Maker Pro
Maker Pro

Multiple LED Driving

bigkim100

Apr 17, 2013
258
Joined
Apr 17, 2013
Messages
258
Ok, I know how to drive individual LEDS, but when it comes to voltage and current limiting of several resistors, I dont really understand doing it correctly

I have a 20 x 23cm illustration transparency in glass that I need to illuminate with leds, normally.
The Illustration is divided into 3 separate parts.
Normally the entire illustration is illuminated in BLUE leds, but each of the 3 separate parts of the illustration need to have its portion of the white leds turned off , and then be illuminated by red leds for that individual section.
The other 2 sections would still be illuminated in white.

Im looking for the number of white LEDs suggested for this size of illustration (20 x 23cm)?
The voltage suggested (I would prefer to work at 6VDC) for all the LEDS, and the resistors suggested for the white LEDS
Dividing up this size of the transparency into 3 sections (*approximately) equal parts, for the RED leds, the number of RED LEDS suggested, the resistors needed to drive them and voltage (again preferring 6VDC)

I just using garden variety LEDs from an assortment received in Ali Express assortment, and the illumination just needs to softly illuminate the image.
 

kellys_eye

Jun 25, 2010
5,131
Joined
Jun 25, 2010
Messages
5,131
A chain of paralleled R/LED arrangement will work, where the resistor (R) is calculated for the chosen LED colour using the standard formula.

Inefficient imho. Use an old Laptop/Monitor with the LCD panel removed (i.e. just use the backlight and the diffusers) and mask the areas to be 'coloured' with translucent coloured film.

Screen frame makes a decent surround too!
 

bigkim100

Apr 17, 2013
258
Joined
Apr 17, 2013
Messages
258
No, the whole dead laptop screen is just overkill.
The led chain current limiting resistance is what Im having a problem with
 

Audioguru

Sep 24, 2016
3,650
Joined
Sep 24, 2016
Messages
3,650
I doubt that LEDs from AliExpress have a manufacturers name, a part number, a detailed datasheet, a brightness level, a beam angle. The LEDs will probably not be matched.
The Name-Brand LEDs I bought from a real electronic parts distributor all match the specs in their datasheet and work perfectly.
 

Harald Kapp

Moderator
Moderator
Nov 17, 2011
12,756
Joined
Nov 17, 2011
Messages
12,756
Have a look at WS2812B based LED strips, easily controllable by e.g. an Arduino, or an ESP32 if you want control via WIFI.
 

bigkim100

Apr 17, 2013
258
Joined
Apr 17, 2013
Messages
258
I doubt that LEDs from AliExpress have a manufacturers name, a part number, a detailed datasheet, a brightness level, a beam angle. The LEDs will probably not be matched.
The Name-Brand LEDs I bought from a real electronic parts distributor all match the specs in their datasheet and work perfectly.
You are correct, they do not, but a general everyday guy, doing general everyday fun stuff, for myself, and sometimes others, they are perfectly acceptable quality
 

bigkim100

Apr 17, 2013
258
Joined
Apr 17, 2013
Messages
258
A chain of paralleled R/LED arrangement will work, where the resistor (R) is calculated for the chosen LED colour using the standard formula.

Inefficient imho. Use an old Laptop/Monitor with the LCD panel removed (i.e. just use the backlight and the diffusers) and mask the areas to be 'coloured' with translucent coloured film.

Screen frame makes a decent surround too!
The one thing i can never get my brain around is that multiple leds in parallel, does each led need its own current limiting resistor???
Thanks, Kim
 

Martaine2005

May 12, 2015
4,355
Joined
May 12, 2015
Messages
4,355
You should put a series resistor for each LED to avoid current hogging and a chain reaction of LED failures.
The red LEDs are approximately 1.8V to 2V. Do some tests for required brightness starting with bright at 20mA and dimming to a few mA.
What 6V supply are you using?.
Are red and white LEDs switched? So white on red off and vice versa.


Martin
 

bigkim100

Apr 17, 2013
258
Joined
Apr 17, 2013
Messages
258
You should put a series resistor for each LED to avoid current hogging and a chain reaction of LED failures.
The red LEDs are approximately 1.8V to 2V. Do some tests for required brightness starting with bright at 20mA and dimming to a few mA.
What 6V supply are you using?.
Are red and white LEDs switched? So white on red off and vice versa.


