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Resistance for Dummies: Lesson 1

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Tate

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Hi tthere. I'm still pretty new to electronics, starting out by learning some simple computer mods. I've been building a computer from sheet aluminum, based on a sweet looking pyramid-shaped CD Player I saw at Radioshack.

Older 3d Model: http://68.45.36.30/giza-2.jpg
Actual thing (so far): http://68.45.36.30/giza-1.jpg

Anyway, I'm wiring up some 3mm 3v LEDs, and I'm trying to figure out the resistance needed. I keep coming accross the formula R = (5 - V) / I, where V is the voltage that I want the LEDs to have, 5 is the amount I'm feeding it (from my computer's power supply), and I would be the LED's mA (.02A per LED).

The problem I'm having is that I think I'm doing something wrong with calculating the total amount of resistance I'll need. For an example, I'm going to have 4 LEDs hooked up in parallel, and I want them to have 1v each. I tried multiplying the amperage by 4 (.08 instead of .02), giving me the formula;

R = (5v - 1v) / .08

...but the LEDs appear to be at full brightness. (In case I'm just picking the wrong resistor, I'm using a 47 ohm, yellow-purple-brown-gold resistor, the closest to 50 ohms that I have).

Please give me any insight you have as to what I'm doing wrong, whether it's in calculating the resistance or just using the wrong components. Thanks in advance. :)
 

jamez18

New Member
I believe that a yellow-purple-brown-gold color band represents a 470 ohm resistor with a 5% tolerance. For 47 ohm you need yellow-purple-black-gold. It is best to use a multimeter to check the reistance instead of just going by the color code
 

Tate

New Member
Ahh... Just realized that, but even with that resistor the LEDs' brightness doesn't seem to be clamped at all. :(

Thanks for pointing out my mistake though. :)
 

nettron1000

New Member
The problem I'm having is that I think I'm doing something wrong with calculating the total amount of resistance I'll need. For an example, I'm going to have 4 LEDs hooked up in parallel, and I want them to have 1v each. I tried multiplying the amperage by 4 (.08 instead of .02), giving me the formula;

R = (5v - 1v) / .08

I think you may be mis-understanding what the formula is telling you.

Its R=(Vs-Vl)/I

R=(5-1)/.02 (Im assuming you got 20 mA from the data sheet ?)

R=200 ohms

This is the resistance that must be placed in series with EACH LED to get full brightness. The nearest resistor to this would be 220 ohms.
Connecting LEDs in parallel with one resistor is a bad design practice.
 

Tate

New Member
Thanks nettron! I hope it turns out as well as I've been planning. It's taking a lot more work than I expected. :D

I think I understand now about that formula only working for one LED, also. Thanks for clearing it up. :)

Let me explain what I'm really trying to do, though, so you can see why I had it set up the way I did.

These LEDs will be lit up with just 1 volt all the time (so they appear dim); this 1 volt will come from the 5v power supply. There will actually be 12 LEDs in parallel.

Then, there is an alternate source of 5 volts that will indicate a status - when the status is on, it will power the LEDs to their full brightness of 3 volts. I planned on just having another resistor that would make that alternate 5 volts into 2, so when it's on, the total going to the LEDs would be 3 volts.

Sorry if that sounds confusing, it's kind of hard to type out like this. :D

Anyway if you can give me some suggestions on alternative approaches to achieve the same product, I'd be very appreciative. :)
 

nettron1000

New Member
Yep your right im confused. :(

These LEDs will be lit up with just 1 volt all the time (so they appear dim); this 1 volt will come from the 5v power supply. There will actually be 12 LEDs in parallel.
Just wondering...these 12 LEDs, would they be monitoring the DATA and CONTROL pins on the parallel port by any chance?
 

Tate

New Member
nettron1000 said:
Yep your right im confused. :(

These LEDs will be lit up with just 1 volt all the time (so they appear dim); this 1 volt will come from the 5v power supply. There will actually be 12 LEDs in parallel.
Just wondering...these 12 LEDs, would they be monitoring the DATA and CONTROL pins on the parallel port by any chance?
Nothing that complex.

The LEDs will be bordering the entire case... sort of a cosmetic thing, really. I have an LCD display from matrixorbital.com that can output 5 volts when something happens (This is a software controlled event... Example: When I get email, the LCD display could flash the 5 volt output on and off.) So the LEDs will just be dim most of the time, and whenever that 5 volt output from the LCD activates, I want the LEDs to light up to full brightness.

BTW, some interesting stuff on your site. :)
 

nettron1000

New Member
To get some idea of what we're talking about i just slapped this together.





