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Quick circuit question

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notasparagus

Member
Joined
Aug 27, 2002
Location
Puget Sound, WA, USA
I am building the circuit below (really basic I know) and just wanted to make sure I am doing this right. I will be using the 5 volt line to run them in parallel, "them" are rated at 3.7 volts. Is a quarter watt resistor high enough? I wasn't sure how much current a molex has. Also, what pin is the 5 volt line, if it is a standard. Below is a rough sketch of the circuit.

Thanks for all your help!
 

Diggrr

Underwater Senior Member
Joined
Nov 29, 2001
"them" will be just fine on a 1/4 watt resistor. Diodes take up a truly puny wattage (amps x volts = watts).
The amperage through a device is determined by the device, not the line supplying it. With the proper resistor to match voltages, you could hook your diode to a 400 amp car battery and be just fine, though it would melt/explode a wrench you dropped across the terminals.
If you'd like to test the capacity of the resistor, you can carefully feel if it's hot after being on for a few seconds, if it is, add another resitor in parallel with the first (but I still don't think it'll be a problem).

On the molex, use red wire for the +5 volts, and either black for the - ground.

Be safe, have fun!
 

Mpegger

Member
Joined
Nov 28, 2001
I strongly suggest you wire the LEDs in series and not in parrallel. When wired in series, the current draw of each LED will not be the same, and depending on whether your running them close to or at thier rating, may cause differences in brightness, or worse, early death of a LED.
 
OP
notasparagus

notasparagus

Member
Joined
Aug 27, 2002
Location
Puget Sound, WA, USA
Mpegger said:
I strongly suggest you wire the LEDs in series and not in parrallel. When wired in series, the current draw of each LED will not be the same, and depending on whether your running them close to or at thier rating, may cause differences in brightness, or worse, early death of a LED.

I thought if you run them in series, the voltages are different, right? So that is why I was wiring them in parallel. It seems you said if they are in series they will have problems.
 

Diggrr

Underwater Senior Member
Joined
Nov 29, 2001
In series, each diode will see only 2.5 volts and can be run without a resistor (on a 5 volt line only), though they will be dimmer than if run at their full rated voltage of 3.7 .

In parallel with the appropriate resistor, they will both run at the voltage they're supposed to (and at the rated brightness). If current draw is different for each diode (a very tiny difference if any) then the line will be able to handle both with ease. It's not near the current draw of a harddrive, for example.

I do suggest you do the math to make sure you're not getting out of the resistors' range for wattage. A milliamp is .001 amps.
 

Mpegger

Member
Joined
Nov 28, 2001
Sorry, I misworded my reply. It sould be...

I strongly suggest you wire the LEDs in series and not in parrallel. When wired in parrallel, the current draw (mA)of each LED will not be the same, and depending on whether your running them close to or at thier rating, may cause differences in brightness, or worse, early death of a LED.

You can easily run the LEDs in series from the 12v line. Remember, the circuit will only use whatever power (mA) it needs/is designed for, so you wont burn it out from running it off the 12V line. Just use the approipiate resistor and all will be fine. I suggest taking a look at the LED 101 page for a better explanation of how to find resistor values. If your still confused, just post the values of the LEDs you want to wire up (V, mA) and I'll figure out a good setup for it.
 
OP
notasparagus

notasparagus

Member
Joined
Aug 27, 2002
Location
Puget Sound, WA, USA
They are 3.7 volt, 20 mA Led's. However, I am still confused, I was under the assumption that running in parallel is better than running in series. How come the current draw is not thee same?
Anyway, I was going to run them off the 12v (before I planned on doing it off the 5v) and got a resistor value of 270. Does this sound right? Thanks for your help!

Update:
I wired up the molex connector, with a 68 ohm resistor off he 5v line and it remains at 5 volt when I tested it with my multimeter. Any ideas whats going on?
 
Last edited:

Overload

Member
Joined
Dec 18, 2000
Wiring your LEDs in parallel will NOT damage them in any way, and I think in your case is the better way to go. Just remember that the currents for each LED will add together and will require a different sized resistor. Two 20mA LEDs will pull 40mA from you 5v rail. so the resistor should be 32 Ohms.

If you try to read voltage with no load you will get the same voltage no matter what size resistor you have.
 
OP
notasparagus

notasparagus

Member
Joined
Aug 27, 2002
Location
Puget Sound, WA, USA
Ah, ok. Thanks for your help. I forgot to double the current when I was finding the resistor, so I guess I get to practice my soldering skills some more. :)
 

Diggrr

Underwater Senior Member
Joined
Nov 29, 2001
Actually, after some PMing, I've found you can easily parallel your LED's, but use a single resistor for each of the diodes to keep them from self destructing (sometimes they do, sometimes they don't) but it's best to be safe and ensure their long life.

Here's my revised schematic, assuming you use the correct resistor for the full voltage of your supply line for each diode, in your case each diode 'leg' is setup for 5 volts. You said you have a resistor for a single diode, get another and follow the drawing.

Sorry for what I said before, maybe I was just lucky in that my diodes have never overheated and failed. Now I know better.