driving high voltage LEDs

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

driving high voltage LEDs

Post by YahooArchive »

I am trying to use an Armmite board in a application where I have to drive 6 12
Volt LEDs. These LEDs have a common ground, so I need to source 12 Volts. I
think that I need about 20ma @ 12 Volts.

I assume that I will need a driver IC, or a relay. These are static LEDs, so
switching speed is not a problem.

I could rewire the LEDs, so I could sink the current and use an open collector
driver that's compatible with the Armmite I/O.

Can you give me any suggestions on what driver to use?



YahooArchive
Posts: 1462
Joined: Fri Oct 19, 2012 5:11 am

Re: driving high voltage LEDs

Post by YahooArchive »

The ULN2003 driver should work and it has been around for years. It has 7
drivers in a DIP package. There is a similar one that has 8. You will have
to re-wire.

The datasheet is here: http://datasheetcatalog.org/datasheets/ ... 337_DS.pdf

YahooArchive
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Joined: Fri Oct 19, 2012 5:11 am

Re: driving high voltage LEDs

Post by YahooArchive »

The ULN2003 looks like a good choice. I'm trying to find a spot in the
Prototyping area on the Armmite board to locate the driver. It appears that I
can install it in the I/O area from I/O 9 to I/15, and let pins 8 and 9 slip
into the GND bus. Since I need 6 outputs, I will need to use I/O 12 or I/O 13 in
addition to I/O 9, I/O 10, I/O 11, I/O 14, and 1/O 15. Since I/O 12 & I/O 13 are
Open Drain Outputs, I might need a pulldown resistor unless the ULN2003 is a
sufficient load to insure a low on that pin.

I don't see any other place to locate the ULN2003. Any ideas?

YahooArchive
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Re: driving high voltage LEDs

Post by YahooArchive »

You've got the right part of the 2803 for 8 lines.

You can't drive it with an open-drain output, as you need to pull these inputs
high to turn on the transistors.

I'd suggest IO0-7, just connect IO0 to GND, and don't try to turn it on as an
output, the default state is input.

You definitely don't want to tie the common to the 3.3V supply as that is a
diode-or of whatever you're driving, and if that's a high voltage or inductive
load, you'll blow out the ARMmite.

YahooArchive
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Joined: Fri Oct 19, 2012 5:11 am

Re: driving high voltage LEDs

Post by YahooArchive »

I'm going to order some ULN2803. I only need 6 drivers, but I'll have 2 spares
if something else comes along. I was inserting the IC backwards, with the
outputs connected to the I/O pins. I could cut the Common pin off the IC, but if
I install it with pin 1 to I/O 8 that puts IC pin 9 connected to I/O 0. The
ULN2802 is an 18 pin part, with 9 pins per side. The Common pin 10 will be left
unconnected.

YahooArchive
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Joined: Fri Oct 19, 2012 5:11 am

Re: driving high voltage LEDs

Post by YahooArchive »

I was going to say that the ULN2003 will turn the output OFF, if the input is
disconnected.
All you have to worry about is exceeding the threshold voltage of, I think 2.4
Volts for on.

YahooArchive
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Joined: Fri Oct 19, 2012 5:11 am

Re: driving high voltage LEDs

Post by YahooArchive »

20 or so years ago, it took a while to understand the datasheet for the 2003.
Note that it doesn't require power, but it looks like it does. It only requires
a ground connection. The Power pin is only there to connect the other side of
the coil suppression diode.

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