led driver circuit

Hi Dave, Do you have any idea what the junction voltage is on your LEDs? Do you have parts like wires and resistors so you can do some experimenting? If I was going to do this I would start by putting 2 LEDs in series with a 300 ohm resistor and then hook it to the battery to see if it lights. You will have to mind the polarity of the LEDs so that they are hooked anode to cathode. If it lights, I would experiment with increasing the resistor to 470 ohms and then to 560 ohms. We are looking for the point where we feel that the LEDs are still bright enough for your purpose. I would then make up 4 more series strings using the other LEDs and a similar resistor for each string. I would then put all 5 strings in parallel across the battery. This will power all 10 LEDs safely and at a minimum current drain on the battery. Make sure your battery has internal control circuitry or you might discharge it below the level where damage can occur to the battery. If each string can be powered at 10 mA and your battery is in good shape you should get close to 2 days of light before it quits.

John

Back again, If you can't get 2 LEDs in series to light up you will have to put the LEDs each in parallel across the power supply and wire each one with a resistor in series. Here again you can experiment and increase the resistor until the light of the LED is dim but still usable for your purpose. What ever current one LED uses will be multiplied 10 times when all 10 are hooked up. No complicated driving circuit should be needed for your project.

John

In this case I would be tempted to replace the 10 LEDs with a single 1 or 3W LED, it will fit easier and can be driven from the 3.7V LIPO with a simple constant current generator with minimal power loss, anything else will probably dissipate as much heat in the resistors as the LEDs them selves.

something like this should last close to 7Hrs on that battery http://www.newark.com/cree/mx6awt-a1-r250-000d51/led-high-brightness-cool-white/dp/25R3684?ost=MX6AWT-A1-R250-000D51

the trick is to not let the battery get too flat or it will destroy it (The battery), so some kind of cut off circuit will be needed

I assume your making some kind of night light ?

peter

I have a basic multi meter, bread board, soldering iron and perf board.

I have a led work lamp which has a broken case like this, https://www.torchstar.biz/detail.php?id=820 it has a battery cut off and charge circuit built on to the control board could this be used??

and i was hoping to replace the led bar with individual leds

i was going to build a car shaped night light out of an old rc car

Hi Dave - I do not have enough information to tell you. The LiPo battery that you mentioned, does it come from the work lamp so that it is matched to the charge circuit? You will have to perform the experiments on the individual LEDs or go with the single brighter LED as outlined by Peter. Are you replacing the work lamp LED strip as it is too bright and concentrated to make a good night light? I hope you can make this work as it seems like a project your son will really like.

John

Hi John, yes the Lipo came from the work lamp i fingered it would be better to use the circuit and battery together than use the battery on it own and charge it separately.

your right about the Led bar when it tested it, it practically lit up the entire bedroom

so i decided to go with the 5mm white leds, just so they will fit in to the car

thanks

Dave

That sounds good Dave. The battery and charging question is resolved. Let me know how your testing of the 5mm LEDs go on the 3.7Volt source.  Since you are going to be dealing with relatively low mA I do not think there will be any problems with heat or a large impact from using resistors to limit the current as opposed to using the current generator circuit that Peter mentioned. It just depends how fancy you want to get.

John

Hi Peter - Whenever I try to answer one of these questions I am always pleased when one of the fellows, like yourself, with more knowledge and experience add their insight. I have come back to your recommendation for a constant current generator to power the LEDs to try to learn something new. I have attempted to build a constant current generator using a MOSFET and a 741 OP amp. While I can get it to work just fine at higher voltages I have not been able to get it to work under the circumstances that Dave is encountering. He has a 3.7 volt battery and the White LEDs have a forward junction voltage of 2.8 volts. Did you have a circuit in mind for this application or were you thinking of using a larger supply voltage than 3.7 volts? I am guessing that my circuit is not working as there is only 0.9 volts to work with between the LED's and the battery's 3.7 volts. I know you are currently busy with your project so there is no rush in answering this.

Thanks John

I have been thinking about this as a side project, but I'm having trouble understanding it and i have to admit I'm getting back into electronics that i haven't seen since college and your help would be appreciated.

thanks Dave

One of the things I have played with in the past is this

http://www.newark.com/texas-instruments/lm3410xnopb/ic-led-driver-constant-current/dp/75M6032

Requires minimal components and will do the job nicely running of a LIPO, read the PDF tech speck, there is an example circuit in there just what you need

Regards

Peter