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Author Topic: Another led wiring question.  (Read 718 times)

Peter34

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Another led wiring question.
« on: May 27, 2019, 07:13:38 PM »

I want to use 6 Leds on boat, that I want to be able to Dim.
I have 2 blue, 2 white and 2 green.
Currently I have the wired In series one after the other running from an old brushed esc. Whilst this seems to be working perfectly fine, in that I can control the brightness of the Leds from a rotary switch on my handset Im not sure If its the correct way.
After reading lots of threads on here about Leds it seems I should be using some resistors somewhere.
Is it OK to leave as is or should I change it.
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malcolmfrary

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Re: Another led wiring question.
« Reply #1 on: May 27, 2019, 07:33:35 PM »

I want to use 6 Leds on boat, that I want to be able to Dim.
I have 2 blue, 2 white and 2 green.
Currently I have the wired In series one after the other running from an old brushed esc. Whilst this seems to be working perfectly fine, in that I can control the brightness of the Leds from a rotary switch on my handset Im not sure If its the correct way.
After reading lots of threads on here about Leds it seems I should be using some resistors somewhere.
Is it OK to leave as is or should I change it.
Yes, you should.  If it is an old PWM ESC, it will be offering full voltage all of the time that the pulse is "on".  If this results in the forward voltage at any LED being exceeded, its life will be significantly shortened as the brightness is increased.  The LEDs will not have the same cooling down/recovery time and will eventually suffer.
If you check the specifications for your LEDs you will notice that they all have different current requirements and different forward voltages.  Using a varying pulse from an ESC is basically getting away with it using pure luck.
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roadrunner440

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Re: Another led wiring question.
« Reply #2 on: May 27, 2019, 07:41:21 PM »

i would do resisters..or they might not last long..each color has a current limit like malcolmfrary said ..say like one of my blue smt led's is like 2.3 volts at 25 mv..so if im running 7.4v sticks I needed a 220 ohm resister.personaly I put them on the b+ leg of the led..once you have put in resister's you could get a potentiometer. run via servo to dim/increase brightness....here is the calculater I use.http://led.linear1.org/led.wiz/
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JimG

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Re: Another led wiring question.
« Reply #3 on: May 27, 2019, 08:12:20 PM »

You have 6 LEDs in series, if each is say on average 2.5V this will need a 15V supply. If the esc is putting out 12V then no need for resisters, LEDs do not suddenly take too high a current if they are supplied with the correct voltage. Resisters are needed when the voltage supplied is higher than the voltage for the LED and is used to drop the voltage over the LED to the needed value thus limiting the current. I have used 12V LEDs which use sets of 4 in series, each taking 3V with no resisters in the circuit. One model has one of these cut up to give individual micro LEDs and they are connected in parallel anjd run off of a 2 pencell NiMh, no resisters, none overbright , none burning out.
Jim
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malcolmfrary

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Re: Another led wiring question.
« Reply #4 on: May 28, 2019, 08:40:08 AM »

12 volt LEDs usually have a built in current limiter, even if it can't be easily seen. 
A lot of simplified circuits use the internal resistance of the supply battery and/or the wiring resisance as their current limiter.  Replacing such a battery has a good chance of the user saying "My goodness, thats brighter than it was before" before needing to figure out the best way to fit the replacement LED.  Building your own circuit to be reliable does need to have a means of limiting the current through the LED.  Without that, you are just running on hope and good luck.
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Ken G121

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Re: Another led wiring question.
« Reply #5 on: May 28, 2019, 10:15:00 AM »

Use a switcher
First selection gives (through resistor) current flow of say 10 ma
Next switcher selection gives through resistor of smaller value 20 to 25 ma.
You would need use the type of switcher that removes one supply as it adds the next
I would have thought that if you wire the leds in parallel you could put individual resistors on the negative side of each led to give correct current for each type.
Should also be possible to use a sequential switcher that adds circuits and does not remove any. In this 2 identical circuits should be able to be used if one circuit provided say 10 ma adding the next circuit should provide another 10ma giving 20 total
I think the secret is to use the empirical method ( posh name for trial and error). Sit in workshop with a selection of resistors (20 to 300 ohm) ( if you want 170 you can use 150 and 20 in series. Sit with a meter set to ma and play. Alternatively through the switcher provide a different voltage supply. Personally I have used usb power supplies ( eBay less than 2). With meter on volts find 5v + output ( negative is usb outer case) on back of board solder wires to case lugs ( usually 4 attachment points) and positive. 5 volts is OK for leds ( with the correct resistor) and the beauty is you get 5 volts  with a 6, 7.2, 7.4, etc so if you change your battery say 2s to 3s it will not affect your lighting ( same applies when wiring up things like radar motors)
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andyquirot

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Re: Another led wiring question.
« Reply #6 on: May 28, 2019, 06:52:26 PM »

Hi there
Just a quick note you need to look up what the voltage is for each colour led as different colour's will require different resistors values .
Regards Andy Q
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Peter34

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Re: Another led wiring question.
« Reply #7 on: May 28, 2019, 08:41:42 PM »

Thanks for the replys. Another question if I may. I cant remember the specs of the Leds I have as I brought them ages ago, I know I could buy new ones but bare with me.
If for example I take one of the blue ones and put for example 5v in to the +leg then I attach my multimeter to the other leg and the ground from the battery the meter displays 2.5v am I correct in that this is the forward voltage of the led.
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Ken G121

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Re: Another led wiring question.
« Reply #8 on: May 28, 2019, 09:42:00 PM »

