Hello everyone. This is my first post and I’m an absolute electronics newbie looking for help!
I just received the DFROBOT FIT0710 50cm LED Strip purchased from this site. The specs tell me it’s a 5 volt device and consumes 6 watts from the 160 leds in the strip. From my basic understanding that means it draws 1.2 amps. There’s no mention of current limiting resistors so I don’t know if they need to be added or if they’re already incorporated into the strip. What would be the best way to work that out? I don’t want to power it up and blow them.
Additionally, I’m planning to cut the strips into 5cm segments which will give me 16 leds per segment. As above, if they need current limiting resistors I would need to know the forward voltage of the leds to work that out but I can’t find any data on that. Can anyone assist?
Welcome to the forum
That’s a great question in most cases LEDs do need current limiting resistors as it’s exceeding the current limit that damages them. Michael recently put together a guide that goes over how to calculate these values in your circuit.
There are a few exceptions to this, some addressable RGB LEDs (like neopixels) actually have a small constant current driver circuit built into them that ensures the current stays at the correct limits.
This LED strip should have the current limiting resistors built-in, I don’t suspect it has many small current driver ICs but it may, in either case, you should be fine without adding extra current limiting provided you keep to 5V operating voltage.
The DFRobot web site says that this strip can be cut every 5cm (another part of the page says it can be cut into 2.5cm pieces) with no mention of current limiting resistors.
So I think one could assume the resistors are part of the strip.
You may have to cut a 5cm piece off and try it. It may be an advantage to have a variable bench supply to do this initially.
Thanks for the welcome guys and thanks for your reply and video tip, it was most informative.
I put a 18650 Li-Ion cell (4.2v) across the strip and it lit up nicely and stayed lit with no smoke so that’s a good start. I then meaured the current with my multimeter whch showed a draw of 0.76 amps. The strip has 160 leds so if I understand correctly that means each led draws 4.75mA. seems low for white leds that are quite bright.
I think the current through the LEDs is pretty non linear. The difference in current between your 18650 cell and 5V could seem to be spectacular and easily become 1.2A. This is partly due to diode non linearity and the voltage drop across the diode remaining pretty constant while the voltage drop across the resistor has to be more significant and has to change by a fair percentage.
Current measurement at these lower voltages should be done using the 10A range which I think you have done as the internal meter resistance of the lower ranges is high enough to play a significant part in current limiting (voltage burden). This resistance will be effectively in series with the circuit under test.
Thanks Bob. Your explanation is above my electronic knowledge pay grade at the moment but I do appreciate your reply.
Ahh, now I see what you mean about non linearity
That diagram captures the challenge well and also illustrates why current limiting resistors are often the go-to solution to keeping LEDs in the sweet spot of their operating range. It’s not the most power-efficient design, but it’s a very straightforward way of capping the amount of current our LEDs will draw.
Thanks @Robert93820 for explaining the problem succinctly, non-linearity is a tricky one to get into for beginners.