# Basic question about the right resistor

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?

funkyguy4000:
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

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.

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?

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

funkyguy4000:
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

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 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.

then the leds in a parallel circuit.

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

Max Continuous Forward Current(IFM): 20mA

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

funkyguy4000:
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:

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

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

funkyguy4000:
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.

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

mmkay! sounds good. So with that, what resistance would I need?

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

so i'd have a 7.2 v supply and they are using .2 A so I would need. (7.2 / .2 = R ) a 36 ohm resistor?

funkyguy4000:
so i'd have a 7.2 v supply and they are using .2 A so I would need. (7.2 / .2 = R ) a 36 ohm resistor?

Not quite...
7.2V(battery) - 3.2V(LED voltage drop) = 4V (across the resistor)

4V / 0.02A = 200 Ohm
4V / 0.015A = 266 Ohm <-- I'd use a 270 Ohm

Oh okay!
What would happen if i used a 220 ohm. I don't have any 270's on me

funkyguy4000:
Oh okay!
What would happen if i used a 220 ohm. I don't have any 270's on me

Wait, you can calculate that yourself now that you have been shown the example calculations. You want to learn not just wait for answers don't you? Mr. Ohms law will thank you.

Lefty

I'd go worst case: High voltage in LiPo, low Vf on the LED, test them all & match the resistors:
(4.2 - 3.2)/0.02 = 50 ohm for 20mA
(4.2-3.8)/0.02 = 20 ohm for 20mA

Another option: Add a boost regulator and kick the voltage up to a known steady level until the battery is drained.

Just add 2 caps and a small inductor.