component questions

Questions on other types of hardware and getting it talking to the ARM CPU
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Joined: Fri Oct 19, 2012 5:11 am

component questions

Post by YahooArchive »

?Recommended Darlington part # that the 4mA from the controller can
drive the base to trigger on condition?

To drive load larger than the 4mA drivers, a Darlington transistor is a
good alternative. To simplify things transistors amplify current, with
a ratio of beta or hfe. This measure will give you an idea of how much
current you can expect to get given an input base current. Power
Darlingtons are 2 transistors connected to get higher beta while having
large current capabilities, with beta of 1000 or more.

So how to find this data-- I find that Digikey is a excellent website
for finding parts, even if you're not going to buy there, they have a
well organized wide selection, with quick links to spec sheets.
Sometimes the results will be overwhelming with too many options, but
you can narrow selections by any number of parameters. Digikey does
change a handling charge for small orders, so you need to group you
purchases. So my search at Digikey turns up more choices than I need,
so I did a quick search at Radio Shack and it showed a TIP120, though
their spec was pretty lame. Armed with a target device I went back to
Digikey and looked up TIP120 and was able to pull up a full spec sheet

For me, I just keep a list of components that I need and add them to an
order when I really need something. I no longer live in Silicon Valley
which has a variety of electronics surplus stores that carry components,
or as I remember in the Bowery in New York which I visited while in
college, and felt like a kid a candy store with all the electronic
surplus places (don't know if they are still there).

But even in my small town there is a Radio Shack that often carries what
I need, if not its an hour trip to Sandy's in Reno, and if its not
there, then its Digikey time.

Back to the TIP120
hfe 1000 or more, so 4mA out of an ARMmite can drive 4A or more
Vce of 60V so it can drive some pretty hefty devices (lamps)
Vbe of 2.5V at high current loads

So to drive this connect an IO through a (3.3-2.5)/4ma = 200 ohms
a bigger resistor is OK if you don't need 4A on the drive.

If you're driving a relay, solenoid or motor remember catch diodes to
shunt current back to the high voltage supply when turning off.

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