Model Boat Mayhem
Technical, Techniques, Hints, and Tips => The "Black Arts!" ( Electrics & Electronics ) => Topic started by: Brian60 on May 14, 2015, 09:39:39 am

I have a roll/tape of smd's that you can cut sections off to use. Each section has 3 smd's and two sm.resistors.
The resistors are marked 151 which equates to 150ohms, so far so far so good, 3 smd's with 2 resistors totalling 300ohms to drop the voltage from 12 volts. They run in parallel by the way not in series.
But I want to use them singly and putting the details into a smd calculator it gives the following, 12v supply dropped to 3volts for a white surface mount led gives 420ohms.
What I am confused about is why does one smd require 420ohms when 3 in parallel only need 300ohms?

It might be to do with the load on the line. Tiny though it is ?
cheers
ken

To drop the voltage over the LED you are making a voltage divider using the resistance of the LED in series with the external resistor. 3V over the LED and 9V over the resistor, so a 1 to 3 ratio of resistance.
When you put 3 LEDs in parallel you are reducing their resistance, the external resistor must then also be reduced to keep the same ratio to divide the voltage by the same amount.
Jim

Hi Brian,
1 LED with 1 x 420ohm resistor with 9 volt drop across the resistor = 21mA current.
With 3 LED in parallel each drawing 21mA total current = 63mA whilst the voltage across them remains constant @3 volts.
You therefore still need to drop 9volts but at the higher current: 
9volts / 63mA = 142.8 ohms.
A single 150ohm is the closest standard value and would be fine.
For single running the 420 ohm resistor suggested is not an (E12) standard value**, but you can use a standard preferred value of 470 ohm, which will reduce the current slightly to 19mA, but I doubt you would see much difference in brightness.
** 420 ohm is available in the E24 range but not all places stock the E24 range... most only stock the E12 range.
I hope this helps.
Best regards.
Sandy. :))

** 420 ohm is available in the E24 range but not all places stock the E24 range... most only stock the E12 range.
OOps!!!... slight brain fade on my part.
420 ohms is a NONSTANDARD value, so...
This should have read 422 ohms is available in the E96 range but most places don't stock the E96 range... most only stock either E12 or E24 ranges.
Sorry for any confusion.
Best regards.
Sandy. :((

Thanks all, your replies have been most informative. :))