Semaphore signals

[tramcar trev]

Greg, mate you always give good simple robust advice but I have a question concerning resistor values. Why are the standard values as they are? Why not in multiples of even decimal value eg 1, 5, 10, 20, 25 etc ohms?

 

[ge_rik]

Thanks Greg

That's a neat approach - easily memorable

Rik

 

[gregh]

Greg, mate you always give good simple robust advice but I have a question concerning resistor values. Why are the standard values as they are? Why not in multiples of even decimal value eg 1, 5, 10, 20, 25 etc ohms?

I THINK it's this reason---
The best accuracy that early manufacturerers could make resistors in was +/- 10%. The standard values are approximately 20% apart. ie each one is ABOUT 1.2 times the previous value.

 

[rjstott]

Greg is right. Resistor values use [size=14pt]Renard numbers from the R6 series for 20% tolerance. These are then conveniently roun[size=14pt]ded.[/size][/size]

One thing to be aware of if you use DCC power is that many LEDs although being diodes can not withstand large reverse voltages so it is wise to include a normal diode (such as 1[size=14pt]N4002 which can tolerate 100V reverse) in parallel with the LEDs but in the reverse direction. This would limit the reverse voltage to that of the forward drop across the 1N4002. Alternately use a single diode rectifier to produce DC, again a 1N4002 would be suitable, being able to supply 1 amp continuous. A final option is to replace the [size=14pt]1N4002 in the first option with a second LED which will also limit the reverse voltage (i.e.each LED protects the other). See these solutions here [/size][/size]http://www.bowdenshobbycircuits.info/page10.htm#lineled.gif. < Link To http://www.bowdenshobbyci...age10.htm#lineled.gif.[size=14pt](Scroll down to AC powered LED)
[/size]

[size=14pt]Richard
[/size]
[h2][/h2] [h2][style="background-color: #888888;"][/h2]