LEDs are dimming with throttle in micro-heli problem

[Danny Troy]

I posted this question in the Micro-heli forum, before I was advised we actually had a "night flying" forum, so I'm reposting here:

I added 4 LEDs on my Esky Lama. One through the bottom of the canopy in the front, one at the end of the tail boom, and two just behind the body. It looks great, especially in a darkened room. I tapped into the on-board 2-cell lipo pack.

Here's the problem: The LEDs glow nicely when the heli is sitting with no throttle, but as the throttle is increased the LEDs dim, and at full throttle the LEDs are totally off.

I was thinking about adding a second 2-cell pack just to power the LEDs. Probably something in the 350mah range, or less, but I'm worried about what happens to the pack if the LEDs are on too long and the pack goes below the recommended minimum....any ideas?

Thanks,
Danny

[Wayne V]

What size resistors are you using on the LEDs?, maybe they are too big and as the voltage drops from the motor being loaded it is not providing enough current or voltage to light them.

[Danny Troy]

I'm not using resistors, which I believe are not needed since the LEDs are connected in series. I could try a series/parallel arrangement, which would double the voltage to each LED. As it is, each LED is being powered by approximately 2 volts at full battery charge. Is this too low, even though I'm not using a resistor?

Danny

[UAVPilot]

Red LED's need about 2.3 volts. Green about 3.5 volts, blue and white about 4 volts. Usually depends on the Type of LED's.

I find it's always best to use Resistors on each LED, and tap the power from the BEC from the ESC. This will keep the voltage at a constant 5 volts. Usually best to run a separate BEC just for the LED's.

[Danny Troy]

Thanks for the heads up on the voltage required for the LEDs.

I'm using two red LEDs and two yellow, so it looks like using a series hookup isn't giving enough voltage. They look OK at the 8.4 volts, and dim as soon as the voltage drops. It looks like I'll have to re-think the setup. Since it's a micro heli I'm trying to keep the weight of the lighting system down to a minimum, so I'd like to avoid adding another BEC. Adding resistors would be much lighter, if I go with a parallel setup.

Danny

[Diggs]

http://led.linear1.org/led.wiz

Try that. Should show you how to wire them up and what resistors to use.

Diggs

[UAVPilot]

Thanks for the heads up on the voltage required for the LEDs.

I'm using two red LEDs and two yellow, so it looks like using a series hookup isn't giving enough voltage. They look OK at the 8.4 volts, and dim as soon as the voltage drops. It looks like I'll have to re-think the setup. Since it's a micro heli I'm trying to keep the weight of the lighting system down to a minimum, so I'd like to avoid adding another BEC. Adding resistors would be much lighter, if I go with a parallel setup.

DannyYou will still get a voltage drop with a parallel set up. It won't be as bad, but it's still not good for the LED's if the resistors are not rated for the maximum voltage.

Each LED should only use about 30mAh max for the best life span.
If you run the LED's directly off the battery, as soon as you hit the throttle the voltage will drop and so will the mAh that the LEDs use, down to like 5 or 10 mAh, then they won't be very bright.

If you run a resistor that lets them be bright when the motor is running, then you will be drawing to much current on the LED's when you shut the motor off, greatly reducing the life of the LED's, as short as a few hours, even minutes if your not careful.

I'm not sure what the Lama BEC is rated for,or familiar with the radio setup, but I would be willing to bet that it would handle 4 LED's ( about 120mAh) with little trouble. All you would need to do is run a Y harness from one of the servos or the wherever else you could tap into the RX power supply, and this will give you a constant 5 volt supply. Makes it much easier to figure your resistors needed.

Also, A separate BEC does not need be heavy. I commonly use cheap GWS speed controls (http://www.bphobbies.com/view.asp?id=V533528&pid=spc001) to power my LED setups. I tap into you Main battery plug on the existing speed control used for the motor , then I just cut the motor leads off the GWS speed control for the LED's, and just use the BEC power supply for the RX for the LED's instead. The BEC for even the small GWS 2 amp ESC has a 1 amp BEC on it, enough to power at least 30 LED's, and they only weight a couple grams after you cut off the motor wires,....... much lighter then an extra battery.

Here is a link (http://www.superbrightleds.com/leds.htm) to some LED's, click on the specs to find the forward rated voltage.
I believe red and yellow are both 2.5 volts max.

Below is a pic of some of my night flyers,..... lotsa LED's :)
About 16 per plane, and I wire the lights to run about 25mAh each.
I usually run off the power from the BEC for the RX, But the BEC is rated for 1.5 to 2 amps on the speed control and I can only run three servos. If I run 4 servos I have to run another BEC for the LED's


..

[UAVPilot]

below are some examples using 4 LED's at 2.5 volts/ 25 mAh each with the Link provided by Diggs in his post, I have used that same calculator before also.

Note the simple setup with 5 volts from a BEC.

Then note how the resistor requirements change as the voltage drops when going from 8.4 volts to 6.5 volts as the battery runs down.

....

[Wayne V]

To make it easy just add a 330 ohm resistor and hook them up in parallel and you should be fine, this is what i use on 7.4-11.1v and have never had an led burn out. I can see why your LEDs are going off, the leds you said are 2v and with 4 in series the voltage drop would be 8v, so soon as you drop below 8v the lights go out. One thing to remember is the voltage is the drop of the device, to hook up LEDs try using current not voltage. A 2v led will also work on 12v it will even work on 120v, you just need the right resistor.

