hiya all,

I am Rebekah Anderson. I am a RC truck modeller.

Currently I am working on a project which has prompted me to try electronics. As it is 20 years since I did electronics at school I'm a bit stuck.

The circuit I am building is a 20 led (1.8mm) strip in parallel working off 6-12v
I figured out the resistor to be 10 ohms (please correct me if I am wrong)
I am also adding an LDO but not sure how to set up the circuit.

I am hoping that one of you guys can help me with a circuit diagram and help me understand it so I can learn and apply for future project.

This is the circuit I etched. It's going to be redesigned but shows the lighting arrangements which is what I wanted to show here. The rest is open to redesign.

All advice is welcome

Regards

Becky

 

Welcome to the forum!
What colour LEDs (different colours have different Vf values) and what is the maximum current rating of each LED?
Since you mention redesign, have you considered connecting the LEDs in groups in series (say 3 or 4 LEDs in series, depending on type)? That would be more power-efficient than having them all in parallel. LEDs connected directly in parallel can be problematic, since their individual Vf values will be different (even if from the same manufacturing batch). This results in some passing more current than others; hence having different brightness and the possibility of a domino-effect failure mode.

 

Hello,

Can you post a schematic of your intended circuit?
It is not a good plan to have the leds directly parallel.

Bertus

 

I'm open to suggestions.
Here are the details

 

I don't have the schematic to hand but the PCB in my original post should give you an idea.

 

It looks like you have 20 LEDs connected in parallel and sharing a resistor, is this correct? if so your circuit will not work properly. Also 10 Ohms it too small a value especially when you have a supply that can range between 6 and 12V.

If you have a pen and paper, please draw a quick schematic.

 

Worst drawing ever I know but I'm no expert. Couldn't draw the symbol for the LDO.

 

Ok, you've missed a wire of the middle 12 LEDs but either way; this circuit won't work i.e. some LEDs will be too bright and may fail, others may not light at all, until the others fail then they will receive all the current and fail as well.

If you need the circuit to operate between 6 and 12V you will require a resistor per LED because the variance in current for low value resistors will be too great over this voltage range. What colour LEDs are you using?

 

White LED.
It did work fine till I burnt out the resistor because I choose the wrong one (1 Ohm) lol

 

White LEDs have a forward voltage of around 3.5V, so with a 12V supply 8.5V will be dropped across each resistor and each LED requires 20mA. 8.5/0.02 = 425 Ohm, the nearest standard value is 430 Ohm per LED. This means that at 12V each LED will get 20mA but at 6V each LED will only get around 6mA. But this is OK for you application.

With your single 10 Ohm resistor the total current at 12V will be 8.5/10 = 850mA. If this were equally shared by the LEDs (which it wont be) 850mA/20 = 42.5mA per LED which is more than twice their rated current and therefore destined for failure.

 

So if I had a 25 Ohm resistor then that would 17 mA per LED?

 

No. Since all LEDs have slightly different forward voltages it is impossible to say what the current per LED would be. This is why LEDs should never be connected in parallel.

 

So what is the minimum amount of components I can use for this circuit?

And I have to fit them in series.

 

What about the low drop output, pointless?

 

For your supply voltage range you need 1 x 430 Ohm resistor per LED. That is all there is to it, I cannot make it any simpler.

You cannot connect even 2 LEDs in series because they would require a forward voltage of 3.5V + 3.5V = 7V and your supply can be as low as 6V.

 

No. Since all LEDs have slightly different forward voltages it is impossible to say what the current per LED would be. This is why LEDs should never be connected in parallel.

i've done it, absolutely no variation from a LED bag

 

i've done it, absolutely no variation from a LED bag

Good for you. It must therefore be true for all LEDs.

 

Good for you. It must therefore be true for all LEDs.

it is for most modern LED.

you can look a 10W or 20W power LED they are parallel too.

 

it is for most modern LED.

you can look a 10W or 20W power LED they are parallel too.

I don't understand your point. Are you recommending that the OP uses 1 resistor for multiple parallel LEDs? If so why?

Resistors are so cheap it costs virtually nothing to add one per LED and ensures that all LEDs receive the correct current.

I have nothing more to add to this thread.

 

I'm confused.
According to post #4

So the LED is a flashing type, emitting R,G,B and Y light (sequentially?). But in post #9 you say it is a white LED.