Electrical – Amp draw of multiple chains of LEDs

amperageled

Can someone help me work out total amp draw for LEDs with an in-series resistor?

I am getting conflicting answers depending on where I look.

  1. One way tells me the amp draw is the total amp draw of all LEDs combined, same as supply voltage.
  2. Another way seems to indicate that the total amp draw is singular, as in 3 LEDs with an in-series resistor (each LED being 20mA) will still only be 20mA.

EG:

12v + —->|—->|—->|—///—- –

12v DC
LED = Vf3.8/If20mA
R = 33ohms

Would the amp draw be 20mA or 60mA?

Further on from that, for multiple chains like that running from the same source, EG, 20 chains in parallel of x3 LEDs (each with in-series resistor),
400mA or 1.2A?

Hopefully I have explained my question right, but probably got some vernacular wrong, so I apologise in advance.

Thanks

PS: Apologies for the Resistor not showing correctly, having trouble getting the ASCII to display correctly

Best Answer

Lets look at your setup with voltage vectors..

schematic

simulate this circuit – Schematic created using CircuitLab

Since, NOMINALLY, you have three 3.8V forward voltage drops in your chain, that leaves only 0.6V remaining from the original 12V across your resistor. The current in the chain is therefore 18.182mA. That current is common to the entire chain.

If you have N chains the same in parallel the total current will be N times 18.182mA.

HOWEVER:

This simple math does not take into account any variance in the forward voltage of the LEDs or any variance in the supply voltage. Further, the forward voltages will change with temperature.

The forward voltage of LEDS can vary significantly. If the forward voltage is one single point less at 3.7V the current will be 27mA, If it drops to 3.5V the current will be 45mA... and on and on.

That variance can mean your actual LEDs may burn out, or have a short life with this setup.

LED strings are best driven with a constant current source, not a voltage source. If that is beyond your capabilities I suggest you reduce your strings to two LEDs per string and use a larger resistor, 220R, to soak up the variance in the LEDs.

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