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### Topic: Basic question about the right resistor (Read 3563 times)previous topic - next topic

#### funkyguy4000

##### Feb 08, 2012, 04:37 pm
Hello,

So I recently purchased 2 3v coin cell batteries from sparkfun and I need to use one for helping a certain lady with a dress.
She wants to put some UV blacklight leds in the dress so i'm helping her.  I'm going to wire them all up in parallel circuits and use smd resistors so that is smaller.
I have a question though.

What is the correct resistance for this?
The forward voltage is 3.2 - 3.8 V , think it would work with 1 3v coin cell?

If so what sort of resistor would I need?
Accelerate to 88 miles per hour.

#### retrolefty

#1
##### Feb 08, 2012, 04:46 pm

Hello,

So I recently purchased 2 3v coin cell batteries from sparkfun and I need to use one for helping a certain lady with a dress.
She wants to put some UV blacklight leds in the dress so i'm helping her.  I'm going to wire them all up in parallel circuits and use smd resistors so that is smaller.
I have a question though.

What is the correct resistance for this?
The forward voltage is 3.2 - 3.8 V , think it would work with 1 3v coin cell?

If so what sort of resistor would I need?

Won't work, two 3 volt coin batteries wired in parallel is still just 3 volts and your led(s) requires a minimum of 3.2 volts before it's forward biased and turns on. If you wire the two coin batteries in series you gain 6vdc, which is enough to forward bias an led. But you haven't stated how much the recommended forward current is for those specific LEDs, so not enough information to calculate resistor size.

Lastly but most difficult is I suspect those small coin batteries cannot support the current requirement for even one LED let alone more then one. Those batteries are designed to flow only a few milliamps at most and will be discharged very very quickly trying to power LEDs.

Lefty

#### funkyguy4000

#2
##### Feb 08, 2012, 04:48 pm
It says that the reverse current is <= uA  (i got these from china)

I meant to say i want to wire the battaries up in series and then the leds in a parallel circuit.
Accelerate to 88 miles per hour.

#3
##### Feb 08, 2012, 04:49 pm
3V is not high enough to turn on an LED with a 3.2 Vforward.
6V would be enough. In which case, assuming the LED needs 20mA for full brightness: (6V -3.2V)/.02 = 140 ohm or higher, say 150 as a standard value. For each LED.

A fairly large coin cell battery:
Lithium Battery CR2032 ?20mm 225mAh 3V
will not last very long, maybe 10 hours with just 1 LED.
5.5 with 2.
3.5 with 3.
Is the coin powering an uC as well?

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### funkyguy4000

#4
##### Feb 08, 2012, 04:51 pm
yea i'd need a microcontroller.  Hmm sounds like i'll be using a few AAA's seeing as how it can't find a smaller solution
Accelerate to 88 miles per hour.

#### retrolefty

#5
##### Feb 08, 2012, 04:54 pm

It says that the reverse current is <= uA  (i got these from china)

I meant to say i want to wire the battaries up in series and then the leds in a parallel circuit.

Recommended foward current not reverse current is the specification you need to find.

Lefty

#### funkyguy4000

#6
##### Feb 08, 2012, 04:56 pm
This is the description.  It has a max peak current

- RoHS : Yes
Life Rating : 100,000 Hours
Emitted Colour : Purple
Size (mm) : 5mm
Lens Colour : Water clear
Peak Wave Length (nm) : 395 ~ 405
Forward Voltage (V) :  3.2 ~ 3.8
Reverse Current (uA) : <= 30
Luminous Intensity Typ Iv (mcd) : 3000(Typical) ~ 5000(Max)
Viewing Angle : 20~25 Degree
Max Power Dissipation(PM): 80mW
Max Peak Forward Current(IFP): 75mA
Max Continuous Forward Current(IFM): 20mA
Lead Soldering Temperature : 240 Degree (<5Sec)
Operating Temperature Range : -25 ~ +85 Degree
Preservative Temperature Range : -30 ~ +100 Degree
Accelerate to 88 miles per hour.

#### Grumpy_Mike

#7
##### Feb 08, 2012, 04:56 pm
Quote
It says that the reverse current is <= uA

Reverse current is what it acts like when it is being a normal diode, nothing to do with the light emitting part.

Quote
then the leds in a parallel circuit.

Parallel LEDs is only possible if each as it's own resistor.

#### Grumpy_Mike

#8
##### Feb 08, 2012, 04:57 pm
Quote
Max Continuous Forward Current(IFM): 20mA

That is the current to design it for, although you might want a touch less, say 15mA.

#### retrolefty

#9
##### Feb 08, 2012, 05:01 pm

This is the description.  It has a max peak current

- RoHS : Yes
Life Rating : 100,000 Hours
Emitted Colour : Purple
Size (mm) : 5mm
Lens Colour : Water clear
Peak Wave Length (nm) : 395 ~ 405
Forward Voltage (V) :  3.2 ~ 3.8
Reverse Current (uA) : <= 30
Luminous Intensity Typ Iv (mcd) : 3000(Typical) ~ 5000(Max)
Viewing Angle : 20~25 Degree
Max Power Dissipation(PM): 80mW
Max Peak Forward Current(IFP): 75mA
Max Continuous Forward Current(IFM): 20mA
Lead Soldering Temperature : 240 Degree (<5Sec)
Operating Temperature Range : -25 ~ +85 Degree
Preservative Temperature Range : -30 ~ +100 Degree

It also says this:

Quote
Max Continuous Forward Current(IFM): 20mA

That is the value to use if you want to have maximum safe brightness from the led.

So (6v - 3.2) / .02 amps = 140 ohms for the current limiting resistor. And yes each parallel wired LED will require it's own resistor.

Lefty

#### funkyguy4000

#10
##### Feb 08, 2012, 05:02 pm
Yea, I can get the resistors and everything i'm just wondering how i should wire this whole thing up
Accelerate to 88 miles per hour.

#### retrolefty

#11
##### Feb 08, 2012, 05:19 pm

Yea, I can get the resistors and everything i'm just wondering how i should wire this whole thing up

Were are best at answering specific questions, can't help you too much with 'this whole thing' as it include information of what you want the arduino code to be able to do with the leds, how many leds, how long you want the thing to operate given a certain battery capacity, etc.

Next world hunger solved right here on the arduino forum.

#12
##### Feb 08, 2012, 05:22 pm
You could also go with a 3.7V flat LiPo battery.
Get a 2000mAH battery, will run quite a while.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### funkyguy4000

#13
##### Feb 08, 2012, 05:30 pm
mmkay! sounds good.  So with that, what resistance would I need?
Accelerate to 88 miles per hour.

#### retrolefty

#14
##### Feb 08, 2012, 05:40 pm

You could also go with a 3.7V flat LiPo battery.
Get a 2000mAH battery, will run quite a while.

A single cell lipo would be pretty marginal working with 3.2 to 3.8 volt forward voltage drop leds as voltage drops pretty linearly with charge state on lipos starting at 4.2 and going down to 3.0, making sizing the resistor rather difficult. I would use a two series cell lipo at minimum.

Lefty

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