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DIY power supply
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Pirateguy:
couple of quick questions:

when a polyfuse is triggered, what exactly does it take to reset it?
could i put in a NC button to interrupt the line to reset a polyfuse?

do polyfuses work any differently on negative voltage?

can i run a 7905 on the -12 rail normally as per datasheet, or does it need something anything else?

exe:
I think polyfuse is just a PTC -- a resistor that quickly increases resistance as it heats up. It doesn't care which way current flows imho.

I don't think it can be reset with a button. The only way to reset it is to cool it down.

I suggest read a datasheet on it, it answers all your questions. I think this is a good start: http://www.littelfuse.com/products/resettable-ptcs/surface-mount.aspx . Be sure to see different plots and numbers in the datasheet. Like, maximum guaranteed current that will not trip it (and how it depends on ambient temperature!), minimum guaranteed current that will trip it, etc.
drussell:

--- Quote from: Pirateguy on September 20, 2018, 08:27:50 am ---couple of quick questions:when a polyfuse is triggered, what exactly does it take to reset it?
--- End quote ---

Removal of the overcurrent and time.  (On the order of a few seconds.)


--- Quote ---could i put in a NC button to interrupt the line to reset a polyfuse?
--- End quote ---

No, that is not how they work.  :)

https://en.wikipedia.org/wiki/Resettable_fuse


--- Quote ---do polyfuses work any differently on negative voltage?
--- End quote ---

No, they are not polarity sensitive.


--- Quote ---can i run a 7905 on the -12 rail normally as per datasheet, or does it need something anything else?
--- End quote ---

You most certainly can use a 7905 on a -12 volt rail.  You're going to be limited as to the current you can draw on your negative side due to the 0.8A maximum output of the power supply compared to the huge currents available on the positive rails, but within that limit you'll be fine.  (Obviously the -5 rail will have even less than 0.8A available since the linear 7905 is going to dissipate a good chunk of your available ~10 watts.)


--- Quote from: exe on September 20, 2018, 09:51:52 am ---I think polyfuse is just a PTC -- a resistor that quickly increases resistance as it heats up. It doesn't care which way current flows imho.
--- End quote ---

Correct.  They are actually known as PPTCs.  The extra P stands for Polymeric.  They are made from a special crystalline organic polymer that gives them the same kind of sharp current characteristic as a zener has on the knee voltage.  Instead of a slowly changing, smooth curve like a regular PTC has, a polyfuse stays basically the same resistance until it heats enough for the polymer to change from a crystalline state to an amorphous state, which raises the resistance very rapidly, limiting the current.  Once the current is reduced sufficiently, the polymer recrystallizes as it cools and the "fuse" is "closed" again (resistance returns to close to the original value, though it can take several hours to return fully to its original value.)
Pirateguy:
right, so the only action i need to undertake is to disconnect the load and wait.

thanks :)

is 2,7 ohm enough for the load resistor?
Pirateguy:
one day i'm looking for an ATX, the next a friend brings by 2 of them + another device that contains one :P
i got a small 150watt one, a large 500watt one and what looks like a 10watt one that has -24v on it for some reason
from a 'media center extender'.

anyhoo i was looking at this guide to converting atx psu and came across something odd.

--- Quote ---Again the voltages that can be output by this unit are the same as before 24v (+12, -12), 17v (+5, -12), 12v (+12, 0), 10v (+5, -5), 7v (+12, +5), 5v (+5, 0). Note that some ATX12V power supplies with a 24-pin motherboard connector may not have the -5V (pin 20) white lead. In this case use the older ATX power supplies with a 20-pin connector above if you need the additional -5V supply.
--- End quote ---

this suggests a +5v and +12v rails can somehow be used to create 7v??

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