Power Supply Question
John Couture
Posts: 370
Hey all,
In defining the specifications for a class using the SX tech board or a breadboard, I need to get some power for the 7805 chip on the board.
1) Can I use a the power present on the USB port?··(similar to how I charge my cell phone)
2) The 12vdc(?) that is present on an RS-232 port?
2) The 12vdc that is inside the powersupply of a pc? (in other words, opening the case and connecting to one of the power connectors)
Again, just doing simple circuits for teaching/demonstration, not like we are powering overhead cranes or anything.
The reason I don't want to use wall warts is the darth of outlets on the desks (each desk has a PC).· In worse case I'll buy a "Y" cable or power strip but that takes up a lot more room.
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John J. Couture
San Diego Miramar College
In defining the specifications for a class using the SX tech board or a breadboard, I need to get some power for the 7805 chip on the board.
1) Can I use a the power present on the USB port?··(similar to how I charge my cell phone)
2) The 12vdc(?) that is present on an RS-232 port?
2) The 12vdc that is inside the powersupply of a pc? (in other words, opening the case and connecting to one of the power connectors)
Again, just doing simple circuits for teaching/demonstration, not like we are powering overhead cranes or anything.
The reason I don't want to use wall warts is the darth of outlets on the desks (each desk has a PC).· In worse case I'll buy a "Y" cable or power strip but that takes up a lot more room.
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John J. Couture
San Diego Miramar College
Comments
Alot depends on how much current you require.
A PC USB port does provide +5V, but be warned I recently damaged one of the USB ports in my computer because had the +5V pin shorted to ground.
The +12V on the serial port probably will not have enough current for your needs (only a couple milliamps).
The PC power supply may have the opposite problem, too much current. If something gets shorted it could burn the wires and/or damage the PC.
Andre' runs his XGS Pico kit from a common 9Volt battery. To get longer run time, you might want to use 6 AA batteries. That would be this simplest method.
Bean.
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If you really need to pull the power ouf of the USB port then I would suggest picking up an external POWERED USB hub. That should be safer than relying on the power from a PC USB port (laptops in particular). They are cheap now and if something goes wrong you may only damage the USB hub and the PC may be ok. Cheap insurance.
I ran into one USB device that was a power hog (sucked power from the USB port) and didn't work reliably. When adding a powered HUB in between it was able to supply adequate power and everything worked ok from then on.
Your best option may just be a wall wart or a battery pack as already suggested.
Robert
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John J. Couture
San Diego Miramar College
I *think* most of these modern switching supplies handle a short circuit gracefully, though they probably drop the power to the PC in doing so. If you are going to try this with kids you better short one out yourself to see what happens. You might ruin the whole computer, as Bean has cautioned. However, I suspect that most PC switching power supplies have over current protection these days.
If you decide to go this route, place a low amp fuse inline before entering the student board. That way a short circuit condition above what might be safe for the PC power supply does not continue indefinitely.
By doing this you can bypass the 7805 all together… or maybe just ruin the PC!
How brave are you?
- Sparks
BTW, at home I use a power supply pulled from an old PC as a "bench supply". It provides plenty of power (both 12vdc and 5vdc) and when you short it out, it shuts down and you have to unplug it to reset it. Thus, I have learned to not short it out very often (grin).
I was going to repackage it in a nice pretty case with meters and all but got lazy and just made up a little box (Home Depot wall outlet box) with several 2.1 power jacks along with a cable that comes out and connects to the old PC power supply. That way if the power supply get fried, I just plug my little box into the next one on the dusty shelf. A 200W PC power supply supposedly has a rating of 20A at 5vdc so they should be able to handle a couple of chips
I'm almost tempted to run a 16GA wire through wiremold along the back of the benches with a fused connection for each workstation and have each bank of benches powered by one of these old desktop power supplies. The district surely has tons of old PC's and getting 10 (some for spares) should not be a problem. My fuse would likely be something like a buzzer that would sound if the line were shorted. My Dad did this with a light bulb on our old HO train set when I was a kid (50-60s) and I came to realize later in life how brilliant and simple the concept was (I shorted out the tracks many times a day and I had a feedback mechanism to indicate why the train wasn't running!).
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John J. Couture
San Diego Miramar College
Else, Get batteries or a wall wart.