Martin
The 6vdc is a high amperage battery, and the lights are switched using a simple logic relay, switched on and off by a circuit
Thank you for your reply
Kim
 

Audioguru

Sep 24, 2016
3,650
Joined
Sep 24, 2016
Messages
3,650
AliExpress, Amazon and ebay all sell cheeep junk that have no detailed specifications. Probably manufacturer's rejects.
Nane-Brand parts sold by a real electronic parts distributor are guaranteed to meet their specs.
 

bigkim100

Apr 17, 2013
258
Joined
Apr 17, 2013
Messages
258
For a Non-Professional guy, just screwing around, The LEDS there are fine for my purpose, and the prices are in Canadian with free shipping
 

Arouse1973

Adam
Dec 18, 2013
5,178
Joined
Dec 18, 2013
Messages
5,178
Give it a go but make sure you connect the current limiting resistor to the anode :)
 

Harald Kapp

Moderator
Moderator
Nov 17, 2011
12,756
Joined
Nov 17, 2011
Messages
12,756
Give it a go but make sure you connect the current limiting resistor to the anode
Why?
It absolutely doesn't matter whether the resistor is connected to anode or cathode. At least functionally.
Things become a bit more interesting when you look at EMC, specially at ESD-proofing your circuit. Then the resistor is best placed at the side where the driving transistor sits to reduce ESD influence on the transistor. But in that case there are again two possibilities:
  • common anode
  • common cathode
Like so:
upload_2022-5-20_6-32-43.png
When you use multiplexed LEDs where a driving transistor sits on both sides of the LED, it becomes even more complex. In that case it is often helpful (although from a purely technical point of view not necessary) to split the resistor like so:
upload_2022-5-20_6-35-51.png
Again note: This is sometimes used to improve resilience of a circuit against ESD. This is not the standard application a hobbyist will use.
 

Arouse1973

Adam
Dec 18, 2013
5,178
Joined
Dec 18, 2013
Messages
5,178
It's more practical than that. If you are making a voltage measurement with a scope it can be quite easy to slip and possibly short out the cathode of the LED or power supply. Thanks Adam
 

Harald Kapp

Moderator
Moderator
Nov 17, 2011
12,756
Joined
Nov 17, 2011
Messages
12,756
@Arouse1973 : Adam, your statement is nevertheless too general: What if the LED is driven by a PNP transistor you would consequently connect the resistor to the cathode, wouldn't you?

it can be quite easy to slip and possibly short out the cathode of the LED or power supply.
Assuming you have an NPN driver transistor (or N-MOSFET), that would resemble the configuration of Q1, D1 in my first schematic. With the anode of D1 connected to Vcc (resisor to cathode as shown), what would happen when you short the cathode of D1 to Vcc? Not much, only the LED will be off and the current through R1 will increase slightly. No harm done.
What if you short the cathode to GND? Then of course the LED would be connected between VCC and GND and burn due to overcurrent.
Let's assume your configuration:
upload_2022-5-20_9-53-3.png
A short from cathode to GND (0V) will simply cause the LED to light up. Here I agree with you. But now a slip from the anode to Vcc will expose the LED to the full supply voltage (if Q1 is on). and the LED will burn, too. I admit this requires Q1 to be on (not required in the first scenario), but what would be the use of measuring across the LED with Q1 off? In that case you would know that the full supply voltage is across Q1's C-E pins and the voltage across the LED is 0 V.

To lessen the impact of clumsy fingers on the LED when doing these measurements, the split resistor technique again would come to the rescue as it limits the current in both cases. The LED will very likely survive the doubled current (since each resistor is 1/2 of the full resistance required) at least for the short time it takes you to notice your slip.

I'm afraid You have not convinced me.
 

Arouse1973

Adam
Dec 18, 2013
5,178
Joined
Dec 18, 2013
Messages
5,178
That's ok, wouldn't it be a dull world if everyone agreed with each other. :)
 

bigkim100

Apr 17, 2013
258
Joined
Apr 17, 2013
Messages
258
Why?
It absolutely doesn't matter whether the resistor is connected to anode or cathode. At least functionally.
Things become a bit more interesting when you look at EMC, specially at ESD-proofing your circuit. Then the resistor is best placed at the side where the driving transistor sits to reduce ESD influence on the transistor. But in that case there are again two possibilities:
  • common anode
  • common cathode
Like so:
View attachment 55205
When you use multiplexed LEDs where a driving transistor sits on both sides of the LED, it becomes even more complex. In that case it is often helpful (although from a purely technical point of view not necessary) to split the resistor like so:
View attachment 55206
Again note: This is sometimes used to improve resilience of a circuit against ESD. This is not the standard application a hobbyist will use.

THANK YOU HARALD!!!!...the second circuit is something that I have been looking for!!!
 
Top