resistors

[chrisatpsu]

for protyping purposes...

i think i've read somewhere that when you dont have the exact value resistor that you're looking for, you can use multiple resistors to get a desired value.

for example, to get 2k ohm resistance, you can connect a 1k resistor with a 1k resistor in series (end to end) to get 2k ohms

and to get 500 ohm resistance, you can connect 2 of the 1k resistors in parallel (both ends are tied together side by side) to get 500 ohms

does this sound right?

[wwwgator]

You are correct.  Resistance is addative when in series,  capacitance (capcitors) are also addidtive when in parralle.

great explanations here

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(Hard to show the folmulas here.....)

[tbone321]

for protyping purposes...

i think i've read somewhere that when you dont have the exact value resistor that you're looking for, you can use multiple resistors to get a desired value.

for example, to get 2k ohm resistance, you can connect a 1k resistor with a 1k resistor in series (end to end) to get 2k ohms

and to get 500 ohm resistance, you can connect 2 of the 1k resistors in parallel (both ends are tied together side by side) to get 500 ohms

does this sound right?

This is correct but unless you need a highly accurate resistance or are moving a lot of current, you can also pick up a few pots (potentiometers) to replace your resistors during prototyping.

[chrisatpsu]

a pot would be a good idea, except i'd be afraid of bumping it all the time.  i'd need fairly acurate resistance when dealing with leds, right?

[tbone321]

There are manydifferent types and some are easier to adjust than others.  You do have some room to play with the resistance as long as the current doesn't exceed what the LED can handle.  The great thing about using pots is that they can allow you to easily adjust the resistance during prototyping and with LED's you can set the brightness that you want by eye and when you get it where you want it, just measure the resitance that you will need from the pot.

[Steve Gase]

so...   i understand that these are the normal levels of resistance used for different 12v led colors.

470 ohms  -  Blue, Green, White, U.V. (Blacklight) & Pink
560 ohms  -  Red, Yellow & Orange

i am interpreting that the different resistors allow them to be brought to the same general intensity level, and compensate for the color differences.  right?

[NavyGator]

You could use a resistance bridge which would give you the ability to adjust the resistance similar to a pot. Or a resistance ladder which would allow you to effectively choose the resistance desired. Both of these can be used in prototyping but are a bit overkill in my mind.

With LED's if you are building a circuit you can order most any resistor or combination of resistors to get the resistance you want. Getting the exact resistance desired is not critical as it is more important to ensure the resistor drops the required voltage or more to limit the current, so the LED is not damaged. If your calculations say you need a 87ohm resistor you will be fine using a 100ohm one instead.

Although a relative term, highly accurate resistors for LED's are not needed when it comes to choosing resistors for our purposes. Using my mechanical engineering background when talking about a highly accurate resistor I think of a strain gauge (really long resistor) which is used in a Wheatstone bridge to determine the amount of strain an object has (how much it has lengthened, compressed or bent).

Here is a Wikipedia link to strain gauge if you are interested.

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Rich

[rimist]

You are correct.  Resistance is addative when in series,  capacitance (capcitors) are also addidtive when in parralle.

great explanations here

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(Hard to show the folmulas here.....)

The section on Inductors is a bit difficult to digest. Is it really all that important to the blinky flashy servo world?

-Rimist (via Tapatalk)

[Dennis Cherry]

so...   i understand that these are the normal levels of resistance used for different 12v led colors.

470 ohms  -  Blue, Green, White, U.V. (Blacklight) & Pink
560 ohms  -  Red, Yellow & Orange

i am interpreting that the different resistors allow them to be brought to the same general intensity level, and compensate for the color differences.  right?

I think you need to read this about color codes on resistors.

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What you show is a 65 Billion Ohm resistor for the 470 ohm (yellow, Purple, Brown) and a 24,000 Ohm for the 560 ohm (Green, Blue Brown).

[rimist]

I believe the colors were leds to be used with that particular resistance, not the color bands of the resistors.

-Rimist (via Tapatalk)

[Steve Gase]

I believe the colors were leds to be used with that particular resistance, not the color bands of the resistors.

-Rimist (via Tapatalk)

I was talking about the colors of LED bulbs, not the bands.  I've heard that a red bulb uses a different resistor than a green bulb. 


Long day and my head aches...   Is seem the best summary I've heard is

Resistors are futile.

I don't hang out with EEs very often, so that may be an old joke.

[chrisatpsu]

I was talking about the colors of LED bulbs, not the bands.  I've heard that a red bulb uses a different resistor than a green bulb. 

the reason behind that is the material used to make the led.
the different types of materials will emit a different color of light.
it's the different materials that is the reason why you use a different resistor for each light...

reference
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[rm357]

The different color LEDs have different operating voltages. If i remember correctly, a red led will come on around 1.2v, but most white LEDs require more than 2.2v.

With the RGB LEDs, as the voltage drops, the output will get redder...

RM

[NavyGator]

I might have misunderstood what you were trying to say here......

With the RGB LEDs, as the voltage drops, the output will get redder...

Not necessarily, the only way that would happen is if all three legs of the RGB LED were connected across the same potential difference. There are also other concerns but having it hooked up like that would not allow you to control the colors independently. This would then negate to purpose of having an RGB LED by not allowing you to control the color.

Rich

[rm357]

If you take a smart string node, set it to white, then start decreasing the supply voltage, it will start getting a red tint to it as the voltage decreases. At some point, the g and b will drop out completely and the red will slowly fade away.

Because the LEDs need different voltages, they need different resistors to help set the current to appropriate values to get white light at a given supply voltage. Resistors are linear, but the different voltage requirement for each color led makes the whole thing non-linear. Since the curves don't match, you get slight changes in the color until you start seeing the dropout...

I watch this every time I turn off the power to my cheaper pc power supply.

This is another reason that PWM is better for RGB led dimming. 50% on 50% off = half bright, no matter how you slice it...

RM