leds Dimmer

[TeddyTyler]

the leds are suppose to be rated up to 12 volts, is this why the leds go dimmer when the motor goes faster?

[ToddO]

LED's (Light Emitting Diodes) will not tolerate being directly driven by a voltage source without some current limiting. You normally place a resistor in series with an LED to limit the current to the desired level. If you have an LED based product rated up to 12v, then the product likely already has currently limiting build in to it.

The battery voltage sags (due to internal cell resistance) as the current draw increases. Higher motor speed would mean more current and therefore some reduction in the battery voltage. If the driving voltage across a LED & resistor decreases, then the current through the LED will decrease. Deceasing the current will decrease the brightness.

Best Regards, ToddO

[The Other Dave]

There are many LEDs that have an integral current limiting
resistor in the same package. You can tie them directly
to source voltage, up to a point.

[TeddyTyler]

Originally posted by ToddO
LED's (Light Emitting Diodes) will not tolerate being directly driven by a voltage source without some current limiting. You normally place a resistor in series with an LED to limit the current to the desired level. If you have an LED based product rated up to 12v, then the product likely already has currently limiting build in to it.

The battery voltage sags (due to internal cell resistance) as the current draw increases. Higher motor speed would mean more current and therefore some reduction in the battery voltage. If the driving voltage across a LED & resistor decreases, then the current through the LED will decrease. Deceasing the current will decrease the brightness.

Best Regards, ToddO



Your answer sounds good also, if this is true I cant do any thing to fix it,right or wrong, unless by replaceing the leds with a lower voltage rating.

[KillerWatt]

the built-in resistor's ohm value is correct for a good, full 12 volts,DC.......you probably have a few volts less when motor is running...a better approach is to use a LED without any built-in device, then calculate what external resistor you need for the actual, motor running voltage availble.....you can then tollerate an overbriteness till motor is on, or switch the LED on after motor is on......the reduced value resistor should keep your LED at near max rated briteness with less dimming effect .........kw

[ToddO]

You will not be able to increase the LED brightness (with a given battery pack & motor setup), unless you have access to the current limiting resistor. If you can get to the resistor, then you can remove it and solder in a new value to increase the brightness at your lower voltage level.

Best Regards, ToddO

[TeddyTyler]

:p What I did find out by trial and error, was that a fresh charged battery pack when first turned on with the motor full throttle keeps the led bright for about 1 minute of running time and then it gets dimmer after about 2 minutes only at full throttle , however the led gets brighter when I lower the throttle. I could live with this set up for flying at dusk. [SIZE=3][COLOR=darkred]

[Mr.RC-CAM]

however the led gets brighter when I lower the throttle.Hmm, I would be interested in hearing the part number to your "12V LED." Frankly, your symptoms sound like it is being over-driven by excess current.

Generally, if an LED gets brighter as the voltage is reduced, part threatening thermal issues have occurred. Too much current is bad.

The battery voltage sags (due to internal cell resistance) as the current draw increases. Higher motor speed would mean more current and therefore some reduction in the battery voltage. This could be the explanation, but if true, reducing the throttle would be speeding up the model in this scenario. But, battery sag is certainly an issue, so full brightness will diminish after a few moments of high-speed flying.

What LED are you using?

[KillerWatt]

of course...when the heavy amp pullin motor speed is reduced, the battery pack's voltage goes back up to it's near no load voltage 'cause the LED's pull almost no current ( 0.02 to 0.04 amp)..untill the pack is run down to BEC cut-off...so full throttle causes low LED and off/low throttle causes hi LED briteness until pack is run dead ......again...use a different, bare LED ( 1.5 to 3.5 volts) without any built-in resistor, calculate resistor needed for particular motor's 3/4 throttle to full throttle voltage available, and your LED's briteness will be more uniform and regulated.................... kw

[Mr.RC-CAM]

Ohhh, I think I found my confusion. Not much info was given, so I assumed that the LED was across the motor's leads. This would allow motor speed to vary light intensity on a standard ESC. That is what I thought was going on in this case. My comments were based on that scenario.

Although it was not stated, I guess that the LED is actually going directly to the battery pack instead. THAT makes all the difference and neatly explains what is going on.

Perhaps TeddyTyler will chime in and fill in the blanks for me.

If constant LED brightness is needed, then use a standard LED with 150 ohm current limit resistor. Connect it to the ESC's BEC (5V) output. Current draw is minimal and it will not upset your R/C gear.

But, if the LED is going across the motor/ESC leads, then my prior comments are worth reading again.

[TeddyTyler]

the LED is actually going directly to the battery pack instead. THAT makes all the difference and neatly explains what is going on.

Perhaps TeddyTyler will chime in and fill in the blanks for me.

If constant LED brightness is needed, then use a standard LED with 150 ohm current limit resistor. Connect it to the ESC's BEC (5V) output. Current draw is minimal and it will not

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You are right and I didnt think of putting the led on the battery side, Thats the answer Thanks for the input. the leds are 12 volt hooked up across the[ motor leads .

[Gene Bond]

I've found that with the LED's I have, I can run 2 in series, with a 10 ohm resistor to the BEC connector. I have 3 pairs wired like this in the ming of my PFC. It works quite well for a dusk floating session...

Play around with the values, starting at about 330 ohms and reducing it, while monitoring current, since no 2 types of LED's drop the same voltage....

[TeddyTyler]

Originally posted by genebond
I've found that with the LED's I have, I can run 2 in series, with a 10 ohm resistor to the BEC connector. I have 3 pairs wired like this in the ming of my PFC. It works quite well for a dusk floating session...

Play around with the values, starting at about 330 ohms and reducing it, while monitoring current, since no 2 types of LED's drop the same voltage.... ====================In the use of leds is it a good practice to wire them up in series ? as per note above=============

[Mr.RC-CAM]

Is the use of leds is it a good practice to wire them up in series ? as per note aboveIt is a fine way to connect them. This is also very efficient (power-wise). Of course the source voltage must be greater than the combined LED forward voltages.

My math suggests that two standard LED's, when wired in series and connected to the 5V BEC output, would use a 75-100 ohm series resistor.

[Gene Bond]

Yes sir, you are correct.... following an overnight 'life test' at 65ma, I went for it, as they were about 2x as bright as I started, with 47 ohms / 40ma... They are bright enough to make me see spots if I look directly at them from 2' away...

They do a great job, I flew for 25 minutes at dusk tonight and the QC pack was still 7.6V when I threw it on the charger.... Gotta love the floaters...

[Gene Bond]

Photographic evidence