# Simple but strange watts amperage and voltage problem on LED strip

In the specs it says the power is 5 watts, voltage can be from 7.4v to 13.5v, and never to draw more than 300mA or I will damage it. I plan on using a 3S LiPo (11.1v) to power it. 5 watts at 11.1 volts is a 450mA draw, though. How is this possible? Is something wrong in the specs or is it safe to power it with 3S?

Hi
The issue here is cooling or to be more accurate risk of overheating
The actual LEDs are rated at 5W but this rating comes with caveats in the manufacturers data sheet regarding mounting to dissipate the heat generated
HK are saying that with their construction current over 350mA will cause overheating and damage.
You can use any size battery BUT you must limit the current in some way. Usual methods are a calculated fixed resistance or a constant current driver
Personally I fitted four 3W circular LED’s in series with a wire wound resistor on a 4S battery
I finalised resistor value by deciding how hot I wanted them to run whilst still not exceeding the maximum specified current.
Hope this helps
Steve

So I can’t plug this straight into a 3S battery’s balance lead using jumper cables?

I don’t have any spare resistors. Can I plug this straight into an 3S LiPo that can discharge basically unlimited (60 amps) current?

Hi @RobbieGM
Just seen your posts of last few days.
I’ll have a look over the weekend and recommend a resistor for you
Where are you based? in UK?
Cheers
Steve

US. I’d like to do as little soldering as possible, but I will if I have to.

A little while ago sparkfun posted a great quick tutorial on how to calculate what resistor you need to limit the current through your resistor. But apart from the resistor value its also important to check the power rating (Wattage) of the resistor to make sure it will not fry. As this is particularly true for high power LED’s like the one you suggested.

To find out what resistor you need you need to know the supply voltage (battery voltage) and the voltage drop (but in this case hobbyking dont provide that on the datasheet) so we will just have to ignore this for now. By ignoring the voltage drop it just means you will over spec the system which is fine as its better to be safe than sorry

So to find what resistance we need we use: $$R=\frac{V_s-V_f}{i}$$
where, R = Resistance
i = LED forward current in Amps (found in the LED datasheet)
Vf = LED forward voltage drop in Volts (found in the LED datasheet)
Vs = supply voltage

which can be simplified to $$R = \frac{V_s}{i}$$

In your case $$R = \frac{7.4}{0.03} = 246 \Omega$$

So you round this up to the nearest resistor you can find which would be 250 ohm. Since the LED bar is 5W, then [this Resistor off ebay should work.

https://rover.ebay.com/rover/1/711-53200-19255-0/1?ff3=4&toolid=11800&pub=5575243709&campid=5337987456&mpre=http%3A%2F%2Fwww.ebay.com%2Fitm%2F5-250-Ohm-10W-Watt-Axial-Ceramic-Cement-Resistors-5-Pcs-YM-%2F172386240709

Thank you! Will I need to solder this? I assume I will but I’m not sure.

Hi
Yes, you will need to solder
This style is easier to mount by bolting it down, however it depends on your layout, @Tropnevad suggestion may suit you better

http://uk.rs-online.com/web/p/panel-mount-fixed-resistors/8916445/

Good luck, don’t burn your fingers
Cheers
Steve