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Author Topic: The resistance equation  (Read 546 times)
Obese Wan Kenobese
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« on: April 13, 2018, 07:16:47 PM »

My search isn't revealing anything effective for planning resistors despite many saying it's easy. I don't believe I'm too stupid. Calculus was fun, but where's the straight forward equation for simple LED resistors?

And what's with buckpucks? Do I still need a resistor if the saber has a buckpuck?

Ok, so my hope is to make a tri cree 410nm violet with quick connects. I've ordered the saber to come with buckpuck, tri guardian blue and a tri consular green quick connects. I was hoping to make my own violet(not red/blue mix) to be compatible. But what resistor do I need?

And what if I make a tri cree with different colors? Can I wire them in series for quick connect if the saber has a buckpuck?
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The first step to gaining knowledge is recognizing your own ignorance.
Serenity is patient, not passionate satisfaction.
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DavidYaw
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« Reply #1 on: April 15, 2018, 01:46:49 AM »

Where's the straight forward equation for simple LED resistors?


It is a simple equation, just applied in a way you're not used to.

V=IR. Voltage = Current * Resistance.

So what's the voltage? It's not the battery voltage. In fact, it's not a constant number at all. The voltage you use here is whatever's left after the voltage drop across the LED. So what's the voltage drop? That's why it's not a constant: The voltage drop depends on the current that's flowing through the LED. If you read the datasheet for the particular LED you're using, you'll find a graph of current vs. voltage. In this datasheet (http://www.cree.com/led-components/media/documents/XLampXPE.pdf), it's on pages 16 and 17 for the various colors.

So, you pick a current and voltage from that graph, subtract the LED's voltage drop from your battery voltage (7.2v for 2 Li-Ions, 6v for AAAs), and plug those values into V=IR, and there you go, you have the resistance you need.

And what's with buckpucks? Do I still need a resistor if the saber has a buckpuck?


Most power sources we deal with every day are what's known as constant voltage power sources. Whether they're supplying a lot of power or a little, the voltage is the same: Whether the AAA batteries are driving your saber for 2 hours, or your TV remote for 6 months, they're still 1.5v each.

What a buckpuck does is it creates a constant current power source. Regardless of how many volts it needs, it will always supply the same amount of amps. In the case of the ones that UltraSabers uses, 1000 milliamps. If you think about that graph of voltage vs. current, you can think about the buckpuck as constantly measuring the current, and adjusting the voltage so that the current is 1000 milliamps.

A resistor and a buckpuck accomplish the same job: Regulate the amount of current flowing into the LED. You don't need both.
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Obese Wan Kenobese
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« Reply #2 on: April 15, 2018, 05:54:24 AM »

Thank Yaw very much!!!!  Cool  Wink  Cheesy
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Selfish passion is unquenching.
What strength have you when you are a slave to your passions?
What power without strength of character?
To self centered rage, you will be chained.
The dark side is no victory.

The long path to peace is through balanced emotions.
The first step to gaining knowledge is recognizing your own ignorance.
Serenity is patient, not passionate satisfaction.
Find harmony by understanding chaos.
Become one with the force, which will never die.

Obese Wan Kenobese
Knight Commander
*

Force Alignment: 312
Posts: 1149


Fretful Instigator of the Prismatic Order


« Reply #3 on: April 21, 2018, 04:44:50 PM »

So, I'm still lost. Sorry.  Roll Eyes If I'm working on an LED that is designed for 800 ma, not 1000ma, does that need a resistor when I link it to the saber that has a buckpuck for 1000ma?  Cry If the buckpuck supplies 1000ma, I would assume the need to tone it down to 800 before reaching the LED. So which resistor do I need to build in to my LED set up to attach to the existing quickconnect Ultrasaber with buckpuck?

edit: I see the data sheet says red and amber are 500 and 700 ma forward current. I suppose the 800 ma LED I'm to be wiring should be as safe as red or amber and I should be safe without both puck and resistor, as you said.
Logged

Selfish passion is unquenching.
What strength have you when you are a slave to your passions?
What power without strength of character?
To self centered rage, you will be chained.
The dark side is no victory.

The long path to peace is through balanced emotions.
The first step to gaining knowledge is recognizing your own ignorance.
Serenity is patient, not passionate satisfaction.
Find harmony by understanding chaos.
Become one with the force, which will never die.

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