Getting constant voltage

[abuthemagician]

I have a project (Wii Wireless sensor bar) that i am working on for a friend. I plan to power 4 infrared LEDs with a 3.7v 810mAh lithium Ion cell phone battery charged by a dismantled charger made for that phone (you can charge the phone or a spare battery). What i want to do is use the device (4 powered LEDs) whether it is plugged in or not. I have a feeling that there will be two different voltages depending on if it's plugged in or not. What is the simplest way to get a constant voltage to the LEDs?

 

[justDIY]

your leds will appericate a constant current much more than a constant voltage. how are the leds wired, all in parallel, or series or what?

just calculate your resistors for the highest voltage you'll be using, the leds will still work fine at the lower voltage as well.

charging the battery:
4.2v - 1.8v = 2.4v / 0.03a = 80 ohm

low battery:
3.0 - 1.8 = 1.2 / 0.03 = 40 ohms

that is your range - pick a resistor closer to the top rather than the bottom, otherwise your leds might burn out while running on the charger

 

[abuthemagician]

Thanks for answering so quickly. In case you wanted to know here is what i will do (only using the voltage from the charger):

assuming the batteries voltage label is correct (will have to measure it) it says its a 3.6v 700mAh battery. That leads me to a 2 x 2 array (1.2v 30mA LEDs) with 2 47 ohm resistors.

Also how do i calculate how long they will last on the battery?

 

[Hero999]

Battery capacity is measured in amps per hour, so your 700mAh battey can supply 700mA for 1 hour, 350mA for two hours or 1.4A for ½ hour. If you know the average discharge current (a reasonable assumption is it being the median of the current at full charge and low charge) then it's fairly easy to calculate how long it will last for.

 

[abuthemagician]

So at 60mA (4 30mA LEDs in a Parallel 2x2 array = 60mA) i would get 11 hours on a 700mAh battery?

 

[Roff]

Battery capacity is measured in amps per hour, so your 700mAh battey can supply 700mA for 1 hour, 350mA for two hours or 1.4A for ½ hour. If you know the average discharge current (a reasonable assumption is it being the median of the current at full charge and low charge) then it's fairly easy to calculate how long it will last for.

Nitpick: Battery capacity is measured in amps times hours.

 

[abuthemagician]

so then how do i calculate?

 

[Hero999]

Ignore him, you got it right.

 

[ericgibbs]

so then how do i calculate?

As Ron says, AHr.

Divide the Amp Hour rating of the battery by the current you are drawing from the battery, the result is the expected time.

NOTE:
This only applies to a fully charged battery in good condition, over time after many re-charges the efficiency of the battery will degrade.
Li-Ion batteries also degrade with time, used or not, from the date they are manufactured.

Also it only an approximation to the Amp Hour rating, look on the web for Li-Ion battery discharge curves

 

[audioguru]

Like I said on the other forum. DO NOT connect LEDs in parallel.

 

[justDIY]

there's nothing wrong with leds in parallel, so long as appropriate measures are taken to insure they get a safe current (each one needs its own resistor in this case)

 

[Roff]

there's nothing wrong with leds in parallel, so long as appropriate measures are taken to insure they get a safe current (each one needs its own resistor in this case)

I think AG was talking about the strict definition if "parallel".

 

[audioguru]

I think AG was talking about the strict definition of "parallel".

Yeah:
3.6V battery.
1.2V LED with a rating of 30mA.
47 ohm current-limiting resistor.
It calculates to 51mA for each LED if they each have their own current-limiting resistor. Too much current.

If two LEDs are strictly in parallel then one would probably hog the current and burn out, then the other one would also burn out.

 

[Hero999]

I can't remember where but I've seen a schematic on a constant current SMPS datasheet using Luxeons, I don't know if it's acceptable to do it with them though.

 

[Nigel Goodwin]

I can't remember where but I've seen a schematic on a constant current SMPS datasheet using Luxeons, I don't know if it's acceptable to do it with them though.

If it's constant current you put the LED's in SERIES - putting Luxeons in parallel is just looking for an expensive heap of junk!.

As already suggested, NEVER put LED's in parallel, use seperate current limiting for each LED, or series of LED's.

 

[Hero999]

I agree, I don't condone such a prattice but Philips obviously do.

Here's the tread where.
https://www.electro-tech-online.com/threads/led-driver.30054/

 

[Nigel Goodwin]

I agree, I don't condone such a prattice but Philips obviously do.

Here's the tread where.
https://www.electro-tech-online.com/threads/led-driver.30054/

Slightly different though! - they aren't putting LED's in parallel, but in series/parallel - and the app note says that's it's OK to do so as long as you use series strings of at least three LED's.

 

[Hero999]

I don't see how that's any worse than connecting two or more in parallel.

 

[Nigel Goodwin]

I don't see how that's any worse than connecting two or more in parallel.

According to Philips the spread of three in series is enough to compensate for any differences between the chains. Easy enough to test, get 6 LED's and a couple of ammeters - one in series with each chain. Measure the currents with both chains fed via a single current limiter - then try it with two LED's then with one LED. Try the LED's in various combinations.

But it sounds fairly plusible?, current balancing resistors work by the increased voltage drop across them as the current increases - LED's don't have a constant voltage drop, it increases slightly with current - three in series will provide three times the voltage increase as the current increases through that chain. This means the other chain(s) will start to take more of the current.

 

[Hero999]

But what happens if you're unlucky and you've got one chain containing all the LEDs with low voltage drops and another chain containing LEDs with high voltage drops?