There are 2 ways to go about wiring LEDs: series and parallel. In most cases, if you have a constant current driver, you’re going to want to wire them in series. If you’re using a constant voltage driver, chances are, you’ll be wiring in parallel. You may even need to combine both methods in order to hit a certain voltage or current to match to a particular driver (see my post on matching COBs to drivers for more info on this). This information applies to LEDs of all types, whether they’re COBs, boards, strips, or whatever.
Wiring LEDs in Series
Series wiring is used most often with constant current drivers. When you wire in series, you add the forward voltages of each LED in the circuit but the current fed to each LED remains the same. If you have 3 LED COBs, each with a forward voltage of 36V at a given current, when you wire them in series, the total voltage drop of the circuit becomes 108 Volts. If, for example, your LED driver produces 1400mA of current within a voltage range of 100-150V, then as long as your total voltage drop of your circuit is within the 100-150V range (our circuit of 108V would work), then all 3 of these COBs will receive the full 1400mA.
Here’s what 3 COBs wired in series looks like:
If you look closely, you can see that each COB has 2 contacts; one has a “+” sign to indicate it is the positive side, and the other is unmarked, which is negative. To wire in series:
- Take one of the leads from your LED driver (it doesn’t matter which polarity) and wire it to the corresponding input of your first COB (e.g. – positive lead to positive input OR negative to negative input).
- Take the other LED driver lead and wire it to the corresponding input on the last COB in the chain.
- In my example above, the 2 white wires are the leads from my LED driver. I have wired the positive driver lead to the positive input on the left COB, and wired the negative driver lead to the negative input on the right COB.
- Now, all you need to do is connect the first LED in the chain to the last LED in the chain, by interconnecting the positive and negative terminals of each LED in between. If you started by connecting the positive side of the LED driver to the first COB like me, then you will wire the negative side of the first COB to the positive side of the second COB. It seems counter-intuitive to wire a negative to a positive, but this is how series wiring works. Once this is complete, continue wiring the negative of one COB to the positive of the next, until you reach the end of the line, where your other LED driver lead is connected.
Wiring LEDs in Parallel
Parallel wiring is most often used when working with constant voltage drivers. A lot of people are now using constant voltage drivers and wiring up their COBs in parallel, since the drivers are usually cheaper and people are more comfortable working with low voltages like 36V, as opposed to high-voltage series circuits that can be 200V+. One drawback is the fact that wiring COBs in parallel does make them vulnerable to thermal runaway.
Thermal runaway refers to the process that occurs when a COB heats up, causing it to draw more current, which heats it up further, drawing even more current, and this loop continues until it destroys itself. Unless you implement something like a resistor to limit the maximum current, there’s nothing stopping the COBs from pulling as much current as the driver will provide if the COBs go into thermal runaway, or the voltage output of the driver rises. That being said, in my own testing, current levels have always stabilized at reasonable drive currents and I have only seen thermal runaway occur at very high currents that nobody is going to run at (3+ amps per COB!).
Now, when you wire in parallel, the forward voltages of each LED no longer add together like they do in series. If your driver outputs 36V, then every single COB that you have hooked up to it in parallel will have that same 36V across it. Instead, the current is what becomes split among the LEDs in a parallel circuit.
For example, your COB data sheet may tell you that when you apply 36V, each of your COBs will pull about 2,400mA of current. So, if you have 2 of these COBs on a 36V constant voltage system, your driver would need to be able to supply at least 4.8A of current. If it can do more, that’s fine – the COBs will only pull what their particular unique current-voltage curve dictates, depending on what voltage you run them at. They may each pull 2,400mA when you put 36V across them, but jump to 2,700mA each when you put 36.5V across them.
You can also wire COBs in parallel on a constant current driver. When you wire in parallel on a constant current driver, you don’t have to worry about the COBs pulling any more current than the driver is rated for, but the current will not necessarily be evenly split among the COBs. You could have 2 identical COBs in parallel on a 700mA constant current driver, and one COB could be pulling 500mA while the other only pulls 200mA, due to small differences in the composition of the LEDs in each COB. See my post on constant current vs. constant voltage for a more in depth look at this.
To wire in parallel:
- Instead of creating a long single chain of COBs, you simply wire all the positive sides together, and all the negative sides together.
- In the picture above, I’ve wired the positive and negative leads of the driver to the first COB on the left, and then connected them to their respective polarity down the line to the last COB.
- Below is a paint schematic of a parallel COB circuit. I won’t be doing graphic design anytime soon.
Combining Series and Parallel Wiring
There may be instances where you need to combine series and parallel in order to match a certain number of LEDs to a driver properly. Generally, it’s better to just add more drivers to keep things simple and in series, but if need be, you can work some magic to make what you’ve got work.
If, for example, I wanted to run 8x CXB3590s (36V) on my driver that’s rated to do 1400mA between a voltage range of 71V and 143V, I could not hook them up in serial. Hooking all 8 up in serial would give me a total voltage of 288V, which is way out of range. What I could do, though, is wire 2 strings of 4 series-connected COBs in parallel. Each string would have a voltage of ~144V (a little less due to the low current) and would draw 1400mA of current. If these 2 strings were then wired together in parallel, the voltage of 144V would remain the same, but the current of 1400mA would be divided among them, giving each COB in each string 700mA. Check out my miserable paint depiction of this type of circuit below:
And that’s the basics of wiring LEDs. As always, if you’ve got any questions or comments, please share them!