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HelicopterOff Topics › Help ... Computing current limiting resistor for LED
08-19-2014 05:24 AM  3 years agoPost 21
Girard Ibanez

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AirWolfRC,
Now you have 13 sub-strings (180 ohm resistors) on the running light circuit.

And you have 12 sub-strings (180 ohm resistors) on the brake light circuit.
There are 13 sub-strings on the running light circuit.

There are 25 sub-strings on the brake light circuit.

I look this over and I don't think I can do it with only 2 diodes.

The brake circuit lights all the sub-strings whereas the running light circuit lights the both outside and middle section.


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08-19-2014 06:03 AM  3 years agoPost 22
Girard Ibanez

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AirWolfRC,

I see what your saying about reducing the diodes.

Thanks for that tip.


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08-19-2014 09:31 AM  3 years agoPost 23
Climax

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You could certainly reduce the number of diodes!

Sometimes doing the calculations is fun... Although it's often OK to make an educated guess as in many applications it's not that critical.

The relationship between current and intensity is by no means linear. In fact for many modern devices it can be difficult to notice much difference in brightness between low and high currents.

You do need to be slightly careful of overdriving LEDs, as rather than getting noticeably brighter they just get hotter and have shortened lives. I guess what I'm saying here is that you might end up with a resistor that seems to works as the LED is producing the intensity you want, however it's being overdriven and could in fact be as bright with a lower current. So try and stick to the manufacturers suggested forward current and sanity check this if you do estimate/guess your resistor values.

It's for these reasons that many designs that need to control LED brightness use PWM modulation. i.e. They are always switched on at their optimal brightness/forward current, but when dim they are only on for a small fraction of the cycle etc. Many chip manufacturers now make easy to use PWM LED controllers, as used in domestic lighting, TVs etc. etc.

I'm not suggesting this here, simple resistors are fine!

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08-19-2014 02:04 PM  3 years agoPost 24
Girard Ibanez

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This is GREAT!

I enjoy seeing the idea from a different perspective and improving on the design.

The LEDs in this design are running at the manufacturer's recommended current.

I also thought about using switching power supply to cut down on heat (loss of energy) but this involves more circuitry. But again, I just wanted something simple.

Thanks ALL for your thoughts and assistance.


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08-19-2014 02:19 PM  3 years agoPost 25
Girard Ibanez

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I was searching for a part number for a 1 amp schottky diode and was wondering if the voltage rating mattered. Hence this 12 volt application and using a 30 volt 1 amp schottky diode.


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08-19-2014 03:25 PM  3 years agoPost 26
AirWolfRC

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About the lowest voltage rating you will find on a schottky is 30 volts . . . so anything with the current rating will do.

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08-19-2014 05:33 PM  3 years agoPost 27
Girard Ibanez

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In this design and using a schottky diode, would there be any reason to still account for the voltage drop of the LED arrays affective by the blocking diodes?

Provided I decide to use a resitor value with a 10% tolerance. Or should I go with a tighter tolerance.

Just looking from an engineering perspective vs a DIY.


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08-19-2014 05:43 PM  3 years agoPost 28
AirWolfRC

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One engineering rule is "don't go precision unless it's needed"
The schottkys will give about 0.5 to 0.7 volt drop compared to 1.5 volt for a silicon power diode.
The production variation on LEDs is another variable.
The design placement of the LEDs is yet another.

Use the design and values I gave earlier.
Build a prototype and tweak it.
If anything, you may have to adjust the value of one resistor.

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08-19-2014 06:58 PM  3 years agoPost 29
Girard Ibanez

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Would using SMT resistor of 1/4 watt handle the wattage as regular resistors given their physical size in comparison to the typical resistor?

Looking at making a cleaner wiring job (asthetics). Soldering the SMT resistor directly on to one leg of the LED.

On another note, why do some design have the resistor on the negative side of the LED and others on the positive side of the LED. From what I understand it how we interprete current flowing from the negative to the positive.


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08-19-2014 07:38 PM  3 years agoPost 30
AirWolfRC

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SMTs assume production assembly equipment.
SMTs use the circuit board to help with power dissipation.
For short runs, you could glue or tape the SMTs in place and hand solder.
You could also use SMT LEDs.

For short runs, through hole mounts work better for assembly.
On another note, why do some design have the resistor on the negative side of the LED and others on the positive side of the LED.
It doesn't matter.

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08-19-2014 09:47 PM  3 years agoPost 31
Girard Ibanez

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I am planning on soldering the SMT resistor directly to the LEDs. Using an aligator clip to hold the SMT to the LED leg then solder the Vs to the SMT resistor.

The LEDs are pressed into a piece of sheet metal. I am concern about the heat disappation since the SMT resistor is not mounted on a circuit board which also acts as a heat sink.

Or should I just use the conventional resistor.


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08-19-2014 09:58 PM  3 years agoPost 32
AirWolfRC

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Conventional resistors will be a LOT easier to work with.

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08-19-2014 10:00 PM  3 years agoPost 33
Climax

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I agree, sounds a bit fiddly!

Also, if you have any vibration the metallization on the parts can fail when you use them like this, it's only a thin layer. I've had this happen with SMD capacitors when using them for makeshift decoupling.

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08-20-2014 09:55 PM  3 years agoPost 34
jcrack_corn

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you will probably overcook at least a couple of smt resistors also or form soldier bridges....def not worth the headache, especially in what looks to be a fairly large part/device with plenty of room to plan for component placement.

are you going to encase in epoxy or some other resin if this is truly an automotive application (plan for reduced heat dissipation if so)? vibration and moisture are tough

------------------------------------------------------------------
do it inverted
------------

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08-21-2014 02:41 AM  3 years agoPost 35
Girard Ibanez

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you will probably overcook at least a couple of smt resistors also or form soldier bridges....
Only if soldering is NOT your line of work. Oops I forgot I'm a hobbyist.


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HelicopterOff Topics › Help ... Computing current limiting resistor for LED
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