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Can I power white LED Backlight Module from 3V watch battery
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Can I power white LED Backlight Module from 3V watch battery

by haleymcl on Thu Dec 02, 2021 4:49 pm

I ordered ten of Part No. 1621 - White LED Backlight Module - Large 45mm x 86mm SID 5.5.1.

I want to power it using a 3V watch battery, either 2032 or 2025. I tried touching the leads directly to the opposite sides of the 3V battery, but it won’t light up. I have successfully powered a different kind of single LED light using this method before. Is it not possible to do with the LED Backlight I ordered from you? Would a smaller size of that LED Backlight work?

haleymcl
 
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Re: Can I power white LED Backlight Module from 3V watch bat

by dlleigh on Thu Dec 02, 2021 6:43 pm

Powering a device like that from a coin cell is not a good idea. Coin cells like the CR2032 are not designed to continuously output the 20 mA of current necessary to power this back light. For brief periods of time (say, less than a second) you can draw substantial amounts of current from a CR2032. If you try to draw a lot of current for longer than that, the cell's voltage will decrease and won't be able to overcome the LED forward voltage drop. Here's a data sheet for an Energizer CR2032 that tells how much current you can draw for continuous and pulsed loads, and what that will do to the voltage and lifetime.

I'd suggest using a lithium-ion rechargeable with this back light, or maybe three AA or AAA alkaline cells in series. Note that you will need to use a series resistor between the battery and the back light, otherwise it will draw too much current and might be damaged. The back light appears to be just an LED with no current limiting resistor. The value of the resistor you use will depend on the voltage of the battery you are using for power. Let us know what you plan to use and we can help you calculate the value.

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Re: Can I power white LED Backlight Module from 3V watch bat

by haleymcl on Fri Dec 03, 2021 5:13 pm

The object of the game is to power this 45mm x 86mm backlight with the smallest and lightest power source possible so that it can be fully self-contained and portable. In addition, I need to be able to completely hide the batteries and resistor behind the backlight and wrap the leads around back to plug them in so that nothing is visible from the front other than the lit panel itself.

What would you recommend? What size and type of batteries, how many of them, and what value of resistor? Anything else I need to secure the batteries and plug the leads in?

haleymcl
 
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Re: Can I power white LED Backlight Module from 3V watch bat

by dlleigh on Fri Dec 03, 2021 6:15 pm

Some of this depends on your budget. Size optimization and cost optimization don't always yield the same outcomes.

If you want coin cells that will source sufficient continuous current to run the back light, I'd look at products from Illinois Capacitor, which you can find on Digikey. Here's one example that will source up to 58 mA continuously, and even more for pulse applications: https://www.digikey.com/en/products/detail/illinois-capacitor/RJD3048HPPV30M/

These are rechargeable and you can use one of the LiPo chargers sold by Adafruit. They are a little thicker than a CR2032 (about 5.5 mm vs. 3.2 mm), but you can get them in a leaded version that doesn't require a holder. However, these are not cheap. Digikey sells the one I linked for about $14, quantity one.

Adafruit sells LiPos that aren't very thick that also might work for you. Take a look at these and see if you can find any that will meet your size requirements: https://www.adafruit.com/category/574

Before we address your other design questions, why don't you tell us more about your application. What do you hope to achieve? What do you intend to use it for? How do you want the device to act? How long would you like it to operate?

dlleigh
 
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Re: Can I power white LED Backlight Module from 3V watch bat

by haleymcl on Fri Dec 03, 2021 7:07 pm

Before I go into how I intend to use the unit, could you help clarify your first suggestion. The link you sent to a product on Digikey didn't work. Can you resend a different link or instructions on how to search the Digikey site for the product you were suggesting?

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Re: Can I power white LED Backlight Module from 3V watch bat

by dlleigh on Mon Dec 06, 2021 1:01 pm

Try this link: https://www.digikey.com/en/products/detail/illinois-capacitor/RJD3048HPPV30M/6159143

Or search the Digikey site under "Illinois Capacitor" and then select "Batteries Rechargeable (Secondary)". I get 19 results for lithium-ion coin cells with different sizes, capacities and features.

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Re: Can I power white LED Backlight Module from 3V watch bat

by haleymcl on Mon Dec 06, 2021 1:53 pm

That new link to the coin cells worked! Thanks! As for how I intend to use the backlight device, I plan to use it as a tracking marker in video images to assist in creating digital visual effects. That's why I need the battery apparatus to be completely concealed behind the backlight panel so that nothing is visible to the camera other than the lit white panel. Keeping the unit as lightweight as possible is very important because I need to be able to mount several of them on a wall, pillar, or other vertical surface using only sticky acrylic glue dots that can later be peeled off the wall leaving no residue and doing no damage to the surface or paint. It would be ideal if the unit could operate for up to eight hours continuously, but four hours would be good enough and I might even be able to get by with only two hours in some circumstances. I would appreciate any advice on technical specs such as voltage, milliamps, resistors, etc., necessary to power the backlight unit for several hours at a time.

haleymcl
 
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Re: Can I power white LED Backlight Module from 3V watch bat

by dlleigh on Tue Dec 07, 2021 12:57 pm

The Adafruit LiPo cells in the link above are fairly thin, ranging from about 5 mm to 10 mm, and have quite reasonable capacity. They don't weight very much and should be easy to mount to the back of the back light with tape or glue.

Their lifetime in your application will depend on how bright the back light needs to be. You don't need to run it at the maximum of 20 mA if you can do with less light output. The datasheet recommends from 10 to 20 mA, with a typical number of 15 mA. Let's assume a current of 15 mA for the following calculation:

If your LiPo cell has a capacity of 500 mAh (read as "five hundred milliamp-hours"), then it can supply one milliamp of current for 500 hours, or 10 milliamps of current for 50 hours, or 15 mA for about 33 hours, or 25 mA for about 20 hours. There's a limit to how much current you can draw, but 15 mA should be OK for the cells we're talking about.

You will need a series resistor between the LiPo cell and the back light to prevent overcurrent. At 15 mA, the forward voltage of the back light LED is about 3.2 volts, per the data sheet. The LiPo's nominal voltage is 3.7 volts, so the resistor will drop 0.5 volts. A resistor that drops 0.5 volts with a current of 15 mA has a value of about 33 ohms. You may want to experiment with some values around there if you decide you want the panel to be a little bright or dimmer. Using Ohm's Law and information from the data sheet, a 20 ohm resistor will allow 20 mA through the back light and a 55 ohm resistor will allow 10 mA through. You wouldn't want to put more than 20 mA through it, but you could use quite a bit less current if your application needed less light output. A 900 ohm resistor will set the current to about 1 mA, which according to the data sheet will light the panel to about 2% of its full brightness.

If I were building this, I'd use the same little JST connectors that Adafruit uses on its LiPo cells. They also sell the male mating connectors, which you can solder onto the back light wires to make it easy to take off the batteries and recharge them. You'll also want one of the Adafruit LiPo chargers, or maybe a few depending on how many of these you build.

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Re: Can I power white LED Backlight Module from 3V watch bat

by haleymcl on Tue Dec 07, 2021 7:45 pm

Wow! Thank you so much for this wealth of information! So helpful! I am still digesting it all and checking out the possible options. I'll need to do a little more thinking, but when I formulate what seems to be the best configuration, I would love to run it by you and make sure I'm on the right track.

haleymcl
 
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Re: Can I power white LED Backlight Module from 3V watch bat

by dlleigh on Tue Dec 07, 2021 8:37 pm

Happy to help. I'll keep watching this forum for when you post again.

dlleigh
 
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Please be positive and constructive with your questions and comments.