current-limitingvoltage
I'm trying to limit a current of a 5V power supply to 100uA. I was first thinking of putting a 50k resistor. That will never allow above the treshold.
However, the load requires at leas 4.5V and varies in the amount of current in needs. So this is obviously a too simple approach for the problem.
How can I extend this circuit to limit the current and keep the load voltage current independent?
Have you tested the two alkaline batteries? I'd bet they are slightly above 1.5V each, bringing your total voltage up to the LED forward voltage. However, when the LED starts conducting, its low impedance pulls the battery voltage down slightly, and the battery itself acts as the current limiting resistor.
As to the second part of your question, I can't say whether its always better to add the resistor and use more cells. If you're making an LED throwie, then it definitely isn't. If you want a consistant runtime that doesn't rely on hitting a bullseye between battery voltage and current draw, then probably.
However I'd like a solution that can sustain 4.5V regardless of the current (within the limits above)
If you are happy with a constant voltage of 4.5V then you should consider a low drop out voltage regulator like this: -
It can be setup to deliver up to 4.5V and at 1A to the load and will only need 4.645V (typically) on the input to sustain the output voltage. Note that it needs a minimum load for it work correctly (1mA) but this shouldn't be a problem given your requirements.
This is typical of a series of many voltage regulators with low drop-out voltage - I'm not saying use this one - I am saying be aware of what TI and other folk may be supplying. It's likely you can find one with current limit circuits built-in. You could also apply a bit of current limiting in this device by having a resistor in series with the voltage feed - this should be chosen to develop just enough voltage across it (at the required load current) to sustain the output. Should load current increase, output voltage will drop.
Best Answer
Presumably you are okay with a small drop, provided it is less than 0.5V.
Here is a circuit that will limit at 100uA and will drop less than 100mV before it limits.
simulate this circuit – Schematic created using CircuitLab
The MCP6001 is an inexpensive rail-to-rail input/output op amp that will operate from a 5V supply. The op-amp will saturate at ground until the load current reaches about 98uA nominally (with the values shown). The supply thus 'looks like' 5V with ~1K in series (the MOSFET contributes less than 10 ohms with Vgs =-5V), so it will drop between 0 and 100mV for load resistances of infinity down to 50K.
For lower load resistances the circuit regulates the output current to ~98uA.
The circuit draws about 200uA from the 5V supply, in addition to the load current of 0~100uA.