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LED circuit question

post #1 of 11
Thread Starter 
I'm going to be replacing the blue leds on my Rheobus Extreme with some diffused green ones, that have a 1.8-2.0 forward voltage. I just finished measuring the leads of the blue leds, and each one showed ~3 volts, at full power.

Is that a valid measure, I mean, do I need to test the power getting to the LED, not what they are using?

In the LED calculator I tried both 6 & 3 volt source, and really am not sure which it is. I'm hoping it's 6, cuz I have those resistors on hand already. yuk yuk yuk.

ledmodvolts.jpg


Thanks,
post #2 of 11
Quote:
Originally Posted by cyclometric;13664522 
I'm going to be replacing the blue leds on my Rheobus Extreme with some diffused green ones, that have a 1.8-2.0 forward voltage. I just finished measuring the leads of the blue leds, and each one showed ~3 volts, at full power.

Is that a valid measure, I mean, do I need to test the power getting to the LED, not what they are using?

In the LED calculator I tried both 6 & 3 volt source, and really am not sure which it is. I'm hoping it's 6, cuz I have those resistors on hand already. yuk yuk yuk.

ledmodvolts.jpg


Thanks,

It would be easier to get new leds as a lot of them run on 3v now also I would replace 1 blue led with one of your leds and 1 resistor for 3v
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post #3 of 11
Thread Starter 
Quote:
Originally Posted by liamstears;13666308 
It would be easier to get new leds as a lot of them run on 3v now also I would replace 1 blue led with one of your leds and 1 resistor for 3v

Well, green leds tend to run at lower volts than blue, and also the blue that come stock are super bright, and I'm putting in diffused green ones.

But if I understand the last part of what you say, I can't think of replacing 1 led at a time, because there are two leds in each little circuit, each dropping 3V. I think you're confirming what I want to do, that is, replace the blues with greens and put a 100 ohm resistor inline.
post #4 of 11
Quote:
Originally Posted by cyclometric;13666964 
Well, green leds tend to run at lower volts than blue, and also the blue that come stock are super bright, and I'm putting in diffused green ones.

But if I understand the last part of what you say, I can't think of replacing 1 led at a time, because there are two leds in each little circuit, each dropping 3V. I think you're confirming what I want to do, that is, replace the blues with greens and put a 100 ohm resistor inline.

in normal operation you would use 1 resistor with 1 led but it all depends on how the leds currently work in the Rheobus

if each led in the circuit is seperately ran from its own 3v then do it like I said but if the two leds are connected together in series to run on 6v you would have to do the same, just check the circuit and see if the leds are connected together and that will tell you how to do it :-)
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post #5 of 11
Thread Starter 
Quote:
Originally Posted by liamstears;13667342 
in normal operation you would use 1 resistor with 1 led but it all depends on how the leds currently work in the Rheobus

if each led in the circuit is seperately ran from its own 3v then do it like I said but if the two leds are connected together in series to run on 6v you would have to do the same, just check the circuit and see if the leds are connected together and that will tell you how to do it :-)

The part I was and still am unsure about is if I add the volts of the two leds together, to figure out the "source voltage" of this mini-circuit with 2 leds, or if both showing 3 meant that was the source voltage. That's how I figured on 6 volts, 3+3, but I wasn't sure if it is a steady source ... but in my foggy image of how leds function, each one "eats up" 3 volts, so it'd have to be 6 total, yes?
post #6 of 11
Quote:
Originally Posted by cyclometric;13667838 
The part I was and still am unsure about is if I add the volts of the two leds together, to figure out the "source voltage" of this mini-circuit with 2 leds, or if both showing 3 meant that was the source voltage. That's how I figured on 6 volts, 3+3, but I wasn't sure if it is a steady source ... but in my foggy image of how leds function, each one "eats up" 3 volts, so it'd have to be 6 total, yes?

I dont understand that lol

Can you post a hi res pic of both sides of the pcb around the leds?
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post #7 of 11
Thread Starter 
Quote:
Originally Posted by liamstears;13668261 
I dont understand that lol

Can you post a hi res pic of both sides of the pcb around the leds?

Here's one of the front, showing a couple sets of them. I took some of the back, but there aren't any lines between the leds on that side, it seems on that side, that's where the connections are drawn for the pots.

ldesvl.jpg

ledsmore.jpg
post #8 of 11
Quote:
Originally Posted by cyclometric;13669073 
Here's one of the front, showing a couple sets of them. I took some of the back, but there aren't any lines between the leds on that side, it seems on that side, that's where the connections are drawn for the pots.

Its hard to see what going on there but by the look of things the positive side of each led is directly connected to eachother marked on the image below as a red line

If that is right then you will need to replace each led with a green led and a resistor

To get the right resistor you need to measure voltage across the yellow and red lines I have marked and if thats 3v use 1 led and 1 resistor for 3v per led you want to replace

ldesvl.jpg
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post #9 of 11
Thread Starter 
Quote:
Originally Posted by liamstears;13669158 
Its hard to see what going on there but by the look of things the positive side of each led is directly connected to eachother marked on the image below as a red line

If that is right then you will need to replace each led with a green led and a resistor

To get the right resistor you need to measure voltage across the yellow and red lines I have marked and if thats 3v use 1 led and 1 resistor for 3v per led you want to replace

So that confusion might be my fault -- because I drew the poles connecting to each other that way in my first post, but in actuality, the circuit is regular, i.e.

source v --> led1 anode ---> led1 cathode ---> led2 anode ---> led2 cathode. ---> ground
post #10 of 11
Not sure if you still need help with this but judging from the pcb layout for the led's it looks like you have two leds in a series circuit with one resistor, also the input voltage from the power supply should be 12v. Assuming this is correct for the leds you want to install with a voltage drop of 2v and a current rating of 20mA a 430 ohm resistor would work, or if the led specs a 30mA rating a 270 ohm will work. Hope this helps!
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