Model Railway Forum

I think we all know realise the potential need for a resistor when using leds.Unless I have misunderstood, quite possible, these are used to restrict the voltage going to the led? However, I seem to think a current restricting resistor is also needed but have never seen this mentioned. Many leds only need 20mA.

Can anyone clarify?

As far as I am aware, the item be it a motor, LED or anything else will only take the current it needs. (This is how my brother (An ex. electrician) explained it to me when I once mentioned about having too much current and he said not to worry (Though I don't remember what we were talking about at the time)).

Hopefully someone can explain more because electronics is an area which I am still learning as it is not my expertize. To my brother it was not only his trade but his hobby as well so he has done the lot from witing windmills on windfarms to building computers etc. He loves his electronics!

I do know that even if the LED happens to require a very similar voltage that one is supplying it is good practice to add a suitable resister anyway unless it has a resister built in. It is because LED's are quite sensitive things.

LEDs are current driven devices and as such need the current limiting, hence the current limiting resistor. The confusion comes with voltage and many people think the purpose of the resistor is to limit the voltage. The voltage is only a factor in the calculation due to Ohm's Law which which relates voltage, current and resistance in the famous formula V=IR. In order to calculate one factor you need values for the other two hence why data sheets for LEDs have values for forward voltage( Vf ) for different LEDs and also max. forward current (If) as well as ideal current. However it is the control of the current that affects the output of the LED and most modelers find them too bright so increase the resistance to lower the current thus affecting the brightness.

This also explains why even a battery or power supply needs a resistor even if it has the correct voltage, in the case of a battery you would be entirely dependent on its internal resistance which may not be enough. As the calculations at voltages near the LED Vf pose problems working out the resistor, the usual advice is to use a higher voltage in the supply dropping the voltage and limiting the current with a resistor as this makes the Ohm's Law calculation more sensible.

Hope this helps

Richard

Mountain Goat wrote: ↑Mon Jan 17, 2022 10:51 am As far as I am aware, the item be it a motor, LED or anything else will only take the current it needs.

MG you are partially right in that incandescent bulbs and motors are resistive devices and therefore self limit the current so only drawing the current they need, however voltage becomes the limiting factor there. LEDs are not resistive devices and cannot of their own limit their current draw hence the need for a resistor.

Richard

Tricky Dicky wrote: ↑Mon Jan 17, 2022 11:02 am

Mountain Goat wrote: ↑Mon Jan 17, 2022 10:51 am As far as I am aware, the item be it a motor, LED or anything else will only take the current it needs.

MG you are partially right in that incandescent bulbs and motors are resistive devices and therefore self limit the current so only drawing the current they need, however voltage becomes the limiting factor there. LEDs are not resistive devices and cannot of their own limit their current draw hence the need for a resistor.

Richard

Best expaination on this subject I ever read Richard.

Tricky Dicky wrote: ↑Mon Jan 17, 2022 11:02 am

Mountain Goat wrote: ↑Mon Jan 17, 2022 10:51 am As far as I am aware, the item be it a motor, LED or anything else will only take the current it needs.

MG you are partially right in that incandescent bulbs and motors are resistive devices and therefore self limit the current so only drawing the current they need, however voltage becomes the limiting factor there. LEDs are not resistive devices and cannot of their own limit their current draw hence the need for a resistor.

Richard

Thanks. I am learning....

Conmaira wrote: ↑Mon Jan 17, 2022 9:42 am I think we all know realise the potential need for a resistor when using leds.Unless I have misunderstood, quite possible, these are used to restrict the voltage going to the led? However, I seem to think a current restricting resistor is also needed but have never seen this mentioned. Many leds only need 20mA.

Can anyone clarify?

Richard (TD) has given the Current flow requirements for LEDs and an excellent explanation.
Your 20milliamp is around the MAXIMUM current a standard LED should be feed with. The true maximum rating is shown in the LEDs specification data and will be shown as IF (Current Forward). My advice would be to reduce this by at least 50% or more if possible.
A standard LED will usually illuminate to an excellent light level with at least 10milliamp and many will work at half that. This is all controlled normally via the Ohm value of the series resistor.
One resistor in either the Anode or Cathode lead of the LED (Only the one is needed). You can also series wire LEDs by wiring one Cathode to the next Anode lead and then add a single series resistor for the chain. When LEDs are wired in series their combined total (VF of the LEDs) should normally be at or below the DC supply volts.
This simple to use LED resistor calculator is a very useful tool to have Bookmarked...http://www.hebeiltd.com.cn/calculator/v5/led.php