--- In Electronics_101@..., John Woolsey <jwoolsey@...> wrote:
I am working on my project. Although I want to make a USB I/O device, I
have realized I just don't have the confidence for it. So I have decided
to buy a USB I/O device and get on with things. I will come back to this
later once I am done more of the project.
So I am trying to find Criteria for the USB device. At the current time
my list is:
Must have Linux drivers (accessible from Java).
Must have Pulse Width Modulation (probably two per device).
Reversible ports would be good Input or output on each pin.
Should have at least 16 ports.
It would be nice if you could time ports to flip on for a short period
(to not burn out solenoids).
It would be used for:
Turning on and off lights. (I think 9V bulbs. Might be 12V).
Sense switches closing.
Firing Solenoids at 48V for short periods so they don't burn out
Holding Solenoids via PWM to keep them fired but not burn them out.
For 48V I am looking to get a power supply and then use a power transistor.
I have looked over the internet and I have found tons of boards that
fit. But I have no experience on which one to choose or what is the
"best board". I am in Toronto ON Can so if there is a supplier there
that would be great.
So advice me please. Any suggestions would be helpful.
thanks
John
If you go with the Ardino board discussed previously, you can easily partition your project and use two boards. That will require the host software to talk to 2 /dev/ttyUSBx ports but that's not a big deal.
Another approach is to buy a USB->RS485 device and use the RS485 protocol to connect to MANY boards. You might consider the ROBIN protocol for this:
As to the 48v stuff, you probably don't want to use a transistor. Consider instead, a MOSFET. With the transistor, you will have the problem that Hfe (gain) for power devices is pretty low. So, you will look at the Darlington drivers and find that they drop a bunch of voltage and dissipate a lot of power as a result.
My favorite MOSFET for just about anything is the IRL540N:
Check figure 3 to see how much gate voltage (Vgs) you will need for the current you intend to carry. If you use a 5V Arduino then you can add a 1k pullup resistor on the pin that connects to the MOSFET gate. This will be pretty slow turn-on so don't expect to do PWM in the high number of kHz. Slow turn-on implies a long time in the linear region which brings along increased I^2*R loss and excessive heating.
If you really want to drive the MOSFEZT, find a MOSFET driver chip. It will dump a capacitor full of charge into the gate in a real hurry and the MOSFET switching speed will be much higher. You will probably change to an IRF540N but expect to need at least 10V of gate voltage.
Richard