No forward voltage is liable to be about 3 . If you connect to 7.2 v battery 7.2-3 = 4.2 (call it 4). Current (I)= v/r or r=V/I
 so for 20ma ( o. 02 a), 4/0.02= 200ohms
So put resistor in circuit ( say in negative line) connect positive side of led to battery positive, connect negative side of led (after resistor) to digital meter (set to ma ) connect other lead on meter to battery negative. Led will light and meter will read current in ma. You want 20 to 30 milli amps. Adjust value of resistor to get what you want 20 ma is almost as bright as 30 but the Led will last longer.
Current is the only thing that matters, if you want to use the same led on s 24v supply, 24-3= 21v, 21/ 0.02= 1050 ohms. You are looking to supply each led with a current of about 20 ma (30 max) the higher the voltage the bigger the resistance value to control the current to the desired value.
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Fred Ellis

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Re: Another led wiring question.
« Reply #9 on: May 29, 2019, 08:42:38 AM »

Hi
Sorry to break in on this, but being the dimwit that I am I see that some say put the resistor in the negative side , so is this only when you run the LED's in series? (never could understand parallel and series even at school)
If you are only running one LED would you have to put it in the positive side?


Again sorry for putting in on this


Fred
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malcolmfrary

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Re: Another led wiring question.
« Reply #10 on: May 29, 2019, 09:51:28 AM »

Hi
Sorry to break in on this, but being the dimwit that I am I see that some say put the resistor in the negative side , so is this only when you run the LED's in series? (never could understand parallel and series even at school)
If you are only running one LED would you have to put it in the positive side?


Again sorry for putting in on this


Fred
It does not matter which, as long as it is there.  If you felt the urge, you could have one each side, as long as the values add up to the needed value. The current starts from the positive pole of the battery, goes through the resistor, then the LED, whatever is doing the switching, and completes by getting to the neg pole.  The resistor, LED and switching device can be in any order given that the switching device might need to be at one end or the other of the circuit.
Series means you go through each component in the chain (or series) one after the other.  Parallel means that you go through all of them at the same time.


Quote
If for example I take one of the blue ones and put for example 5v in to the +leg then I attach my multimeter to the other leg and the ground from the battery the meter displays 2.5v am I correct in that this is the forward voltage of the led.
No. 
From the description, your meter is acting as the resistor, and is displaying the voltage across itself at a very low current because voltage meters are generally very high resistance.  Because 2.5 volts is coincidentally half of 5 volts, the voltage shown happens to be, at that very low current, the same as the volts across the LED.  The way to test is to hook up the LED and the resistor, check that the LED is glowing as expected, then connect the voltmeter ACROSS the LED to read the actual voltage.
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andyquirot

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Re: Another led wiring question.
« Reply #11 on: May 29, 2019, 07:10:09 PM »

Hi there
try using this link it is the easy way just enter the values.


http://led.linear1.org/1led.wiz


Hope this helps.
Regards Andy Quirot
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Peter34

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Re: Another led wiring question.
« Reply #12 on: May 29, 2019, 08:16:34 PM »

Thanks for link. I now know what resistors I need..
If I split the Leds into pairs  2 of each colour and add the correct resistor is it then OK to run them off of the one brushed esc?


If not what do you recommend for controlling the Leds from my handset on dial to control brightness.

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Peter34

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Re: Another led wiring question.
« Reply #13 on: May 29, 2019, 09:15:41 PM »

Sorry for the double post but ran out of edit time.
Thought I'd mention I connected the multimeter to the led and a random resistor I had lieing about to a 3 cell lipo. The meter read 7.8. Which I guess is 7.8ma?. Without the resistor the meter read 19.5.
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malcolmfrary

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Re: Another led wiring question.
« Reply #14 on: May 30, 2019, 09:59:39 AM »

Sorry for the double post but ran out of edit time.
Thought I'd mention I connected the multimeter to the led and a random resistor I had lieing about to a 3 cell lipo. The meter read 7.8. Which I guess is 7.8ma?. Without the resistor the meter read 19.5.
Sorry, that doesnt tell anything useful. 
Was the meter on volts or amps?  Was it in the one after another chain (series as it should have been for measuring current) or was it connected across one of the components (looking for the volts across that component)?  Is it a switched range multimeter or a self-ranging one?  Information needed to make sense of the numbers read.


This ESC - is it forward only or does it do reverse?  LEDs have a very limited tolerance to revered voltage.


Quote
If I split the Leds into pairs  2 of each colour and add the correct resistor is it then OK to run them off of the one brushed esc?
Yes.
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Peter34

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Re: Another led wiring question.
« Reply #15 on: May 30, 2019, 10:13:44 AM »

I have the following meter. rapid test mas830l.
I had the led attached to plus on battery, resistor on minus leg of led. Red lead of meter attached to resistor and black lead to minus of battery.


Esc is set to forwards only.

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malcolmfrary

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Re: Another led wiring question.
« Reply #16 on: May 31, 2019, 09:35:02 AM »

That, if the meter is set to volts, measures the voltage across the resistor.
A good tutorial here - https://www.youtube.com/watch?v=bF3OyQ3HwfU - well worth looking at.
A useful read here - http://www.aeroelectric.com/Reference_Docs/Books/Practical_Electronics_Handbook_6th_ed.pdf  - the interesting bit is around page 116.
To measure the current, the meter should be connected between the battery negative and the resistor where the resistor used to connect to the battery and be set to measure current.  That lets you know the mA flowing through the circuit.
To know the voltage across the LED the meter should be set to volts and connected across the legs of the LED.
Trying to get a measurement while dimming using an ESC won't work with a digital meter - due to the way they work, they cannot get an accurate reading off a pulsed supply because it keeps changing.
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