Correct resistor value?

[fractal5]

http://www.z80.info/z80test0.htm

Does 470 Ohm seems reasonable for the LEDs on the left hand side in this diagram?

A typical LED draws around 20 mA, so this should give 9,4 V. But the Z80 is driven by +5 volts.

What resistor value should I use?


Also why is "E" used instead of "Ohm" in this diagram? After googling this it seems it simply means Ohm, and that "R" also means the same.

E -> R -> Ohm, are all equivalent (correct me if I'm wrong).

 

[jpanhalt]

You are right about the "E" taking the place of ohm or Ω. Maybe it is avoid imagined confusion. "R" is commonly described as the decimal point. Thus 4R7 = 4.7 Ω. Why the author of your link chose 470E instead of 470R or just 470 is a mystery.

John

 

[granddad]

In the circuit shown the Z80 is sourcing the drive current, 470R would be about right , give ~ 10 mA per pin , consider if 3 leds (or more) were all on... using a lower value resistor , may come close to maximum mA of the cpu. check the Z80 data sheet for absolute maximums.

 

[Nigel Goodwin]

You are right about the "E" taking the place of ohm or Ω. Maybe it is avoid imagined confusion. "R" is commonly described as the decimal point. Thus 4R7 = 4.7 Ω. Why the author of your link chose 470E instead of 470R or just 470 is a mystery.

No mystery about it, he's not a native English speaker (his email address is .DK), a number of countries on Continental Europe use E as a short cut for ohm instead of R as English speakers do.