LED Schematic Question

mjh36

mjh36

Well-known member
It's about how some schematics have a resistor before or after the LED. I don't understand the science of why that is. Why on one where the resistor is after the LED, it doesn't blow up? Example here is the Complex Overdrive and Danube Fuzz:

Danube
20210417_142125.jpg

Complex
20210417_141911.jpg
 
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Remember the resistor is used to limit the current running through the LED. Think of the Led and resistor as being in a closed loop circuit. As long as the resistor is somewhere in that circuit, Ohms law states Voltage = Current x Resistance, so at a constant voltage if the resistance goes up the current must decrease. I struggled with this myself in the beginning. hope This helps.
 
TGP39 said:
Remember the resistor is used to limit the current running through the LED. Think of the Led and resistor as being in a closed loop circuit. As long as the resistor is somewhere in that circuit, Ohms law states Voltage = Current x Resistance, so at a constant voltage if the resistance goes up the current must decrease. I struggled with this myself in the beginning. hope This helps.
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Ahhh i get it now. I always kept thinking one had to "protect" the LED with resistance before it. Would there be any other reason to use one way or the other? Builder's preference? Layout? Even though both ways work, I wonder why there isn't like a "standard". I wonder a lot. 🤔
 
The orientation depends on the layout of the PCB.

If the LED is at the top of the board the resistor usually comes after the LED, because the LED connects directly to the + pad.

If the LED is at the bottom of the board the resistor usually comes before the LED, because the LED connects directly to the SW pad.
 
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