Safety considerations for a smoothed unregulated PSU

I've recently put together an old 220/12VAC transformer (actual gear ratio 16.74:1), 4 schottky diodes as a bridge rectifier (2 × S24 + 2 × S34), and a pack of good looking recycled electrolytic capacitors for ≈2630μF, everything embedded in an old PC power supply box with ground connection.

Gonna be used to feed a CC/CV buck module powering a led up to ≈13W max.

Right now, it doesn't have any sort of protection/safety measures. I am thinking in adding overcurrent protection and call it a day unless someone is able to suggest something affordable and smarter.

Using this linear power supply design assistant I see the peak inrush current at the secondary shouldn't exceed 4.5A, which means less than 269mA of current in the primary for 200V mains input (a low voltage condition) at “full load” (10Ω).

0K, 250+V AC fuse. What current rating should I choose? 300mA seems fine? What with regards to fuse type, fast or slow blow?

Cheers

Use a slow blow , (T) 350-500 mA will stand the inrush current at the start , due to the capacitors in the secondary . But , why Schottky diodes for rectifying ?

2Amps normal diodes are dirt cheap , and they are designed for 50-60 Hz , with controlled avalanche .

I have a set of 2A05 diodes from the disassembled PC power supply, but used the schottkies because of efficiency. Disadvantages?

350mA seems fine, but why slow blow? Maybe fast blow fuses get wear out damage if operated near their rating?

Also been reading a bit about resettable PTCs, but maybe it's not worth a try here, their datasheet parameters seem a bit confusing to me yet (checked: 30R500U, Littelfuse).

Thanks.

Nah, a ½A fast-blow should be enough. I’d even go with a 1A fuse.

You never want that fuse to blow because of too much load current (on linear supplies), but a catastrophic failure (eg, screwdriver crowbarring the mains, insulation blowing out in the transformer, etc.).

Only things to really consider is to put the fuse as the FIRST thing when the mains get into the box. A power-entry module would be best. And the fuse should be before the switch, before any power-connected indicator light, before any long-ish length of wire (ie, don’t run the wire into the box all the way to the transformer, and have the fuseholder at the transformer; put it way farther upstream), etc.

2500µF/A or better is the usual recommended value of capacitance at the output. If you plan on drawing ½A, 1500µF would be a good starting point.

Filters, spike suppressors, etc., are nice, but I’d just get a filtered power-entry module instead.

If you’re just throwing it together from parts you already have, consider it junk and treat it accordingly. If you want something good, stick with premium parts.

Also, I wouldn’t waste my time with individual diodes, but would just get a bridge-rectifier. Easier to wire up and replace if need be.

Overkill. Linear supplies are hideously inefficient. I²R losses (“copper losses”), magnetic hysteresis (“iron losses”), power-factor losses, etc.

You’re worried about losses due to evaporation in a leaky bucket.

As far as I am aware, transformer losses are minimal, next there's the bridge and then the capacitors. The low efficiency factor of linear supplies is the regulator, which I am not using. I feed this into a switching buck converter. Combined efficiency should be above 80%, I believe.

Ah, that looks like a nice setup. You kinda made it sound like you unearthed some old equipment and frankensteined something together.

And yep, I still got a few surplus supplies from æons ago (TI computers?) that need a step-down transformer, and don’t work directly from the mains. The same supply (brand new, unused) made the rounds from like every mail-order surplus joint around.

Kinda like this: Texas Instruments - 1053201-0002 - Power supply board. Triple output. .

Well, I've narrowed down my choices to:

  1. 10 pcs Ceramic Tube Fuse Axial Leads 3.6 x 10mm 0.5A 500mA Quick Fast Blow 250V @ fenggang18
  2. 10x Ceramic Tube Fuse Axial Leads 3.6 x 10mm T0.25A T250mA 0.25A Slow Blow 250V @ xcailau

Reading on a related little doc (http://sun-pec.com/Specification/Fuse_specs.pdf) these slow blow fuses:

  1. Minimum 4 hours to blow at rated current. My setup (theoretically) only caress those 0.25A in the primary briefly, at 100Hz.
  2. Sorry, the translation is so freakin' bad I'm not gonna bother any more… :facepalm:

Fast blow here I go! LoL!

