Electronics 101 - HELP!!!

[cobraleader]

G'day all,

I need help with a basic electronic circuit.

I have to designed a 3V circuit to run 3.5V LED's (30mA). I want to run everything on 2* 1.5V AA batteries.
I'm told I need to have resistors on each LED, from the calculations I have done, I need resistors of -0.01666 value.
R= (3 - 3.5)/30

Have I done this right ?

I want to run at least 15 LED's in this circuit, however the more I can add to this one circuit the better, as I have drawn up at least 7 different (non connected) circuits - all individual to each other.

Also which is better, everything in parallel or in series ?

I also want to have the option of switching between a battery driven circuit (for short display durations - 1 or 2 hours) and a power pack driven circuit (for long duration - > 2 hrs)

Have I just created the monster from hell ??

Cheers
Mark

[Chacal]

I'm no expert or anything, but...

...3V circuit to run 3.5V LED's (30mA)...from the calculations I have done, I need resistors of -0.01666 value.
R= (3 - 3.5)/30

Have I done this right?

Yes. The -0.01666 value means you don't need a resistor, (the supplied voltage is less than the drop of the LED) as long as the voltage supplied is stable and doesn't go over 3.5V (batteries are usually reliable to keep below their voltage rating, because said voltage is a factor of the chemicals that the battery uses – electrochemically, they can't supply more voltage – they only lose voltage over time as they get spent)

...Also which is better, everything in parallel or in series?

You'll have to go with the parallel arrangement, because if you use a series arrangement you'll be dividing the voltage by the number of LEDs (two will have 1.5V each – not enough for them to light up – three will have 1V each, four will have 0.75V and so on)

[Scott Hasty]

Why would you run a 3.5V LED with a 3V supply. The short of it, no, you don't need a [current limiting] resistor because your supply voltage is less than the LED's forward voltage. Your LED, in this circuit, will illuminate, but they will be dimmer than they are designed to be.

Your formula is correct, but your number for your current is wrong. First, this should be in milliamps (mA). You have 30 listed. This would be 30 amps! It should be listed as .030 A. .030 is a bit hot for some LED's, so I usually plug in 20 mA.

Additionally, why don't you use 6V for your supplied voltage. You would need 125 ohm resistors for current limiting. I also don't recommend using batteries for 15 LED's. With AA's you'll drain them very quickly. C and D cells have signifigant current capacity, but are very large. Wall wart power is your best bet.

Scottie

[en'til Zog]

Hi, cobraleader.

JUST to keep things interesting, I'd recommend using THREE (3) batteries to power the LEDs. That's about 4.5 volts at the best of times, slowly dimming down as the batteries wear out.

Lessee - 15 LEDs at 0.020 amps each, that's a total of 0.3 amps - within the capacity of "C" size cells - a bit big but stashable in a reasonably sized base. Resistors for each LED - hm... that's 1 volt left over, at 0.020 amps or a 50 ohm resistor or slightly larger. (Then again a more common 47 ohm 1/8th watt resistor would work too - a bit brighter for the LED but survivable 0.021 amps.)

OR you could use a set of 3 AA sized NiMh rechargables in the stand - that's about 2 AH or amp hours which would (should) run your beast for around 6 hours.

But Scottie is right - a wall wart or plug-in power supply is a really good choice for general display - where you can plug the whole thing in, that is.

O.K.

Who's next in the "Confuse a Cobra" contest?


(But all these are quite valid ways of powering the circuit.... Just different.)

MODEL ON!

[zaphod]

Emperor Zog:
thanks for "showing us the light".
Pres. Zaphod

[cobraleader]

G'Day all,

Thanks for the feedback. Keep them coming, I'm becoming unconfused by the second.

Would you believe I've had more feedback from you guys than anyone else, except the salesman

Ok back to the drawing board.

Cheers
Mark

[Godfather]

G'day all,

I need help with a basic electronic circuit. {snikt!}

Mark, here are 2 SSM articles that should help: Working With LED & Das Ãœberblinker

[Sparky]

I'm guessing Sam's isn't making their electronics starter kits/workbooks anymore. I believe they were located in the out back. If you can find any of these they're a great way to ease into electronics, since that's where I started. Along with HAM radio. But I still have my Heathkit/Sam's analog trainer, which came in parts and I assembled per the instructions. Doing so helped me replace the power transistor I burned later. It's there in the model room.

Zog, you didn't try to send him a negative resistor? Those I guess we have to keep under wraps, I think they only use them in our physics experiments in college, at least that's the only explanation for the results we got in our labs.

[en'til Zog]

Oh yes... a Negative Resistor is also known as a 'battery'....

I've run into polarized resistors before, by the way.... No idea why but in circuit they just conducted one direction but not the other... Drove us NUTS. (Or was that a short putt?)

Zaphod, my President - Volkswagen LUX! (Fiats are too expensive.)

All youse guys - MODEL ON!