Would that be accurate?


Q1 is a transistor switch that is slightly biased 'on' via R2.

R2 controlls the brightness of the LEDs when the LCD panel is not outputting pulses. You can simply replace R2 with a standard fixed
Resistor when you decide how dim you want the LEDs to be.
 

Tate

New Member
e said:
where does the mobo go? :D
cool lookin case man.
Motherboard and all the other parts (except some fans and the CD drives) sit right on the bottom in brackets. Here is a see-through version, along with a solid one;

http://68.45.36.30/giza-3.jpg
http://68.45.36.30/giza-4.jpg

To nettron;

I'm a graphics person. I know how I want things to look in the end... but as for electronics, I'm just now learning how to use resistors the right way. I'm probably in way over my head with this mod. :(

As far as I can tell from that schematic,the transistor will fully light up the LEDs when the LCD display activates. I Think that is what I need; I was planning to use the 5v from the LCD's output to directly power up the LEDs, but I'm sure a transistor would be better, using the power supply to regulate the LEDs' power. (I think that's what your schematic describes, anyway). The only problem I have now is that I need to know what type of transistor... if there are different kinds.

So as a recap, I want the LEDs to have 1 volt going to them at all times. When the LCD outputs it's 5 volts, I need the LEDs to light up to a full 3 volts (their max). I just want to confirm that the diagram you posted will do that (so I don't go blowing up my precious expensive blue LEDs. :D


I'd also like to thank everyone, especially you nettron, for your quick and informative replies. This forum is filled with people who really know what they're doing, and I'm glad I came accross it. I've been reading some of the threads around here, and even though most of them are too complex for me, at least I might pick up a thing or two that could help me in the future. Thanks guys. :)
 

nettron1000

New Member
The only problem I have now is that I need to know what type of transistor... if there are different kinds.
Any transistor will do but you will need to know some particular things about the transistor you are using. What transistor(s) do you have on hand?



So as a recap, I want the LEDs to have 1 volt going to them at all times. When the LCD outputs it's 5 volts, I need the LEDs to light up to a full 3 volts (their max). I just want to confirm that the diagram you posted will do that (so I don't go blowing up my precious expensive blue LEDs.

Well that schematic is just an educated guess, i dont know everything about your setup. You should test the circuit on a breadboard with a transistor driving one LED, i wouldnt recomend that you blindly build this circuit without testing it first and understanding how it works.

Heres a link that explains what a transistor switch is and how it operates.

http://www.rason.org/Projects/transwit/transwit.htm


PS. do you have the data sheets for those blue LEDs?
 

Tate

New Member
I ordered the LEDs from here - they say they're 3 volt, and I just figured that they're 20 mA LEDs... I don't know anything else about them. :(

I don't have any transistors on hand, since I really haven't done much in electronics past simple hook ups like this using a couple resistors and a switch. I think I might have come up with a new solution though...

After wiring up an LED with a 220 ohm resistor and powering it with 5 volts, it seemed ok but very bright... I'm not sure if it was supposed to be that bright, but I only left it on for a second. This is a test LED anyway :D

Then I put a 560 ohm resistor before it, and that gives me a nice brightness for when the LEDs are dim.

Since my LCD has more than one 5-volt output, I am now thinking of putting that 560 ohm resistor between one of the outputs and the LEDs, and having another resistor between the second output and the LEDs. This way, I can have just 1 of the outputs on for dim LEDs, both for bright LEDs, and neither for complete off (which I really wasn't planning on, but it would be an additional feature I could use). So now all I'd need is to figure out that second resistor's resistance.

I seem to just be guessing now. I think I need to go back and read up on basic electronics some more... every time I try that formula to figure out the resistance, it doesn't turn out the way I would think it should. Thanks for the help anyway. :)
 

nettron1000

New Member
After wiring up an LED with a 220 ohm resistor and powering it with 5 volts, it seemed ok but very bright... I'm not sure if it was supposed to be that bright, but I only left it on for a second. This is a test LED anyway

Yes they will be very bright , they are working at nearly their max brightness when using a 220 resistor . Its even recomended that you not look directly at the LED. But im not sure if you should operate them from the 5 volt supply like that without having some means of controlling the current and voltage, which is why i included the transistor. The transistor will drop some voltage before it gets to the LED and will act as a valve to control current flow.



Then I put a 560 ohm resistor before it, and that gives me a nice brightness for when the LEDs are dim.
Yep, thats the idea, now that you have a "ballpark" figure you can experiment and see what works for you.
 
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