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'[EE] Driving Remote LEDs'
2006\05\26@043133 by Josh Koffman

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Hi all. I have a bit of a design challenge I'm grappling with. Here's
the deal. I have a friend who wants to make some remote indicator
lights. In the beginning they will be controlled by switches, but I
will be building a PIC based system in the future.

We've talked about how to deal with long cable runs in the past, but
this application is slightly unique in that the indicators are
bi-colour LEDs. These are the ones where they are two diodes wired
back to back. At the moment I'd like to avoid having to put in a
remote PIC mainly due to the tight delivery schedule he needs. So how
best to provide protection when the polarity of the line will be
changing? Let's guess that the line length will be between 50 and 400
feet of cat5 cable.

Also, I haven't done the calculations yet for voltage drop. If anyone
has done this in the past, tips on good voltage to use (to deal with
line drop) would be appreciated. In the future I will be moving to a
more solid state design and I'm thinking about using an open collector
or similar driver with a slightly higher voltage to drive the long
line.

Thanks!

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
       -Douglas Adams

2006\05\26@045318 by Wouter van Ooijen

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> So how
> best to provide protection when the polarity of the line will be
> changing? Let's guess that the line length will be between 50 and 400
> feet of cat5 cable.

A 3v9 zener diode + a small series resistor?

> Also, I haven't done the calculations yet for voltage drop.

don't, use a constant-current drive.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2006\05\26@061808 by David VanHorn

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>
>
> don't, use a constant-current drive.


Exactly what I was going to suggest.  That takes all the wiring and source
voltage questions out of the picture (for reasonable amounts of variance
anyway.)

2006\05\26@063241 by Tom Sefranek

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Think current source/sink with a good voltage compliance range,
then line length is a non-issue.

Josh Koffman wrote:

{Quote hidden}

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2006\05\26@082414 by Spehro Pefhany

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At 04:31 AM 5/26/2006 -0400, you wrote:

Hi, Josh:-

{Quote hidden}

A 1000' pair of AWG24 wires has a loop resistance of about 51 ohms at 20°C.
If you're going to be using, say, 470R series resistance, the voltage
drop for 50-400' is hardly significant as far as visual brightness goes.

>Also, I haven't done the calculations yet for voltage drop. If anyone
>has done this in the past, tips on good voltage to use (to deal with
>line drop) would be appreciated. In the future I will be moving to a
>more solid state design and I'm thinking about using an open collector
>or similar driver with a slightly higher voltage to drive the long
>line.

Maybe you could use an RS485 driver such as the SN65HVD20. Or if you feel
lucky, just use a couple of PIC pins and split the total resistance into
four, with a couple of unipolar TVS's to ground in a T. Or just a
single resistor if switches are doing the driving.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->>Test equipment, parts OLED displys http://search.ebay.com/_W0QQsassZspeff


2006\05\26@091950 by Bob Axtell

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The ideal driver is a RS422 driver. These are protected from ESD and can
deliver a lot of
current from a long way. I have personally used them at 1000m. They are
bidirectional so
will work well for the dual-led.

LEDs are current devices. As long as the cables are protected from
transients, the scheme
will work reliably.

--Bob

Josh Koffman wrote:
{Quote hidden}

2006\05\26@150836 by Josh Koffman

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On 5/26/06, Wouter van Ooijen <wouterspamKILLspamvoti.nl> wrote:
> > So how
> > best to provide protection when the polarity of the line will be
> > changing? Let's guess that the line length will be between 50 and 400
> > feet of cat5 cable.
>
> A 3v9 zener diode + a small series resistor?

So the zener would drop down my higher voltage to 3.9V, and the
resistor would act as a current limiter, both on the remote end. That
way I get to use a higher voltage on the sending unit side, correct?

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
       -Douglas Adams

2006\05\26@151033 by Josh Koffman

face picon face
On 5/26/06, Tom Sefranek <.....tcsKILLspamspam.....cmcorp.com> wrote:
> Think current source/sink with a good voltage compliance range,
> then line length is a non-issue.

right...as a non engineer, I don't fully understand this. Can you
perhaps give me a couple more terms to make my Googling a bit more
helpful? And does this go against what the others have suggested, or
is there a difference between a current source/sink and a constant
current source.

Thanks!

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
       -Douglas Adams

2006\05\26@152611 by Josh Koffman

face picon face
On 5/26/06, Spehro Pefhany <EraseMEspeffspam_OUTspamTakeThisOuTinterlog.com> wrote:
> A 1000' pair of AWG24 wires has a loop resistance of about 51 ohms at 20°C.
> If you're going to be using, say, 470R series resistance, the voltage
> drop for 50-400' is hardly significant as far as visual brightness goes.

So in theory for my simple version I can basically just ignore the
cable. Everything is going to be very simple in that it will just be
switches and LEDs - there shouldn't be much to get harmed by picking
up interference. Right?

Once I move to the more complex version with actual electronics
driving the long lines, I will have to revisit that protection again.
The RS485 driver idea is kind of cool...in theory it would be able to
control the LEDs directly. I'll have to see what the output of the
drivers is to see if it has enough oomph to light a bright LED. The
advantages are many - I can drive the driver easily from a PIC and if
there is some sort of horrible lightning strike or whatever, the chips
are cheap and can be sacrificial (assuming I use 75176's...I have over
200 just sitting around).

Thanks!

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
       -Douglas Adams

2006\05\26@154203 by Wouter van Ooijen

face picon face
>> A 3v9 zener diode + a small series resistor?
>
> So the zener would drop down my higher voltage to 3.9V, and the
> resistor would act as a current limiter, both on the remote end. That
> way I get to use a higher voltage on the sending unit side, correct?

No, at least I don't think you think the way I think.

Put the small resistor in series with LED, zener diode in parallel with
this combo. Zener shorts a reverse voltage, and limits (by drawing
currunt, hence energy) the 'normal polarity' voltage. The resistor
limits the (peak) current when the zener is limiting the voltage. In
normal use the zener is not active, and the resistor adds a small
voltage drop (assuming a constant current driver).

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2006\05\26@155552 by Wouter van Ooijen

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> > Think current source/sink with a good voltage compliance range,
> > then line length is a non-issue.
>
> And does this go against what the others have suggested, or
> is there a difference between a current source/sink and a constant
> current source.

That's the same thing, and the advice is good. If you power you LEDs
from eg 12V you switch an NPN transistor from the 0/5V output of a PIC
(you use a PIC?) and you put a resistor between emitter and ground,
let's say 330 ohm, which determines the current. The LED is between
collector and +12V. No other resistors.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


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