low enough resistance to blow your fuse or trip your circuit breaker,

Most of this i believe to be current and true, but look around, i am fallible,

Even worse, safety requirements may conflict with the power supply being usable.  The most useful power supplies that I have include floating ungrounded outputs with a separate chassis ground which may or may not be connected to any of them but try explaining such a design to someone who interprets safety regulations as requiring everything to be grounded.

Protection Diodes, Depending on the supply architecture it may be sensitive to negative voltages, or voltages that exceed its supply, in addition allowing supplies running in series to be turned on or off at different times, in most cases you will find a diode close to or exceeding the supplies rated current with anode to negative cathode to positive, this covers running in series and reverse voltages, while more rarer, you can find TVS diodes or similar to handle ESD spikes fed into the output wires, in most cases the output capacitance will handle it, some constant current supplies with tiny output capacitance makes it more of a concern,

It is almost always a bad idea to use more output capacitance than necessary for proper frequency compensation if only to limit the energy available under fault conditions.

Over Voltage Protection, sometimes called a crowbar circuit, essentially a circuit that when the voltage its set for is exceeded, will bring the output to 0 as fast as possible, sometimes at the expense of the supply itself, dead shorting the output, tripping it into constant current mode (or blowing input / output fuse) and ensuring the device being powered cannot be damages,

Over current protection, likely the flip side, when you exceed the current, open circuits the output, to prevent any more current flowing, remembering this is generally instantaneous protection not regulation, and a constant current mode is not the same thing, but as you look around, they can be made more like a circuit breaker where they allow X% over rating for Y Time, with a fancy curve,

It is worth mentioning thermal protection.  I like using integrated regulators as pass elements because they include thermal protection, over current protection, and safe operating area protection beyond what may be designed in.

[tautech]

All wise words from experienced forum members. 

All I have to add:

It is worth mentioning thermal protection.  I like using integrated regulators as pass elements because they include thermal protection, over current protection, and safe operating area protection beyond what may be designed in.

is apply the KISS principle (keep it simple stupid)
LM338 or a similar beast. 

[Weisserrabe]

in Austria we have the "RIS" where you can look up the laws, as soon as a standard becomes part of a law it has to be available for free (download pdf)
if you understand German language this might be helpful for you:

https://www.ris.bka.gv.at/GeltendeFassung.wxe?Abfrage=Bundesnormen&Gesetzesnummer=20002002

https://www.ris.bka.gv.at/Dokument.wxe?Abfrage=BgblAuth&Dokumentnummer=BGBLA_2006_II_33

https://www.ris.bka.gv.at/Dokument.wxe?Abfrage=BgblAuth&Dokumentnummer=BGBLA_2010_II_223

[M0BSW]

 I'm building one at the moment, the design was in one of my old Radio Ham magazines, so I doubt that  it would pass current standards here in the UK , anyway it will be fit for my uses. I to have noticed there definitely a secret society within Electronics on the need to know level, they Know and you have no right to know." Whatever" as my daughters say.
I for one will go along my happy way,use common sense and enjoy the hobby.

[David Hess]

Here in the US these copyrighted rules are passed into law by the legislators so you have to pay real money to see them.  They technically have to be made available for free but you are not permitted to copy them in bulk.