Usual you use slow blow with transformers, they can have a fairly high magnetizing current, in addition to the capacitor in-rush current.

Trying to get the gist of that: “mail-order surplus joint”, :???:

I believe you mean it was a popular choice, the board looks good (18-20VAC seems an uncommon transformer output value, though), yet that compound word still beats me.

mail-order surplus joint = an electronics store that sold surplus and obsolete equipment.

before the internet these stores would mail out cheap catalogs listing their items, and the customer would fill out and mail back a paper order form of the junk they wanted to buy along with a check for payment…

The in-rush current is well taken care of. At a presumed 98% efficiency, primary current will still be within 0.25A max peak value (but close) at full load if no brownout is present (inductive load).

Slow blow are known for their capacity to withstand , for few seconds ,at least ,2.5 times , their rated current . It means that in the first moment , when the capacitor from the output are acting as a short circuit ( only few tens of seconds ) , the slow blow fuse will take the spike of current . A fast , or ultrafast fuse will react instantly , if the inrush current is over the stated value . ALL of the appliaces that involves transformers are provided with this kind of fuses.., just check some , in your house...

In terms of power, the unregulated supply will handle ≈20W at most. At this point, I believe I'm going with a 200mA 250V slow blow fuse, even at a 200V brownout condition that is ≈40W of average power input.

Many choices here: Ceramic Slow Blow Fuse 3.6 x 10mm Axial Leads 125V 250V 0.1A-6.3A 10 - 30pcs @ sunelectronic

Done by now, I believe.

Good and wise choice , Barkuti !

Back in the day (and still some exist today), there were places like Poly-Paks, Micro-Mart, etc., which would sell 7400-series ICs, linear ICs, other discrete parts, and things like power-supplies, wall-warts, relays, buzzers, you name it.

When something would get dumped on the market, it would hit like all those stores at the same time. You could then shop around for nickels’n’dimes for the lowest price (considering shipping, too). Power supplies, gearmotors, etc., were the Big Things that would hit all these places.

Even today, I think places like Parts Express (car-audio-related: speakers, crossovers, tools, alarms, etc.), other niche markets, where you can still put in an order via phone and/or mail.

But those places that’d sell ICs took a beating with online stores, so lots of them are probably out of business. Think it was Micro-Mart which had an assload of 74SC00-series chipe. Same zero-power like 74HCs, but slightly “older”. I stocked up, still got bunches of them.

Ah, Marlin P Jones is still around (mpja.com), and there was a Dick Something whose buzzword was “fair dinkum!” (fair deal) for lots of discounted stuff. The former is still around, can’t recall the latter to be able to look it up.

I am going to quote myself from another thread about vintage electronics. This quote contains a small list of old time surplus and electronic dealers.

Don’t recall.

Yep, but don’t think I ordered anything from them.

Yep, had one on Queens Blvd in Rego Park, used to visit there semi-regularly. Later became geared more towards selling stereos and stuff.

Again, know the name, but don’t think I ordered anything from them.

Yeh, pretty much. :smiley:

Yep. Probably ordered once or twice from them.

Nope, not offhand.

Yep, I used to get the ~50pp catalogues from them. Loved the stuff on the last-pages, real hardcore surplus goodies.

Oh yeah. Pre “The Shack” days…

Miss the Sunday sales flyers with Kornfeld’s “Flyer-side Chats”. They were good reads.

What idiots management was afterward, though. They had their niche, they were growing, and growing, and growing… and then they ditched all those DIYers who MADE RS what it was, and decided to become another “Me, too!” company selling phones and phone-plans, in a market which was already saturated.

Idiots, idiots, idiots…

They deserve bankruptcy.

And then their prices started skyrocketting. Got geared less towards hobbyists and more towards hardcore labs.