What length of hook-up wire (electronic circuit board) is equivalent
to a 470 Ohm resistor? I want to place LEDs at some distance (max 6
feet/1.8 metres) from the TTL outputs that drive them, and normally
one would include a 470 Ohm resistor per LED. But if the connecting
wires (probably the individual cores in flat ribbon cable, like IDE
drive cable) provide enough resistance, there may be no need for the
You would need far, far more than your maximum of 6 ft of ribbon cable core.
You can look up copper wire resistance tables.
eg 2 metres of the finest wire here, 40 gauge, would only have a
resistance of six ohms or so.
So, for driving LEDs, you can ignore the resistance of tracks and cables.
Temperature is of major importance when determining DC resistance.
For 100 per cent IACS copper dc resistance can be found with the following
Rdc=( (Pc/CMA)*((tah+T)/(tah.+20)) )*(length in feet)
Where Pc is the circular mil ohm resistance in ohms at 20 degrees C.
tah is the inferred temperature of zero resistance.
T is the temperature in degrees C at which we wish to find the DC resistance
CMA is circular mil area (diameter of conductor in thousandths of an inch
For the resistance of a copper conductor at 75 degrees C. with a CMA of 1620
circular mils (No. 18 AWG) 1000 feet long the resistance would be:
Rdc=(10.371 ohms/1620 cma)(234.5 degrees C+75 degrees C)/(234.5 degrees C+20
degrees C))(1000 ft)
Rdc=7.8 ohms per thousand feet
This matches the value in Table 8 or Chapter 9 in the NEC.
For resistance of other metals used the circular mil ohms resistance per
foot at 20 degrees C in place of the value of 10.371 used for copper.
Typical vaues for Pc and tah are:
copper (100 per cent IACS) is 10.371 ,tah is 234.5
Aluminum (61 per cent IACS) is 17.002, tah is 228.1
Brass (27.3 per cent IACS) (70 cu, 30 Zn) is 38.0 and tah is 912
Lead (7.84 per cent IACS) 132.3 and tah is 236
Reference: Neher McGrath paper
The Calculation of the Temperature Rise and Load Capability of Cable Systems
published by AIEE (now the IEEE) October 1957
Ah. Now is this something I need to worry about? The LEDs will be
coming on and going off at different times - about four or five on at
any one time. Some will stay on longer than others.
How would this interference manifest itself?
Place an am radio, tuned to a quiet channel, next to the wires and you
will get some audible clicks. If that worries you, use twisted pairs to
each LED or pass the wires through a ferrite core.
Not really something to worry too much about, if all that you are doing
is switching LEDs on and off. Very much something to worry about, if you
are trying to send data at a very high data rate.
Aren't you guys forgetting about "kick back"? if you run a fast spike
TTL signal down 6' of wire and then turn off the LED, you'll develope
some kick-back, right?
He may need to incorporate a capacitor and high ohm resistor to ground -
say a 1Mohm resistor in series with a capacitor (not sure what value).
Just a thought to get a better idea for his circuit.
The source impedance is going to be several hundred ohms - and at the
end of the line is a (conducting) LED that is going to present a very
low impedance to any voltage transients over its knee voltage. You don't
get a lot better than that for minimising reflected power. Add a ferrite
core to the line and the "spike" isn't going to be fast - not that it
would be very fast anyway, because of the source impedance.
Of course if things were different, say a very low impedance source and
a very high impedance load, and the shape of the signals really
mattered, etc, etc, then a lot more attention to the design would be
Peter, I'm open to any ideas! Basically, think of a small circuit
board with CMOS 4099s or TTL equivalent latches. Each output pin shall
drive an LED. I can, of course, choose between different LED types and
sizes, but I want to aim for a 3 mm size, as the application needs
these to be as compact as possible. There are also, apparently, LEDs
now available that incorporate their own resistor. So here we have our
latch with each output pin connected to an LED. Pin goes high, LED
Now extend the distance the LED is mounted from its driver pin by,
say, 6 feet max. I could use ordinary hook-up wire from Maplins. Or I
could use a strip of flat ribbon cable, separating out the inividual
cores at the LED end. Ideally, my long mounting board could have
copper tracks on it, but that is not feasible for a homebrew
Over to you!
Thanks, by the way, for all these useful tips!
The LEDs will be switched on/off at a rate comparable to MIDI when
playing an average piece of music. That is, however many individually
struck notes you would get in a bar/measure, that is the equivalent
rate for the LEDs.
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