Electronics > Beginners
Back before basic (voltage or current)?
FriedMule:
I think I need some totally basic teaching here about what voltage vs current, I know that Voltage is the potential and current is the electrons but here is my lack of knowledge:
Lets take a standard LED, I have seen some dimmer's change the voltage and other changing the current to dim the Led. So is it the resulting just the wattage that decides it's brightness or how do you know what to adjust to dim?
When do you adjust the one instead of the other in other circumstances?
magic:
Well, I can imagine there being two separate questions.
1. Which quantity is tighter correlated with brightness. I'm no semiconductor physics wizard but I suspect it's the current and for a given diode there is some more or less constant probability that each passing electron ends up emitting light rather than dissipating its energy as heat. Voltage OTOH seems more closely related to energy of each emitted photon and therefore wavelength/color. Of course that's a difference between various LED types, I don't think changing voltage applied to one diode has any effect on color.
2. Which quantity is easier to control. For LEDs it certainly is current. If you set constant current, voltage will also remain almost constant and so will wattage. And experience tells that constant current drive is effective at achieving reasonably stable brightness.
On the other hand, minor variation of voltage causes significant change of current, power and brightness. Voltage drive is harder to adjust and harder to maintain over time.
Therefore pragmatism suggests current regulation for LEDs.
FriedMule:
--- Quote from: magic on October 01, 2019, 07:14:31 pm ---Well, I can imagine there being two separate questions.
1. Which quantity is tighter correlated with brightness. I'm no semiconductor physics wizard but I suspect it's the current and for a given diode there is some more or less constant probability that each passing electron ends up emitting light rather than dissipating its energy as heat. Voltage OTOH seems more closely related to energy of each emitted photon and therefore wavelength/color. Of course that's a difference between various LED types, I don't think changing voltage applied to one diode has any effect on color.
2. Which quantity is easier to control. For LEDs it certainly is current. If you set constant current, voltage will also remain almost constant and so will wattage. And experience tells that constant current drive is effective at achieving reasonably stable brightness.
On the other hand, minor variation of voltage causes significant change of current, power and brightness. Voltage drive is harder to adjust and harder to maintain over time.
Therefore pragmatism suggests current regulation for LEDs.
--- End quote ---
Really great reply, thanks! :-)
1) So current adjust the amount of brightness while to little volt just vont make the LED turn on?
2) Always aim for constant Voltage and then make the Current change, because it's easier and more stable?
Is that a fair conclusion from your answers?
TimFox:
For LEDs, it’s better to control the current (required to get the brightness), then make sure that the current source can supply enough voltage (technically the compliance voltage) for the LED at that current.
MosherIV:
--- Quote ---Always aim for constant Voltage and then make the Current change, because it's easier and more stable?
--- End quote ---
?
Ohm's law applies. V = I × R
If the voltage is fixed and you want to change the current, then you HAVE to change the resistance.
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