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Author Topic: 4.7microFarad Capacitor - when to use, how to calculate size needed?  (Read 2636 times)
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I'm attempting to control a RGB led by using a programmable remote control and a 38kHz IR Receiver.  The starting point to my project is http://arduino-cool.blogspot.com.es/2012/09/arduino-rgb-led-managed-by-remote.html, however I'm only using a RGB led that draws currents 50mA Red and 30 mA Green or Blue.  P=IV tells me this led only produces 0.15w - 0.25w power, which is way less than the 5w led used in the given example.  I've already calculated the resistance to be 100 ohm for Red, 167 ohm for Green/Blue when using a 5V power supply.  The Radioshack 38kHz IR receiver (276-640) says to supply with .56mA max under no signal input.  I'm assuming they meant 56mA.  Now 56mA is much higher than what is used in the example being 15mA.  I calculated from I=V/R

I=5v/330ohm   I =15mA  -- Why such a low amperage when IR receiver says to use 56mA?

Since I'm using a low powered RGB led then I assume I don't need the ULN2003A driver to manage voltage to a high powered 5W bulb.  Is that correct?  I will wire up D11 to Green, D9 to RED, D6 to Blue.  

I see a 4.7microFarad capacitor is wired in between the Vs input and GND of the IR Receiver.  I understand that the capacitor stabilizes the current, but why do I need that for an IR receiver?  Is it because detecting wavelengths (nm) accurately requires a stable voltage?  In the example it uses a 4.7microFarad capacitor, so how do I know what size to use?  Does the size of the capacitor depend of the voltage used in the circuit or the amperage required of the hardware?  Why is the capacitor tied between 5v and GND?  Thank You.   If people can't see that I'm controlling the color of the led they might think that it has intelligence such as to change color when it senses an attractive female in presence! smiley-zipper


* IR Remote_esquema.png (303.73 KB, 1600x1547 - viewed 126 times.)
« Last Edit: December 09, 2012, 01:38:11 pm by encryptor » Logged

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Absolute maximum output current for the Arduino pins is 40 mA.  30 mA is a reasonable recommended maximum.  That means your Red LED draws too much power to be connected directly to the Arduino.  You could run the Red LED at 30 mA to see if that is bright enough.

The 330 Ohm resistor and capacitor appear to be more for filtering power than to limit current.  You can probably leave them both out and connect directly to the Arduino 5V line.
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I've already calculated the resistance to be 100 ohm for Red, 167 ohm for Green/Blue when using a 5V power supply.

So why does your schematic show 33 ohms for all 3 LEDs? In any case, you haven't allowed for the forward voltage of the LEDs, or the saturation voltage of the ULN2003A.

The Radioshack 38kHz IR receiver (276-640) says to supply with .56mA max under no signal input.  I'm assuming they meant 56mA.

No. they mean 0.56mA.

I=5v/330ohm   I =15mA  -- Why such a low amperage when IR receiver says to use 56mA?

The purpose of the 330 ohm resistor is not to reduce the voltage, it is to prevent noise on the +5V supply reaching the IR receiver (in conjunction with the 4.7uF capacitor).

Since I'm using a low powered RGB led then I assume I don't need the ULN2003A driver to manage voltage to a high powered 5W bulb.  Is that correct?  I will wire up D11 to Green, D9 to RED, D6 to Blue.

The absolute maximum output current per Arduino pin is 40mA, which is below the 50mA you want for your red LED. So you do need a buffer, unless you are prepared to run your LEDs at lower current.

I see a 4.7microFarad capacitor is wired in between the Vs input and GND of the IR Receiver.  I understand that the capacitor stabilizes the current, but why do I need that for an IR receiver?  Is it because detecting wavelengths (nm) accurately requires a stable voltage?

