No but it doesn't have to be exactly 2A, just as long as it's 2A or more. A 12V battery would also work.
Thank you a lot.
Is there some limit to the "or more"? If I connect it to a 12V output of a PC power supply that probably outputs 14A (though I have no way to measure it) is that ok?
(I've had some failure with LED Ambilight I was making with an Arduino that I think was due to the wrong current.. but I might be wrong I should really read up on this subject a bit more)
Basically, there is no limit. You could use a car (automotive) battery that can output 300A and it would be OK. The important thing is that the power supply be well regulated, meaning keep the output at 12V.
Most PC power supplies require a minimum load in order to regulate, so I don't recommend trying it, especially with an UNO connected.
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When a power supply's label says "12V 1.6A" or "12V 14A" what that means is that the output will be fixed at 12V and the current can be up to 1.6A or up to 14A. The circuit connected to the power supply will draw whatever current it needs from the power supply, no more, no less. The power supply will not and can not force the maximum current to flow into the circuit. The only way it could force a certain current to flow would be to increase the voltage, but the voltage is fixed. We call this a "constant voltage power supply". Most power supplies are of this type, including PC PSUs.
There is a special type of power supply that forces a particular current to flow, and that is called a "constant current power supply". This type is only used in special cases. An example would be high power LED lighting. Power supplies like this will say something like "15-60V 700mA" on the label. This tells you that the power supply will adjust its output voltage between 15V and 60V as needed to get that rated 700mA current to flow.
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Thank you for the explanation.
I've connected the power supply to this board.
I have a little multimeter that says the output is 12.02-12.03V.
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I think this adapter, plus the PC PSU will be ok for your project. But there is a danger, as @jim-p points out, that when all your electromagnets are off, the voltage might not be very stable, but I don't think it would matter much. It's not going to be high enough to damage those TIP120.
Connect your voltmeter and then connect and disconnect one of the magnets a few times. If the voltage stays at 12V, then I would say it's OK but I be much happier if you had a 12V 2A supply.
If your 1.6A supply is outputting 12V, you could use it for testing everything if you just use 3 magnets.
uhh.. you're right, it drops to 20mV when the magnet is engaged.
Are you doing the test with only the magnet connected to the supply, a proto setup, or the entire circuit? How exactly are you doing this?
The magnets and transistors won't be damaged with small changes in the 12V even 11V to 14V would be OK. What I'm worried about is the UNO.
So, for now, do this:
Power the UNO from the USB and only use the 12V for the magnets. However you must connect one of the UNO GND pins the the PC power supply ground. Then when you get a 12V 2A supply you can connect everything to it.
You need to measure that electromagnet current independently from the supply. It is the root factor that limits the operation of the circuit. Find a heavy duty supply somewhere, connect your circuit, except measure amps instead of volts.
Then take the results and mull them over... the supply is also a suspect. Those supplies have output current limiting, On the whole, PC energy needs have declined over the years and many supplies may treat 12V as an accessory voltage. Don't blindly believe the published spec or the one that's written on it. You said previously:
If I connect it to a 12V output of a PC power supply that probably outputs 14A (though I have no way to measure it)
That is problematic. You didn't say whether there was any other way to know. Unfortunately, a simple short circuit output current measurement won't work because it will probably "fold back" under protection. A typical PC supply would NOT be capable of 12V @ 14A.
I guess you've realized... this is only one magnet and you have multiple of them.
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chmm.. I guess one more supply run is unavoidable.. I'll use it to get the MOSFETs and proper power supply (the weak one I found doesn't work)..
For test, use 5V. It will supply the current you need, the magnets will just have less force. You might gain some understanding, then find a proper 12V supply and use it finally.
MOSFETs?
Did you mean to 20mV or by 20mV?
This is the PC power supply? The label on the side should give the available current or power at each voltage rail:
What is the coil resistance of your electromagnet?
Photos of your current, actual hardware test setup would be immensely helpful.
There's a video under the test illustration
it dropped TO around 100 mV - 20mV.
Yes an old PC power supply LC-B350ATX it says +12V/16A
and the magnets are 12V KK-P20/15. I couldn't find the coil resistance value anywhere.