series vs parallel

[john2k]

Hi guys,

I'm slightly confused about the series and parallel wiring of led's. From what I understand so far, a series circuit will divide the overal voltage by the number of LED's and hence have a smaller amount of voltage per LED, whereas the parallel will give the same amount of voltage to each led. What about the current? if I have lets say a 12V 280mA constant current driver that delivers 280mA constant current, I am assuming that on a series the 280mA will be shared out and that in a parallel all led's will get the 280mA of current?

I have an LED unit which has 4 led's inside and the unit is rated at 8W 12V and the LED's are wired in parallel. Because they are wired in parallel does that mean each LED will get 12V? I want to under-drive them to make them less bright as they are way too bright. So i've connected up my 12V 280mA constant current driver which seems to power them up and they seem much less brighter and work fine. But I am just not sure exactly how it is working and if it's correct. Are each of the 4 LED's in the unit receiving 280mA at 12V individually? that seems quite high but yet the led's are being under-driven because the output brightness is far lower than originally.

any advice would be very much appreciated.

thanks

 

[Yamabushi]

Series divides the voltage. Parallel divides the current.

 

[john2k]

Series divides the voltage. Parallel divides the current.

thanks for your reply. very much appreciated. that makes sense :)

My constant current driver delivers 280mA but the output voltage can vary, does this effect led's wired in parallel? my led unit is rated at 8W 12V but if i'm driving it with lets say a constant 280mA current and a fluctuating 12v-14v voltage will that effect the LED's or will the voltage fluctuation not effect the LED's since the current is limited anyway?

 

[Lou Minescence]

Wikipedia.

 

[Mr Happy]

I have an LED unit which has 4 led's inside and the unit is rated at 8W 12V and the LED's are wired in parallel.

Tell us more about this "LED unit"?

There is no such thing as a bare LED that runs from 12 V. Is this an automotive lamp fixture, perhaps?

In general, if you have a 12 V lamp fixture, then series/parallel is irrelevant. Whatever is inside the fixture is inside the fixture and you need not care.

If you have bare LEDs then you do care, but if you have bare LEDs it will not be "rated at 8 W 12 V". That is not how LEDs are rated.

If you have a 12 V fixture and you limit the current supplied to it, you will probably reduce the brightness. You might also reduce the brightness more simply by reducing the voltage supplied to it.

Everything depends on the details.

 

[john2k]

Thanks for your reply Mr Happy, yes the unit is an automotive DRL unit.

The unit has a sticker on it that says 8W 12V. I had a spare unit to play with so I opened the sealed unit up to see how it is wired and how it looks inside. Inside the unit there are 4 high power LED's that look like the luxeon LED's, they are soldered to a heatsink plate and wired in parallel.

The LED driver I have been using which has been working fine has a label that says input voltage is DC 7V-24V and output voltage is DC 3V-21V and a constant current output of 280mA.

Now from what I understand so far, if lets say each of the 4 led's have a 3V forward voltage, then if they were wired up in series, the driver will need to deliver a 12V voltage to them because they will all divide the 12V amongst them and use the 280mA per LED. Whereas since my unit is wired up in parallel I am assuming the driver will deliver only 3V because each of the LED will actually get 3V. Is this understanding correct?

I'm sure if the driver was giving too much voltage to the LED's they would have blown by now surely? i have been using this driver for few months now but need to understand if it's all correctly done and how exactly it is working.

Is there any way to use a multimeter to actually see how much voltage the driver is delivering to the LED unit itself? because if i hook up the multimeter to the output of the driver it says 12V, should I connect it to the LED unit first and then measure the voltage?

Many thanks for your help guys.

 

[Mr Happy]

Now from what I understand so far, if lets say each of the 4 led's have a 3V forward voltage, then if they were wired up in series, the driver will need to deliver a 12V voltage to them because they will all divide the 12V amongst them and use the 280mA per LED. Whereas since my unit is wired up in parallel I am assuming the driver will deliver only 3V because each of the LED will actually get 3V. Is this understanding correct?

No, it is not correct. The LED driver will need to deliver approximately 12 V because the light unit is marked "12 V" and is designed to be connected to a 12 V automotive supply. If you connected it to 3 V it wouldn't work.

If you wired 4 LEDs in parallel and connected them to a 12 V supply they would blow up instantly. There is a missing element here, such as a resistor or a current regulator inside the light unit that you have not seen or described.

As a 12 V 8 W unit, the design current should be about 0.67 A. If you feed it from a constant current supply and give it only 0.28 A then you will throttle the power input and reduce the brightness. In that case the operating voltage will be a bit less than 12 V, but it is hard to say how much. You would have to measure it with a multimeter to find out.