When I studied electricity I always had this question and no one answered it succesfully.
So we have the Ohm lay that states that Intensity = Voltage / Resistance, and then the Electrical Power that states that Electrical Power = Voltage * Intensity.
Which is cool.
Now if I run a phone charger with an output of 5v and a wattage of 1W I can get a max Amperage of 0.20A
And if I run a computer PSU with 300W at 5V I get a max amperage of 60A
But what determines the Wattage itself. Like in this case, if I am trying to shortcircuit the phone charger in theory I would have close to 0 resistance so I=V/R; I=5/0.03Ohm; I=166A, but that is not what actually hapens, with the multimeter it showed that I was getting 0.6A
I am trying to build a hot wire foam cutter and I have tried with a modem psu but the wire didn't got hot at all so I checked the amperage flowing with the multimeter and I was getting 0.6A and the charger itself has an output of 12V and the wire a resistance of 0.03Ohm which should give me 400A if I shortcircuit, not the 0.6A I was getting.
What am I doing wrong/not taking into account?
I is current, not "intensity." The load, ie. the resistance, determines the amount of power drawn from the supply, assume constant voltage. Pretty much every power supply you will encounter is intended to operate at constant voltage. Notable exceptions include battery chargers, because batteries should be charged at constant current, not constant voltage. But this only applies to ones the charge the cells directly, not phones and laptops and things.
No computer power supply is going to be able to source 400A of current, and good thing, because you probably would have killed yourself. It likely has current limiting circuitry so it doesn't blow up when some idiot like you tries to short circuit the supply and turn it on. If it drew 400A through a resistance of 0.03 ohms, you'd be dissipating 4.8 kW in the wire; you already mentioned that your supply was 300W max. A well designed supply will do what yours did. If you were less lucky the supply would be toast, possibly with an electrical fire if you were less lucky.
Use heater element wire, like nichrome or something, which is higher resistance.
>>947619
Obviously I haven't done it with a computer PSU. I tried a modem PSU that probably doesn't have more than 1W.
I am trying to understand how it is possible that despite trying to shortcircuit (which is what you more or less do with a hot wire foam cutter) the modem charger it allow more current.
Since you said that normal power supply units normally operate at constant voltage, in this case 12V. Supposedly if I reduce the resistance(shortcircuiting) I should get higher amperage.
measure the voltage at the output of your power supply as you short it. it will be very low.
P=V*I
if any of V or I is low, the power will be low.
the keyword here is internal resistance of the power supply
most of the voltage drops internally
when you shorted the power supply with a wire, it detected the short and limited the output current, as it's supposed to. you need to use resistive wire, like from a toaster or hair-drier, which should have a resistance of about 6 ohms. this will get you 2A or 24W of heat which is fine for cutting foam.
>Intensity = voltage / resistance
>v / R
>intensity
>W/m^2 = V/Ohms
>>947640
Thanks
>>947616
>studied electricity
>>947632
>I should get higher amperage
just say current
you can't have 0 resistance, everything has resistance, wires have resistance
the more current you pull the hotter the supply will get. heat increases resistance so the current available doesn't increase linearly forever
transformers have a core which will saturate, regulators have a thermal shutdown for protection
there is a limit to everything.
>>947667
Oh look, mr. Fun sucker is here
>>947632
Hotwire cutters that aren't designed by idiots like you don't shortcuircuit becasue they use proper heating wire that has a load, and also is able to heat up.
>>947619
I stands for intensity of current. Blame the French.
>>948468
that makes no sense though the french are stupid. but anyways
>>947616
Use nichrome wire, a mains to 12v halogen electronic transformer and dimmer - match the dimmer to the transformer e.g. both should be leading edge or both trailing edge.
I built a hotwire table for about $20 which was the cost of the dimmer. Nichrome was from a fan heater and transformer was thrown out by someone changing halogen downlights to led.
There are literally dozens of articles for this on a google search.
> that makes no sense though the french are stupid.
J.J. Blodgett: I don't object to foreigners speaking a foreign language. I just wish they'd all speak the same foreign language
>>948498
there's no such thing as leading edge or trailing edge transformers. dont confuse the dude.
>>947667
>you can't have 0 resistance
>what is superconductivity
>>948604
i was speaking metaphorically
Ok, I know a thing or two about foam cutting, as I have around 15 years of experience with it as an RC enthusiast.
Here's my website about it, use google translate:
http://extremflug.de/seite028.htm
In a nutshell:
As a cutting wire, use 0.5mm NiChrome wire.
This will need 2-3A to be hot enough for cutting.
My website mentions how to set up a transformer, and that still works, but it was 10+ years ago, so there is a more modern approach to it.
You should use a 24V power supply like this:
http://www.ebay.de/itm/MW-24V-MeanWell-76W-3-2A-RS75-24-RepRap-CNC-LED-Drucker-Netzteil-Power-Supply-/262283316516?hash=item3d114dcd24:g:R7QAAOSwJcZWfvdI
http://www.ebay.de/itm/AC-DC-Converter-110V-220V-to-24V-6A-Power-Supply-Switching-Transformer-Konverter-/291610070250?hash=item43e5508cea:g:r-kAAOSwuYVWneXG
Make sure it has at least 3A, 6A is better.
Next you need a PWM dimmer like this:
http://www.ebay.de/itm/1-fach-PWM-LED-Dimmer-fur-LEDs-Strip-leisten-lampen-12V-24V-/221950944157?hash=item33ad4ea39d:g:6roAAOSwbdpWVfFP
Make sure it is rated 24V. Put it between the power supply and the wire.
This will most likely work excellent.
>>949004
I forgot: A transformer needs the dimmer between input and transformer, as DC power supply needs the dimmer between the power supply and the load. Not that you get confused.
>>947667
This right here.
If you don't believe him, measure the resistance of a regular incandescent light bulb. I can't remember what it is exactly, but you would think plugging it into a 120V light socket would cause the thing to explode like a bomb (it's like 3 or 5 ohms or something?), but that's because you're measuring it while it's cold.
i built one as well, used a 24v transformer used for doorbell, a regular light dimmer, rotary kind, and i went to the music store and got a pack of banjo wire. worked well.
good luck op
>>948531
I'm not talking about the old style iron core trannies - over in Aus, the electronic transformers (switch mode units) often specify which type pf dimmer is needed - leading or trailing edge.
>>947616
>What am I doing wrong/not taking into account?
Overcurrent protection. Most of today's psus are overcurrent protected. They either limit current or shutdown when you try to draw more than spec allows.
>>949497
that might make sense in the perverted homeland of Josef Fritzl, Australia, but it doesnt make sense anywhere else. because a switching power supply is invariably a regulated supply, so it would counter any attempts to be dimmed, and create an unholy unpredictable mess.
https://www.youtube.com/watch?v=VDekCnihS0M
>>949504
Define regulated. Switcher doesn't have to have feedback you can run a clock for the frequency open loop.
