Lamp Control Whoops!
kendall...@gmail.com
harm to my USB hub and possibly my board. I haven't checked the board
yet. I have two of them so at least I have another board. I'll need
to go buy a new USB hub. When you see this you'll probably think I'm
a total idiot, but hey I'm an amateur and it shows.
To preface this I'm doing some weird circuit engineering because the
LED controller board I'm using controls the ground rather then
controlling the 5DC out. It has a terminal for 5vdc always on and 16
terminals that the cpu can control the ground with. So to control a
tip or a mosfet I have to control the ground for the 5v rather then
control the logic current.
I was pretty dumb for sure because I was prototyping on a bread board
and I think I did a lot wrong. Here it the circuit I tried:
http://i49.tinypic.com/2cwtm49.jpg
When started up the lamps were dim showing that power was rolling
through the Tip 120 at 5vdc. When I turned on the 18 v dc the lights
were brighter as expected however my usb hub immediately lit up and it
zapped the power supply to the hub among other things. Weird thing
here is that the hub started pulling 5vdc from the cpu when I
unplugged the power supply. So I did something pretty catastrophic.
Here are my reactions to this. First the diode definitely didn't stop
the current when I swiched on the 18vdc. Second I thought that when I
didn't even have the 18vdc on that the lamps would not be connected
because the ground was unresisted. I guess current still flows
through the TIP even if there is a readily available ground with no
resistance?
With all the rethinking and such here is what I'm thinking the correct
circuit would be:
http://i45.tinypic.com/30lfp06.jpg
However will the resisters fix my bounce through issue? Will using a
mosfet instead make any difference? I have a ton of tip 120s and I
don't have any mosfets on hand. Any ideas on how to do this right
before I do something dumb again?