Alan,
Isn't that misleading? Both globe and LED brightness are determined by the voltage applied and current drawn. Change either and the brightness will change for both of them.
- Tim
Yes and no. You are thinking of the Power formula P = V * I. An increase of V or an increase of I or an increase of V and I will result in an increase of P. So in that sense you are absolutely correct. However in a specific device or circuit that may or may not be true. Take for instance the LED drivers referenced in the reply to Chris. Increasing applied voltage to them does not increase amperage in the circuit.
In this thread's specific instance, a bulb's luminosity is related to the applied voltage whereas LED brightness is related to current flow.
The current flow through a bulb is fixed by resistance of the filament at a given temperature. Raise the voltage and you raise the temperature making the bulb brighter but also increasing its resistance which in turn limits current flow. If the bulb were burnout proof at some point raising voltage would not cause more current to flow because the filament temperature induced resistance will have risen to a point that prohibits doing so. Since the resistance is internally fixed by the filament size, the temperature (and resultant resistance) is a function of applied voltage, not of available current. That is why bulbs are commonly referred to as a voltage controlled device. BTW the basis for folks using 1156 tail light bulbs for over-current protection.
The current flow through an LED is not fixed whatsoever hence why an external means of limiting current is always required. There is no limit, the bigger the power supply you have the more current you can pass through an LED. At least for a very brief moment!
The only voltage relevant to an LED is the voltage drop across it. You can operate an LED on any voltage you wish so long as the proper current limiting resistor is used. Because an LED is not voltage specific but is current specific they are referred to as a current controlled device.
As for the LM317 circuit, it's been a long time since I looked at one of those but given that it provides a fixed voltage to the output regardless of the voltage applied to the input wouldn't you classify it as a constant current device (for our purposes at least), given that whatever is connected to it (globe or LED) being given a fixed voltage would draw a fixed current? I know it's not a true constant current device but still.
- Tim
In our particular instance the bulb resistance does not vary and the LM317 voltage does not vary so yes, the current remains the same. But that is not the definition of a current controlled device or circuit. Virtually all devices/circuits would qualify as current controlled if we apply that definition. A current controlled device/circuit is defined as one that maintains a fixed current as voltage and/or load varies. Just as a voltage controlled device/circuit is defined as one that maintains a fixed voltage as current/load varies.
The LM317 in and of itself does not have the ability to limit current. That is dictated by the resistance of the load. For instance, if you short together the leads of a LM317 the power supply's full current output to flow through the the device. The resulting puff of smoke proves it is not a current limiting device. Conversely, measuring the output voltage while placing differing loads on the LM317 you see the voltage does not change with load proving it is a voltage limiting device.
So, you are right with respect to the state of any device/circuit at a given moment in time but, with all due respect, wrong in definitions.
