Yes - another noob building a bench supply from an ATX PSU.
My question is this - given that these supplies can happily provide such huge amounts of current, how can I protect my teensy weensy Arduino projects from suddenly getting a jolt of 20A or more juice shoved through them, when I make an inevitable mistake in my breadboard connections ? I don't think I have ever built a circuit yet that pulls more than around 1.5A (at least not by design... By accident? Yeah - well, that's a different story! :-/ )
Putting fuses on each of the 3 output lines (I only intend to use positive 3.3, 5, and 12v) is one way, but I've heard that even 'quick-blow' fuses can run at considerably more than their rated amperage for extended periods before actually cutting the circuit - and then of course, you need to have spares available. DC Circuit breakers are another alternative, but quite expensive.
Is there a simple, equally cheap to implement, alternative to fuses, that would limit the draw on any of those three outputs to (arbitrarily) 2A, and either hold the current at that level, or cut it off (but allow the circuit to be reset without having to replace components)?
I know that I could build or buy a 'proper' bench supply that can be operated at constant voltage and/or constant current, but that's (possibly) a project for some time in the future. At the moment, I am only interested in a solution that can be applied to an ATX conversion, and fits in a beginners budget - i.e. one that forces me to rely on salvaging a PSU from an old junked PC in the first place ;-)
Thanks
Graham

 

You can put electronic fuses in-line with the supply outputs.

Here's an idea that was mentioned recently:
https://forum.allaboutcircuits.com/...er-control-module-design.141962/#post-1198540

 

You can put electronic fuses in-line with the supply outputs.

Thanks for the idea Dennis - it looks like that's just the kind of thing I'm after, but the cost of anything involving MOSFETs is prohibitive here in Australia. Either I pay a premium to purchase locally, or pay extortionate delivery charges (and now import sales taxes as well, since July 1) to purchase from cheaper sources overseas (or wait 2 months for delivery from China, and not be sure to get what I ordered in the first place).
The cheapest P-Channel MOSFETs I can find locally are the IRF9540N at $6 each, but their 'on' resistance is 117 milliohms - whereas the article you reference suggests I would need something lower than 10 milliohms. Also, I can't find any reference to sub 1 ohm resistors rated at 2W... I'm guessing I'd need to build an array of resistors in parallel to get 0.35 ohms at 2W. Based on the components I can find, it looks like I'd be facing something like $25-30 at least, to implement three such e-fuses. :-(
Thanks anyway
Graham

 

Then it looks like you are stuck with fuses.

 

Then it looks like you are stuck with fuses.

LOL - thanks ;-)

 

Atx psus monitor output current in the pulse transformer, it is then rectified to provide a small voltage and is compared with the reference set by a resistor divider on the main chip, usually around 0.3 to 0.7V.

You can alter these resistors to set a lower reference voltage and thus lower your output current sense.

 

Use resettable "polyswitch" fuses.
https://www.ebay.com.au/itm/100Pcs-...458983?hash=item41c4501d67:g:AW4AAOSwsThbPd-O
https://www.ebay.com.au/itm/20Pcs-F...458730?hash=item41c4501c6a:g:t1oAAOSwEehbPd-M

They are available in a great range of currents. I use then from 50mA to 10A range.
(Usually I do not get then from Ebay 'tho)

It is good practice to add one to the final product too.

 

[QUOTE="You can alter these resistors to set a lower reference voltage and thus lower your output current sense.[/QUOTE]
Thanks DodgyDave - sounds like something i could possibly investigate - if i understood the principle of the current sense bit of your explanation I’ll see if i can find some articles online that explain how that works. Thanks for the idea.

 

hi Grum,
The problem is how low do you go with the fusing.?
Even 300mA flowing for less than 1 second can take out a circuit pathway which is designed to take only 100mA.
When you consider the ATX is original designed to drive a motherboard and its peripherals with 'minimal' fusing, perhaps you are over thinking the level of protection required.?
I would fit a 2A fuse for a Arduino, after all a 9Vdc wall wart rated a 2A is often used.
[uses the onboard 5v or 3.3v reg]
Eric

EDIT:
I would recommend not running an Arduino from a 12Vdc supply, the onboard reg gets hot and there have been reported failures,
One solution would be a 7809 or 7808 Vreg using the ATX 12Vdc as an input.
The Arduino will run OK with a 9v or 8v input

 

One solution would be a 7809 or 7808 Vreg using the ATX 12Vdc as an input.

And you gain the lower current limit offered by the regs too. A good solution.

 

[QUOTE="You can alter these resistors to set a lower reference voltage and thus lower your output current sense.

Thanks DodgyDave - sounds like something i could possibly investigate - if i understood the principle of the current sense bit of your explanation I’ll see if i can find some articles online that explain how that works. Thanks for the idea.[/QUOTE]



Here is one example using the TL494 chip, the Current sense is the small transformer under T1 at the top of the circuit, the voltage is rectified by D11, D13 and fed to Q6,7 to switch off the chip .

 

Use resettable "polyswitch" fuses.
They are available in a great range of currents. I use then from 50mA to 10A range.

Thanks dendad - that’s another good idea to look into

 

Here is one example

That’s great - many thanks for the example. After I’ve recovered from taking the grandkids to the zoo tomorrow - I’ll sit down and work through this to understand what is going on.
Thanks again
Graham

 

hi Grum,
perhaps you are over thinking the level of protection required.?
I would recommend not running an Arduino from a 12Vdc supply, the onboard reg gets hot and there have been reported failures,
One solution would be a 7809 or 7808 Vreg using the ATX 12Vdc as an input.
The Arduino will run OK with a 9v or 8v input

Ha ha - thanks Eric - perhaps you are right. I do tend to spend far more time planning and analysing a problem than is probably needed - I blame it on a lifetime working in business and systems analysis :-D
Btw - don’t worry about the arduino supply, I already have a variable regulator box I built to use with the unregulated wall warts, that put out about 18v to a low current circuit, even though they say they are 12v. I also had in mind to use it with solar charged car batteries as another power source for my project bench - until I found out the cost of solar panels :-o
Cheers