You can drive LEDs or COBs with a constant voltage supply using a very simple constant current circuit on the output. This won’t be as efficient as buying a constant current driver, but it can be VERY cheap and easy. The simplest such circuit consists of one resistor!

Say you have a constant voltage power supply that puts out 12 volts. You have some LEDs that run at 2.5 volts at 700 ma. If you connect four in series, that will total 10 volts. So your series resistor would have to drop 2 volts at 700 ma. Ohm’s law says E / I = R, so 2 volts / 0.7 amp = 2.86 ohm. You can’t get a 2.86 ohm resistor, but Digikey has 3 ohm 2 watt resistors for 29 cents.

So lets see how the circuit works with a 3 ohm resistor. Let’s assume the current stays the same, for now. At 700 ma a 3 ohm resistor would drop E = I x R volts, so it will drop 2.1 volts. The LEDs will get 9.9 volts at 700 ma. Pretty close! Actually, the LED current will drop a bit so this is just approximate. Notice that I pick the next higher resistance you can get, so the current would not go up.

The resistor power dissipation would be P = E x I so that is 0.7 x 2.1 = 1.47 watts. This is why I specified a 2 watt resistor. If you use a lower rated one, it would burn up.

Now lets think about efficiency: If you are burning up 2.1 volts out of 12 volts total in your resistor, the efficiency of this circuit is 1 - (2.1 / 12) = 0.825 or 82.5%. But you also have to count the efficiency of your 12 volt power supply. A very good switching supply might have 95% efficiency, so the combined efficiency is 0.95 x 0.825 = 78.3%. You see why people pay a lot of money for a Mean Well constant supply with 95% efficiency!

Because of the low efficiency when you burn up a lot of power in your resistor, you want to run as many LEDs or COBs as you can with the fixed voltage. But you always need at least 10% of your power burned up in the resistor for the sake of stability. If you operate with too little power in the resistor, this circuit can be very unstable, because LEDs or COBs can change their current draw a lot with a small voltage or temperature change.

Don’t forget to use a good heat sink. Power LEDs or COBs MUST have a good heat sink or they will get too hot and burn up. It’s a good idea to measure the heat sink temperature when you first turn it on, and for the next couple of hours. You want to see where it stabilizes. 85 C is probably too hot!

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