Model Boat Mayhem - Forum

Please login or register.

Login with username, password and session length.
Pages: [1]   Go Down

Author Topic: Anyone know about SMD's?  (Read 1209 times)

Brian60

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 3,315
  • Location: Hull,UK-but currently residing in Los Martinez, Spain.
Anyone know about SMD's?
« 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?

Tug-Kenny

  • Global Moderator
  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 7,717
  • Location: Newport. S Wales
Re: Anyone know about SMD's?
« Reply #1 on: May 14, 2015, 10:22:13 AM »


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

cheers

ken
Logged
Despite the high cost of living   .......... It remains popular

JimG

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 578
  • Model Boat Mayhem is Great!
  • Location: Dundee
Re: Anyone know about SMD's?
« Reply #2 on: May 14, 2015, 10:33:34 AM »

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
Logged
Dundee Model Boat club

AlexC

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 137
  • Location: West Coast Scotland
Re: Anyone know about SMD's?
« Reply #3 on: May 14, 2015, 02:04:08 PM »

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. :-))
Logged

AlexC

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 137
  • Location: West Coast Scotland
Re: Anyone know about SMD's?
« Reply #4 on: May 14, 2015, 04:56:39 PM »


** 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 NON-STANDARD 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. :((
Logged

Brian60

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 3,315
  • Location: Hull,UK-but currently residing in Los Martinez, Spain.
Re: Anyone know about SMD's?
« Reply #5 on: May 14, 2015, 05:31:29 PM »

Thanks all, your replies have been most informative.  :-))
Pages: [1]   Go Up