Hi All,
I have a bright white LED that takes 300 mA with a forward voltage of about 3.4 V. I'm powering it from 3 AA cells, and it's clear that a series resistor is not good enough as a regulator: choosing a value to suit 1.5V per cell, I only get half the current once the per-cell voltage has dropped to 1.3V.
So I'm looking for a current-regulator that will give better regulation than this - but it doesn't need to be anywhere near perfect, and simplicity is vital. The low dropout requirement is the main challenge: most circuits that I can think of need a series current sensing resistor, which needs to develop a significant voltage. I'm currently experimenting with something like this:
Vin --------------------+ | | / _|_ \\ \\ / ->
/ _V_ ->
| | | |/ +-------------| |\\>
| GND --------------------+
This gives enough regulation - though a bit more would be nice. Its main deficiency is that it's sensitive to the gain of the transistor. So:
- Are there any tricks to reduce the sensitivity to the gain of the transistor, that don't involve an additional series voltage drop?
- Is the FET equivalent of this circuit any less sensitive to device variability?
- Are there any particular transistors that have more repeatable gain in this sort of configuration? (I'm currently using a ZTX349A, because it was handy.)
- Any better circuits?
Cheers, Phil.