Electronics > Beginners
DIY power supply
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Pirateguy:
just remembered: i still have a couple of UK wall plugs.
harvested 2x 3A fuses from those.
MarkF:

--- Quote from: Pirateguy on August 15, 2018, 01:55:02 pm ---well my ATX is onlu 200watt, but that still plenty to set my projects (and me) on fire.

--- End quote ---

Mine too.  Just trying to drive home the point with those who think fuses are not necessary.   ;D


--- Quote ---to clarify: i want BOTH a selectable fixed voltage that i can dial in with a switch AND a variable output that i can set with a pot.
the variable output at least will have a little panel v/a meter to tell me what is going on and the unit will have a temp display.

if i understand correctly you are saying it is better to have each output with it's own binding posts
cuz i might need to use several at the same time?

--- End quote ---

Exactly.  Separate posts for each voltage.


--- Quote ---i have some breakout boards for stepping down to 3.3 and 5v for arduino. i am considering including one of those
and drawing the 3 and 5v from there instead of directly from the atx.
and for the variable output i have 317 and 338 regulators.

--- End quote ---

Why?  Just use the 3.3V, 5V, +12V and -12V directly from the ATX supply.
You could tap off one of the ATX +12V lines to drive a LM317/LM338 to create a 0 to 10V variable output. (I believe the cutoff for the LM317 and LM338 is around 2V.  So, you won't be able to go above 10V with a 12V input.)


--- Quote ---maybe i should make it so i can switch between a 5amp 338 one and another limited to lower current?

--- End quote ---

You can go a long way with the ~1A polyfuses (or normal fuses) on each output voltage.  I would wait a see what your real needs are and proceed from there with something more at that point.  You may wish to buy a proper lab power supply with current and voltage adjustments.
rstofer:

--- Quote from: Jwillis on August 15, 2018, 05:47:51 am ---Difficult to say what the potential current is. Current limiting is nice to have, but not completely necessary.Unless you create dead shorts your projects will only draw as much current as it requires .You can always put in short circuit protection later if you like.

--- End quote ---

I would think that adjustable over-current protection would be the second most important criteria for a power supply right after voltage.  I don't always care if voltage is adjustable, I just need +- 15V and 5V but current limiting at a low level seems terribly important.  Having current foldback on a 5A supply doesn't do much to protect things.

The other day I was bringing up a Z80 project and there was an address conflict.  In effect, two sources were driving the data bus at the same time.  Not good!  I had set the current limit to 100 mA and, sure enough, the limit kicked in and the output voltage dropped to about 1V.  This potentially saved a lot of chips.

Even adding fuses doesn't do much good if they are of some high value.  I want to limit current at low mA, not low Amps.

If I didn't have the money for a real lab supply, I might try these $6 Chinese supplies just to see if they would work.

https://www.ebay.com/itm/Red-0-30V-2mA-3A-Continuously-Adjustable-DC-Regulated-Power-Supply-DIY-Kit-PCB/201751652278

There are dozens of these supplies around, maybe some of them are satisfactory.

rstofer:

--- Quote from: MarkF on August 15, 2018, 06:34:39 pm ---
--- Quote from: Pirateguy on August 15, 2018, 01:55:02 pm ---well my ATX is onlu 200watt, but that still plenty to set my projects (and me) on fire.

--- End quote ---

Mine too.  Just trying to drive home the point with those who think fuses are not necessary.   ;D


--- Quote ---if i understand correctly you are saying it is better to have each output with it's own binding posts
cuz i might need to use several at the same time?

--- End quote ---

Exactly.  Separate posts for each voltage.


--- End quote ---

Spaced exactly 3/4" on centers so that double plugs will fit.
Pirateguy:
i was thinking i would have a selector switch between differently rated fuses.
one limited to something appropriate for micros and something a little stronger for motors n stuff like that.

i was thinking of using the breakout boards cuz they come with a regulator for each output.
isn't that also a way to limit the current?
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