# LED Circuit Help.

Discussion in 'Circuit Help' started by Kamga S, Jan 4, 2017.

1. ### Kamga S

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Alright so I am making a Thank You sign using of 40 - 50 Red LEDs. Im currently having trouble figuring out the wattage and resistance needed for the circuit. At first i thought to just use a 220 ohm resistor but because there is a voltage drop between each leds im not sure it if will work . All it will be is a 9 volt battery connected to a resistor connected to 40-50 leds in parallel. the LEDs have a forward voltage of 1.8-2 volts and each LEDs requires about 20ma of current. How exactly do i figure out the wattage and resistance needed for this? Help much appreciated.

Kamga S, Jan 4, 2017

2. ### Bluejets

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Firstly, what is the 9v battery type?

Bluejets, Jan 4, 2017
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3. ### Kamga S

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9 volt Alkaline 500 mah and I have decided to go with a 10 watt resistor but still dont know what resistance value to use.

Last edited: Jan 4, 2017
Kamga S, Jan 4, 2017
4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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A small 9V battery probably won't last very long.

Go to the resource section and read the resource on driving LEDs. It has pointers to several LED calculators which will help you in your calculations. You will be best off using one resistor per 3 LEDs.

(*steve*), Jan 4, 2017
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5. ### Hicham

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with such low amp battery, I would avoid using any resistor instead, connect bank of LEDs to form a total voltage circuit of 9v. to do that, you would connect 5 LEDs in series (or 4 depending on the LED voltage tolerance range) and the rest in parallel. This last the battery a lot longer.

Hicham, Jan 5, 2017
6. ### Herschel Peeler

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Something like this?

Herschel Peeler, Jan 5, 2017
7. ### Kamga S

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Is it possible to use something like this?
https://smile.amazon.com/gp/product/B00SY37S00/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

Setting the converter to 2 volts and connecting the LEDs in parallel to the converter?

Kamga S, Jan 5, 2017
8. ### Audioguru

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An LED is driven from current not voltage. Most 5mm diameter LEDs are used at 20mA. Leds have a range of voltage because they cannot be made to be the same so if you connect a 1.8V LED to a 2V power supply then it will soon burn out because the current will be very high without a resistor to limit the current.

Audioguru, Jan 6, 2017
9. ### Kamga S

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What ive decided to do is use an AC power adapter to run the circuit instead of a battery through a buck converter lowering the voltage to 2 volts (what these LEDs are rated for) and the the adapter has a max output of 2 Amps with the circuit im creating will draw about 1.2 amps. Could this work?

Kamga S, Jan 6, 2017
10. ### donkey

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I see what you mean kamga just remember 3 important parts in your design.
output voltage HAS to be DC so if not already then throw in a rectifier bridge.
LEDs are hungry little things.. they will consume as much current as you give them...... then turn black, small flame etc. ensure you include your resistors to limit current.
Lastly limit the amount of LED's in a string to about 3 LEDs and a resistor. otherwise the voltage drop will make the LED's no work

donkey, Jan 6, 2017
11. ### Audioguru

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LEDs are not rated for 2V. They could be anywhere in the range from 1.7V to 2.2V. If all your LEDs have the minimum forward voltage of 1.7V and you apply 2.0V then POOF the LEDs are gone. If they are all 2.2V and you apply 2.0V then they will not light or they will be very dim.
You must limit the current and have about 1/3rd of the supply voltage across the series current limiting resistor to allow for the variation in forward voltage.

Audioguru, Jan 6, 2017
12. ### Kamga S

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Alright I have finally figured out how to set up my circuit. Ill be powering it with my 12 volt AC Adapter but i have one problem. My multi meter reads 11.888 Volts when i connect directly to the adapter(No load). Is this bad? Im just concerned because im unsure if my multi meter is wrong or my AC adapter is bad.

Kamga S, Jan 7, 2017
13. ### Audioguru

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If its output is not regulated then without a load its output will be higher than it is rated. If it is regulated then its output will be about within 5% from 12V (11.4VDC to 12.6VDC). So either yours is regulated or your meter is wrong.

Audioguru, Jan 7, 2017

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