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olddirtyvegan
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CC LED information...
« on: Jan 21st, 2003, 10:48am » |
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Ok, I finished checking out our beloved LED on the color classic front button board. It's a stock 2.5v 1206 package green SMD LED, fed through a 180ohm dropping resistor (0805 package SMD) from a 6v feed line. So, any LED that will work with 6v or under will work if you put in the appropriate dropping resistor. Here is the stock schematic:
Mouser has a 1206 Blue LED (P# 606-CMD15-21UBC @ $2.94EA). It is a 5v LED, so we would need to change R1 to 50 ohms to get the correct drop. For those who are getting new to electronics, here is how we figure that out:
Ohm's law says that R=E/I where R=resistance, E=voltage and I=current. In the stock setup, we know the resistance of R1 (180 ohms) and we know the voltage drop across R1 (3.5v, since we are supplied with 6v and the LED uses 2.5, 6-2.5=3.5). We plug these in to get the current in the circuit, and
180=3.5/I, = 180I=3.5, = I=3.5/180 = I=.020 or 20mA
Now, we want to use a diode that needs 5V, so we plug in our current and voltage information to find the new resistance value of R1. Since we need 5v, and we are starting with 6v, we need to drop 1v on R1, so E=1. I stays the same, at 20mA or .020A. So, we plug them in:
R=1/.020 R=50, so R1 needs to be 50ohm
Anyway, I've got the parts on order and will post my results when I'm done. I'll be documenting the process via photos and text and will give it to Chris and Stuart for the Faq or PCC page. Hope this info helps!
-msr
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olddirtyvegan
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Re: CC LED information...
« Reply #1 on: Jan 21st, 2003, 11:29am » |
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Ok, I didn't have any SMD diodes at the shop, but I did have a bunch of red 2.5v 3mm diodes lying around. I swapped our green buddy for a red LED and we're looking good. All the digital cameras are checked out right now from the shop, but later this afternoon if one comes back I'll throw up a picture. Looks pretty cool, although I think the blue will be better.
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Chris Lawson
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Re: CC LED information...
« Reply #2 on: Jan 22nd, 2003, 7:14pm » |
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on Jan 22nd, 2003, 6:46pm, ccc_trix wrote:| Will all LEDs pull the same current? Should we assume that? |
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LEDs are rated for a specific current and should be driven at EXACTLY their rated current, or funky operation will result. Drive it too low and operation will be flaky; drive it too high and the LED will overheat and explode. Dr. Bob blew one up on TechTV; search for "exploding LED" here on the Fora and you'll find the thread that talks about it.
You need to know at minimum the current rating and voltage of the LED. If you know these two, resistance can be calculated as follows:
(6 - V) / I
where V is the voltage rating of the LED in volts and I is the current rating of the LED in amperes. Most LEDs are rated in milliamperes, so divide by 1000 to get the ampere rating.
For example, the stock LED is a 20 mA, 2.5V part, so we can calculate the resistance as:
(6 - 2.5) / 0.020 = 175 Ohms
Since SMD resistors are usually +/- 5%, the 180 Ohms that Matt measured it to be falls right in the range of acceptable specifications. For a 5V LED also rated at 20 mA, you would use a 50 Ohm resistor, as Matt notes above. If the same 5V LED were rated at 40 mA, you'd need a 25 Ohm resistor.
Would people be interested in a simple CGI script that calculates this for them? You'd put in the voltage and current specs and it would spit out a resistance.
I think this is the most useful approach, as it's easier to buy a resistor to fit a specific LED rather than an LED to fit a specific resistor. It would be marginally more difficult to make the CGI script allow the user to select any two values and then calculate the third, but I'd rather not go to the trouble if I don't have to.
cl
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| « Last Edit: Jan 22nd, 2003, 7:19pm by Chris Lawson » |
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Chris Lawson
CC FAQ maintainer, general Mac guru
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Chris Lawson
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Re: CC LED information...
« Reply #3 on: Jan 22nd, 2003, 7:21pm » |
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on Jan 22nd, 2003, 7:18pm, ccc_trix wrote:calc.exe is good for that  |
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Only runs on VirtualPC
Also, I'd rather not have to open up another app when I'm browsing the WWW - I'm a lazy bastard and would like my browser to do the work for me.
Side note related to that: didja know any browser with JavaScript support has a basic calculator built into it? Enter "javascript:" into the location field and then basic mathematical expressions after that. Press return and the page will display the answer.
cl
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Chris Lawson
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olddirtyvegan
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Re: CC LED information...
« Reply #5 on: Jan 23rd, 2003, 9:24am » |
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Nice work Chris. My blue parts still aren't here, but my red LED is ticking right along.
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Chris Lawson
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Re: CC LED information...
« Reply #6 on: Jan 23rd, 2003, 6:51pm » |
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on Jan 23rd, 2003, 6:30pm, ccc_trix wrote:| If you're that bored add colour codes, please. That's the part that always pains me. (I have a pile of unordered resistors.) |
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Do you mean resistor colour codes? SMD resistors don't *have* colour codes.
If you mean LED colour codes, well, I don't know of any relationship between colour, current rating, and voltage.
cl
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Chris Lawson
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Alan ONeil
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Re: CC LED information...
« Reply #7 on: Jan 23rd, 2003, 8:48pm » |
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Well, concerning the resistor, I don't think it is necessary to solder an SMD one if one doesn't have the skills. Wouldn't a regular size resistor work just as well when soldered to the pads the SMD one would have gone?
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Chris Lawson
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Re: CC LED information...
« Reply #8 on: Jan 24th, 2003, 4:08am » |
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on Jan 23rd, 2003, 8:48pm, Alan ONeil wrote:| Well, concerning the resistor, I don't think it is necessary to solder an SMD one if one doesn't have the skills. Wouldn't a regular size resistor work just as well when soldered to the pads the SMD one would have gone? |
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Two problems.
1) Through-hole parts are, as a rule, VERY difficult to mount on SMD pads.
2) There isn't enough room in the front of the case to mount a through-hole resistor there even if you *could* mount it properly.
And, uh, they *really* aren't that difficult. The 0805 package is nice and big, so they're about the same difficulty level as, say, a small through-hole part would be.
cl
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Chris Lawson
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olddirtyvegan
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Re: CC LED information...
« Reply #9 on: Jan 24th, 2003, 6:54am » |
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That resistor is tiny Chris....
Yeah, it would be pretty tough (much harder than just using the proper SMD) to get a through the hole resistor to work, even a 1/8 watt. I've ordered extras, so jjust email me and I'll send you one.
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Chris Lawson
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Re: CC LED information...
« Reply #10 on: Jan 24th, 2003, 11:07am » |
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on Jan 24th, 2003, 6:54am, olddirtyvegan wrote:| That resistor is tiny Chris.... |
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I'll have to take a look. I don't have a good mental image of the PCB in my head, but I thought there was a resistor on it about the size of the cross section of a pencil eraser. Is that not the one in question?
cl
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Chris Lawson
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Chris Lawson
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Re: CC LED information...
« Reply #11 on: Jan 24th, 2003, 5:34pm » |
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There are plenty of online colour code charts out there.
cl
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Chris Lawson
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