A few questions about powering projects from mains

[JoeP]

I've decided to try and make a 3-24V 2A bench power supply project that's powered from mains, and I'm slightly nervous about picking the right components.
Firstly, what sort of cable should I be using to connect the plug to the circuit? Since the current will be very low at 230V, will I be alright to use some 4 core 22AWG wire I have lying around? If not, what are the minimum requirements, and where should I buy the wire from in the UK?
Secondly, what sort of transformer should I be using? I can't seem to find anything under 5 pounds - is that as good as I'm going to get? Surely there must be cheaper transformers out there that are used in phone chargers etc, or is it better to make my own transformer?
Finally, are there any other precautions I should take (other than ensuring exposed metal is Earthed, and that I use fuses)?

[adamcord]

While you would get away with using 22AWG wire, it strikes me as bad practice to use long term. I'm not sure about the best place to buy cables from in the UK, but probably online.

As for the transformer, if you're stepping down the mains voltage, 5 lbs is about as good as you're going to get. Small transformers, as used in switching power supplies for example, won't work at low 50Hz mains frequency. If the core is too small, it'll saturate and that's bad news.

A fuse is the bare minimum you want in the way of input protection. MOVs and transient suppression caps are nice. Maybe a common mode choke too. 

[JoeP]

What could make 22AWG wire fail in the long term? What's the thinnest wire I should be using?

Regarding the transformer - I should have been more clear: by 5 pounds I meant £5, about $6.50 USD. Is that about right? Would it be good enough to take apart a $3 mains-usb adapter and take that transformer?

[mariush]

There's nothing cheaper than a mass produces mains cable.

See computer / monitor power cables : https://www.amazon.co.uk/s?k=computer+power+cable&s=price-asc-rank&qid=1556054404&ref=sr_st_price-asc-rank

If you don't want the big IEC connector, you could also go with the mickey mouse plug that's typical on laptop adapters and smaller devices, random result from amazon uk : https://www.amazon.co.uk/ABC-Products%C2%AE-Cloverleaf-Adapter-Toshiba/dp/B003VRBFJE/ref=sr_1_8?keywords=computer+power+cable&qid=1556054404&s=gateway&sr=8-8

Both connectors are easy to find, for the regular computer connector, maybe you can find an old or faulty power supply for free and reuse the connector from there, otherwise there's stores like Farnell , RS-Components, tme.eu , mouser etc which stock them... or you can make the cable permanent to your box by cutting the connector and soldering the wires directly to your transformer leads.

NO you cant use transformers from things like USB chargers ... those are switching power supplies and the transformers are custom, designed to work within a narrow output range and maximum current, and at high frequencies, that's why they're so small...

Easiest for a beginner would be to go with classic transformers and make your circuit to adjust voltage (use a linear regulator, or a cheap DC-DC converter from eBay if you don't care about output voltage quality)

If you go with classic transformer, for a 24v DC 2A output, you'd probably need a minimum a transformer rated for 18v AC and minimum ~ 2A / 0.62 = 3.2A ... so let's say you go with a 24v AC 3.2+A ... or 24 x 3.2 = ~ 75 VA minimum

If you go with toroidal transformers, they can be as lightweight as 1 KG ... here's a bunch of them on TME.eu (18v+ , 75VA+) : stupid long tme.eu link

The cheapest one that would fit your needs is 16$ plus shipping, 80VA; 230VAC; 24V; 3.33A; 0.9kg; Ø: 95mm;  : https://www.tme.eu/en/details/tst80w_24v/toroidal-transformers/indel/tst-80-003/
(for the transformer above)
You rectify that 24v AC to DC using a bridge rectifier, which gives you a peak dc voltage of around 24 x 1.414 -2 x Diode drop on rectifier or around 31v  (so be sure to use 50v rated capacitors after rectifier)
The max DC current would be ~ 0.62 x AC Current = 0.62 x 3.33 = 2.05A
You'll pick enough capacitance to keep the minimum voltage above the threshold you need... let's say 26v minimum ... approximate capacitance with C = current / ( 2 x ac freq x (Vdc peak - Vdc min) = 2A/ [2 x 50 hz x (31v - 26v) ] = 2 / 2 x 50  x 5 = 1 / 250 = 0.004 Farads or 4000 uF .. so you'd probably use at least 4700 uF or 5600 uF
and so on...


You could of course replace it with a switching power supply that outputs 24v or more, and then use your own circuit to make it adjustable...

[DDunfield]

What could make 22AWG wire fail in the long term? What's the thinnest wire I should be using?

It's not long term that you worry about. You should use cable appropriate for carrying the available current in a circuit.

I don't know UK standards, but here in NA a standard distribution circuit offers 120v at 15A, adjust following discussion accordingly.

Your supply might draw <1A, however you have to consider a failure at any point ahead of your fuse (say the point of ingress to the equipment cabinet). In NA this would mean the cable would have 15A available. The idea is to insure that the breaker trips before the cable erupts in flames, so you should use cable rated to carry 15A. (certain circuits in NA are allowed 20A breakers and accept standard appliance plugs, so you really should allow for 20A).

If you wish to use smaller cable, correct practice would be to use a fused plug with a current limit appropriate to the cable you choose.

Dave

[aju11]

In addition to fuse you may consider using series Lamp protection.

Place a tungstan lamp in series with your mains supply.
Select tungsten lamp's wattage as per the current demand by your circuit. (Higher watt lamp for higher current circuit)

This will avoid fireworks-like situation if you have serious fault in your circuit.

[ejeffrey]

In the UK plugs are fused so it is ok to use thinner gauge cord wires as long as the fuse is rated appropriately.

Note that you still should be using mains rated wiring in terms of insulation.  Standard hookup wire or low voltage cables are not appropriate for a power cord regardless of wire gauge.

[JoeP]

Many thanks for the pointers. Since posting the question I did realize that the mains devices you get commercially have some circuitry before the transformer, which explains why the one I'd need would be so much heavier.
Is a home made SMPS out of the question?
Also, is there any possibility of avoiding a transformer all together - such as a capacitor divider circuit? Or would that use too high currents?

[mariush]

The transformer has a super important role: ISOLATION .... it protects you from high voltages , mains... it's SAFETY.
Those capacitor dropper based tiny power supplies only work for small currents and they're meant to be used where someone has no way of touching wires .. like let'say something wrapped in plastic and/or glass (led lamp).
It would not be safe otherwise ... and again shouldn't be used for more than 2-3 watts.

[ejeffrey]

I don't think a home made offline SMPS is a good project, certainly not for someone with your apparent experience.  It is a relatively complicated project both in terms of performance, safety, and usually requires custom high frequency transformers which you would probably have to wind yourself while maintaining isolation safety.  If you want to build a switcher, a much better option is to get an off the shelf fixed AC-DC power supply then use that to power a DC-DC converter.

For a basic lab supply to test out home built projects, a transformer + linear regulator is really the way to go.  It is a simple starter project and relatively easy to get the safety side right because the transformer itself is designed to provide isolation.  If you use an IEC mains entry jack with a fuse and a switch, it can really be as simple as two wires.  The weight is an advantage: it keeps the device planted on the bench when you are plugging connectors into it.

Capacitive droppers are not suitable for a lab power supply.  They can only provide relatively small currents and they do not provide isolation, so they are used for devices that are themselves isolated: plastic enclosure, no exposed metal or connectors, or maybe devices that primarily exist to handle or switch mains power such as mains power meters or light timers.