Need to reduce voltage on a 3.7v battery for an LED circuit

I am building a circuit with a 3.7v 130mah Li-Po battery. This circuit will be for some LED lighting using about 6-10 LEDs. Is there a resistor calculator to figure out what to use to reduce voltage so LEDs will not burn out. I have spare electronics that I can salvage the smaller resistors from, but I don't know the color codes for the bands indicating resistance. I appreciate any help. This is my first post, so I apologize in advance if I left anything out.

h34d4ch3 said:

I am building a circuit with a 3.7v 130mah Li-Po battery. This circuit will be for some LED lighting using about 6-10 LEDs. Is there a resistor calculator to figure out what to use to reduce voltage so LEDs will not burn out. I have spare electronics that I can salvage the smaller resistors from, but I don't know the color codes for the bands indicating resistance. I appreciate any help. This is my first post, so I apologize in advance if I left anything out.

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hi and welcome,
What colour range are your LED's, different coloured LED's have different typical forward voltage drops[ operating voltage] also the required current thru the LED's.?

Resistance in Ohms = 1000 times (supply volts minus diode forward voltage) divided by required current in mA.
Google for resistor colour codes.

look at site https://www.superbrightleds.com under led information, should help give you a bit of a start.

An LED sets its own voltage. Your "3.7V" Lithium battery cell is 4.2V when fully charged and should use a circuit to disconnect its load when its voltage drops to 3.0V to 3.2V. If you use 3.5V white or blue LEDs then you will see them dim as the battery voltage runs down.

The series resistor limits the current (not the voltage) to prevent an LED from being destroyed.
Most 5mm diameter LEDs are fairly bright with a current of 20mA. With 10 of them then your 130mAh battery cell will power them for about 39 minutes but you will see them dimming.

ericgibbs said:

hi and welcome,
What colour range are your LED's, different coloured LED's have different typical forward voltage drops[ operating voltage] also the required current thru the LED's.?

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They are white/light blue in color. I obtained them from a LED flashlight. As far as the current I would like to have the LED's as bright as possible without damaging the LED's.

if you have the flashlight assembly still, just look at its circuit to see how it was run. and at the same time, check what the current that
it had running to the leds? that should give you a starting point of the current needed for the leds, i used the 9led flashlight assembly for
my pcb etcher shaker, i used a lm317 to set the current to them as was used in that old flashlight.

The assembly used 3 "AAA" batteries for 9 LED's. "AAA" batteries are about 250-1250mah. Would it be better to use triple A batteries then my 3.7v 130mah Li-Po battery? This light setup is being built into a hat. So I would barely be able to get in 2 "AAA" batteries. I am making a water resistant setup so sweat will not be a factor.

Most white and blue LEDs are 3.5V. A "3.7V" Li-Po battery cell might light them for only a few minutes before they are dimmed too much by the battery voltage running down.

AA old fashioned carbon-zinc cells (from you-know-where) have been on a boat and in a Dollar Store for a few years so they are probably dead when you buy them. Alkaline cells from the same place might be a little better. Alkaline cells from a modern democratic country are much better.
An Energizer AAA alkaline cell has a capacity of 1200mAh at 25mA when its voltage has dropped to almost nothing. Its capacity might be 800mAh when its 1.5V has dropped to 1.0V.

Some Chinese flashlights use a 4.5V old fashioned carbon-zinc battery that has low current. Then they do not have a current-limiting resistor. The LEDs are in parallel so the one with the lowest forward voltage hogs most of the battery current and soon burns out. Then there are fewer LEDs sharing the current so they soon all burn out but the cheap battery runs down and saves the LEDs from burning out.

I think most people who buy a cheap Chinese LED flashlight never replace the battery, they throw away the flashlight instead.

I have circuit that use Vcc=5V

Can i use 4,5 V battery for power supply?

A 4.5V battery has three brand new alkaline cells in series. The voltage quickly falls to 3.6V then drops slower to 3.0V when the battery is almost dead.
We don't know if your 5V circuit still works when its supply is only 3.0V to 3.6V.

h34d4ch3 said:

I am building a circuit with a 3.7v 130mah Li-Po battery. This circuit will be for some LED lighting using about 6-10 LEDs. Is there a resistor calculator to figure out what to use to reduce voltage so LEDs will not burn out. I have spare electronics that I can salvage the smaller resistors from, but I don't know the color codes for the bands indicating resistance. I appreciate any help. This is my first post, so I apologize in advance if I left anything out.

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There is an IC designed specifically for this: stack each set of LEDs in series (like dual five LED strings) and drive them using a constant-current boost converter like the LM2731 or LM2733. These are switchers that can run the battery down to about 2.7V and put out constant current through the LEDs regardless of battery voltage and LED temp. The phone makers use this technique for LED backlights, it's the best solution. It eliminates the power wasted in resistors so increases battery life.

Here is a constant current application for LEDs I designed for the LM2731 data sheet:

h34d4ch3 said:

I am building a circuit with a 3.7v 130mah Li-Po battery. This circuit will be for some LED lighting using about 6-10 LEDs. Is there a resistor calculator to figure out what to use to reduce voltage so LEDs will not burn out. I have spare electronics that I can salvage the smaller resistors from, but I don't know the color codes for the bands indicating resistance. I appreciate any help. This is my first post, so I apologize in advance if I left anything out.

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Do you have any electronics background? Also, I would like to see what circuit you are using. If your circuit is just the battery series connected to a resistor to the LED (the LEDs should be in parallel), then it would be really easy, you just need to know the current rating of the LED's. You can search the internet or do a test for the max current of your LED (you will have to break at least one of your LEDs). What you do is connect the battery, potentiometer and 1 LED in series, start the potentiometer at max (I recommend 10k pot), then go slowly down. as the resistance goes down, the LED will go brighter until it stops emitting light, it means you reached max current. Go measure the resistance of you potentiometer, buy a slightly higher resistor of that value. Now you're operating your LED at brightest.
This method is really non-efficient and you'll feel like a savage doing this, but seeing you don't have much experience in electronics (you don't even know resistor color bands, and you don't need to salvage resistors, you can buy them, its really cheap and you don't need to scourge a heap of trash to find you resistor value), I think this is the best way for you.

For the resistor color bands, you can just google them, its really easy. I recommend you read basic electronics first, its actually available in this site. )