It's because in the applications for which they are normally used (e.g. TV remote control receivers), the 5V supply is normally derived from a standby switching regulator and is likely to have quite a lot of noise on it. Any noise at 38KHz could lead to the IR receiver thinking it was receiving a signal. The 330 ohm resistor and 4.7uF capacitor filter out such noise.
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In the example it uses a 4.7microFarad capacitor, so how do I know what size to use? 
You use at least what it says in the data sheet. You do not calculate this you take the manufacturers recommendations. They have the test equipment and they know what sort of mess on the power line upsets it. Do not try and second guess them.
One team for the set top box manufacturer I used to work for thought they knew better and it caused a massive recall of the set top box, it cost the company a fortune.
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no worries I'm using a 220 ohm resistor on Red, Green, Blue.  Let me point out again that is not my sketch.  I am using a 100 ohm resistor between my 5V source and Vs to IR receiver which calculates to 50mA.  That will be drawn from the 5v pin which is capable of 50mA.  Again what is the max amp one should only draw from 5v?  I asked why only 15mA because my IR receiver is asking for 56mA max.  I do understand lower power means less static, but doesn't it also mean less reception?  The hardware calls for 56mA so isn't that project's IR receiver under powered?
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Why do I bother offering help when you ignore me? I'll say it again: your IR receiver takes 0.56mA max, not  56mA. If you don't believe me, follow the link to the datasheet.
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I do understand lower power means less static, but doesn't it also mean less reception?
Then you know wrong. Static have no correlation with low power.
Listen to dc42 as well.
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Why do I bother offering help when you ignore me? I'll say it again: your IR receiver takes 0.56mA max, not  56mA. If you don't believe me, follow the link to the datasheet.

I read it the first time.  thanks for your reply.  It just doesn't seem right to me though.  0.56mA = 0.00056A.  To my understanding the correction should be 56mA or 0.056A.  Am I wrong or am I being misguided?  please let me know.
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Well the Vishay datasheet has it as .95 to 1.5 ma device Vcc current draw. And that sounds more reasonable then either of the two values being stated here.

http://pdf1.alldatasheet.net/datasheet-pdf/view/26603/VISHAY/TSOP1836.html

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« Last Edit: December 10, 2012, 01:16:53 am by retrolefty » Logged

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0.56mA is not at all unreasonable, this one http://www.sparkfun.com/datasheets/Sensors/Infrared/tsop382.pdf takes less (0.27 to 0.45mA).
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ok R=V/I = 5v/0.56mA = 8929 ohms.  So if I use a 10K resistor I get I = V/R = 5V/10K ohm = .0005A = 0.5mA so that's pretty close to what is recommended (0.56mA).  I will still use a 4.7 microFarad capacitor however I really don't know if it is the correct size for my circuit.  Someone other than manufacturers must know when to choose different size capacitor.  Thanks! smiley-lol
« Last Edit: December 11, 2012, 09:01:38 am by encryptor » Logged

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ok R=V/I = 5v/56mA = 8929 ohms.  So if I use a 10K resistor I get I = V/R = 5V/10K ohm = .0005A = 0.5mA so that's pretty close to what is recommended.  I will still use a 4.7 microFarad capacitor however I really don't know if it is the correct size for my circuit.  Someone other than manufacturers must know when to choose different size capacitor.  Thanks! smiley-lol

I still don't think you understand what the Vs pin for the IR receiver chip is requiring. It just needs a constant voltage applied to it with a capacitor bypass wired to the Vs pin to ground for extra power filtering. The IR receiver does not require a series current limiting resistor wired to it's Vs pin, the device will draw what it requires and only that amount. Putting a large 9k ohm series resistor will just act like a voltage divider and the receiver chip will end up only getting half the required voltage it needs to operate at. So keep that resistor very small (or remove it altogether) as it and the capacitor are just performing a power filtering function not a current limiting function. Led require current limiting resistors, ICs such as that IR receiver chip (it's much more then just a IR diode, there is analog and digital circuitry inside that device as shown it it's datasheet) require a constant voltage to their power input pins.

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Someone other than manufacturers must know when to choose different size capacitor.
No they don't because no one else has the information required to make that calculation.
It is the correct size for ANY circuit, yours is not special.
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thanks for all the help I've been getting guys!  Tis the season! smiley-cool
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