Suitable Pot

[pager8]

Hi All.
I am looking for a suitable potentiometer to control the brightness of 3mm LEDs assumedly wire wound, 6v-9v input controlling down to 2v-3v. What value would I need to look at?
Thanks in anticipation
Pager8 (Geoff)

[A Stuart]

I think we need to know more.
Do you know what current they draw, both maximum rated, and at the minimum brightness you will require?
Also their nominal forward voltage drop?
Is this supply 9V ac or dc?

[bob orbell]

Geoff, Richard put a chart on here some time ago for the required resistor, or do you want to have the option to adjust the the LED to different levels. Bob

[A Stuart]

Haven't looked, but can't help thinking a switch and handful of resistors might be less expensive.

[John Caswell]

A pot will not adjust the brightness of an LED as it is a diode. It is either on or off. You would need to make some sort of digital control to to switch it on/off in varying mark space ratios, ie as lamp dimmers do.
That said there are quite a few LED types from ultra bright to dim on the market, you have just got to find them!

John

[pager8]

Thank you all for your replies, What I was looking for is a way of dimming the brightness of a blue LED to match others, I have several in different Amps etc fed from supposedly 6,3 volts and they are at different brightness levels once resistance is determined I would fit appropriate resistor (Or am I misreading the problem)
Regards Pager8 (Geoff)

[A Stuart]

What am I missing here? Have LEDs changed completely in the last 40 years. Last time I tried playing with what one does, putting 20mA through it was bright, putting (say) 0.25mA through it was dim, no "on-off" behaviour.

[colin.hepburn]

Hi Geoff/all
Here’s a post and a link Richard posted up for an LED calc that’s simple to use shows you the schematic and the diagram wiring
http://www.world-designs.co.uk/forum...ead.php?t=7948
See link here for the calc here
https://ledcalculator.net/#p=50&v=2&c=4&n=1&o=w
hope that helps
colin

[John Caswell]

Cannot fault what Ali says as I have done the same myself, but I have found that it does depend on the LED, hence my comments.
I can only say try it and see.

John

[Richard]

<< Thank you all for your replies, What I was looking for is a way of dimming the brightness of a blue LED to match others, I have several in different Amps etc fed from supposedly 6,3 volts and they are at different brightness levels once resistance is determined I would fit appropriate resistor (Or am I misreading the problem)
Regards Pager8 (Geoff) >>


Hi Geoff, the problem may be the supply voltages are different or the leds are different.


You could correct their brightness’ individually by changing the resistor but the visual matching can be a bit hit and miss if the leds are different types, ie, clear or diffused, standard bright, high bright, super bright, ultra bright etc, the naming is various depending on the maker and seller.


I’d buy a set of matching new leds off ebay such as these,


https://www.ebay.co.uk/itm/352163012...EAAOSwhUpcv19e


Then if the supply voltages are all similar 6.3V make all the resistors similar.


The highest dissipation for any of these resistors is 60mW so small 1/8W or 1/4W resistors will be fine.


3mA will give a nice low level indicator; 1K
5mA is brighter perhaps for a brighter room or if it’s not in your direct vision; 620R
10mA very bright; 300R
20mA for laser eye surgery; 150R


I can’t imagine you’ll need the last 2 options


Attached is my 3 box Pre3 set up with blue leds at 3mA, picture and circuit drawing.


If you want to play with the calc put in Supply 6.3V, LED voltage drop 3.3V (for blue leds or other colours as per the ? mark in circle drop-down) and whatever mA up to 20A max for these types, eg,


https://ledcalculator.net/#p=6.3&v=3.3&c=3&n=1&o=w