[Danny Troy]

Thanks guys for the great advice. So I'll be making a trip to my local RS store for some resistors, in case I can't find what I need in my stock. I'm thinking initially I'll go Wayne's route with the connection directly to the battery, since I already added a 2-pin micro Deans connector for that purpose.

One nice thing about these LEDs is that you can salvage them from items you buy at the .99 store. I bought a load of flasher type devices for bicycle riding, and each contains about 6 or 8 LEDs. So I have plenty of spares in case they wind up blowing due to overvoltage.

Danny

[Wayne V]

Danny, not trying to be a PITA, but your last statement in the post above still leads me to believe you think overvoltage burns out LED's, it doesn't, it's the excess current that kills them, as i said above you could hook a LED up to 120vdc and still not burn it out if you use the right resistor.

[UAVPilot]

it's the excess current that kills them, as i said above you could hook a LED up to 120vdc and still not burn it out if you use the right resistor.If would have to be one BIG honking resistor though ;)

[Wayne V]

Why is that, your only drawing 20-30ma, as always the formula is I=V/R so if i wanted to run a LED on 120vdc with 20ma it would be V/I=R which is 120V/.02A=6000, so a 6Kohm resistor attached to a LED would work on 120VDC, it would also work on 120vac but you need to also add a diode so you only get a half wave. As for the physical size of the resistor you would need one that can handle 2.4watts. To have a perfect sized resistor you are also supposed to subtract the voltage drop across the LED, 2V for the red as mentioned in the post above.

This is why i say to use a 330 ohm resistor for 7.4-11.1v as at 7.4v the LED is seeing 16ma and at 11.1v the LED is seeing 27.5ma which is usually in the safe zone for most LEDs.

[UAVPilot]

Why is that, your only drawing 20-30ma, as always the formula is I=V/R so if i wanted to run a LED on 120vdc with 20ma it would be V/I=R which is 120V/.02A=6000, so a 6Kohm resistor attached to a LED would work on 120VDC, it would also work on 120vac but you need to also add a diode so you only get a half wave. As for the physical size of the resistor you would need one that can handle 2.4watts. To have a perfect sized resistor you are also supposed to subtract the voltage drop across the LED, 2V for the red as mentioned in the post above.

This is why i say to use a 330 ohm resistor for 7.4-11.1v as at 7.4v the LED is seeing 16ma and at 11.1v the LED is seeing 27.5ma which is usually in the safe zone for most LEDs.Gee, your smart,.... formulas and everything.....

Still, most of the 3 watt resistors I have are generally larger then the LED's themselves.

Thats all I meant.....................


A 3 watt resistor (or 2.4 , whatever) is going to be just a bit bigger and heavier the a 1/4 watt resistor that would normally be used for much lower voltages
Actually, I don't have any 3 watt resistors, biggest one I have is only 1 watt, still pretty big, I try not to use them on my planes.
I do have some 5 watt, but they are wire wound and ceramic coated, probably wouldn't work in this case.

But hey, whatever.

[Wayne V]

Well most of us don't have 120v on board our planes anyways, just trying to state a fact that the voltage is not what burns out LED's . Actually the 5watt would work, the LED's only care about the current ( proper ohm value ) not the power rating of the resistor.

Between the partying, drinking, and card games my 3 year college diploma in electronic engineering technician has come in handy once or twice.

[UAVPilot]

Well most of us don't have 120v on board our planes anyways, just trying to state a fact that the voltage is not what burns out LED's . Actually the 5watt would work, the LED's only care about the current ( proper ohm value ) not the power rating of the resistor.

Between the partying, drinking, and card games my 3 year college diploma in electronic engineering technician has come in handy once or twice.
OK........

Again I meant the five watt would not be needed in the case of building airplanes as well as the 1 watt or 3 watt resistor.

I think most that have bothered to read this thread understands what you meant about the voltage not being what burns out the LED's.

Pretty clear that just about no matter what resistor you put in the path of a given voltage, and the voltage will remain the same, it just resist the current draw, maybe that's why they call it a resistor?.

Congratulation on your partying, drinking, card playing and still getting that once or twice used diploma.

Me, I was simply trying to be slightly humorous in my post about using a much larger resistor for 120 volts, while dealing with model airplanes.

I'm sorry your 3 year degree didn't teach you my simple minded bad attempt at humor, my bad.

I'll resist commenting on your comments next time.

I get the point.

I will shut up.

no more out of me.

really.

:rolleyes: ;) :p :cool:

[Danny Troy]

Just an update on my progress. I wound up going the 330ohm resistor route (1/4 watt) and the attached photo shows the results....fantastic. I don't know how long they will last before they burn up, but I have plenty spares, so no worry. Sorry I couldn't get a shot of it while flying, but I would say there's no dimming at all while applying throttle.

Funny, I'm actually using a 12 volt CREE type (ultra powerful) LED headlight on my 48v electric bike, and I'm using a DC-DC converter that has a 24v output, which is what I'm powering the CREE headlamp with. I suppose it knocked down the amperage to a point that is allowing the 12v CREE LED to operate without burning out.....so I guess it's a combination of volts and amps for the proper operation of LEDs.

Anyway, thanks for all the advice and at .99 a pack of five resistors, it's definitely the cheapest and easiest way to go. I wound up buying three packs, so I have 11 resistors left to use on the next project :D

Danny

[Wayne V]

Sorry for not seeing the humor UAVpilot, i was just replying the facts about what you wrote, all in all everyting we posted must have helped cause danny has light now and throttle setting is not affecting the lights.