Voltage Controlled Resistor

[TheKrieger]

I'm designing an embedded system with 4 parts:
-Power supply from a 6V battery.
-6V solenoid.
-5V uController.
-A wireless receiver (Haven't decided which one to use yet).

Here's the problem: The uController can only operate up to 5.5V (and it's not recommended to do so); so I need to lower the supply voltage to a typical 5V. I tried using a Zener diode with a resistor to obtain 5V since a regulator is useless without a higher input voltage. However, the wireless module won't be always on and the current consumption varies greatly, whenever the current consumption drops, it ends up either as a voltage rise or being dissipated as heat by other resistors (Which makes me waste precious battery charge ); if the current consumption rises, the voltage drops.
In other words, what I need is a way to automatically regulate the input current. I immediately thought of a variable resistor which could be made with FET's, PIN diodes or a photoresistor, however, I do not know how to decrease resistance when voltage drops, but keeping voltage within 5V.

What's the most effective approach to solve this problem? The less components needed the better.

Hope anyone can point me in the right direction. Thanks,

 

[jackorocko]

http://www.ti.com/lit/ds/symlink/lm2937.pdf

LDO voltage regulator, first graph on page seven. 5.5Vin before the output voltage drops below 5V. What is the lower limit on the input to the uC?

 

[CocaCola]

Just use one or two series diodes to drop the voltage bellow 5.5 no resistor needed...

 

[TheKrieger]

Jack: the uController lower limit is not a problem since it can work with 2V. However, it controls some transistors and I require 5V. I'll look unto the LDO.

CocaCola: But as I mentioned earlier, if the current is fixed: I consume power whenever I don't need it and Voltage drops when I do.

 

[TheKrieger]

http://www.ti.com/lit/ds/symlink/lm2937.pdf

LDO voltage regulator, first graph on page seven. 5.5Vin before the output voltage drops below 5V. What is the lower limit on the input to the uC?

Jackorocko, I think this may be exactly what I am looking for. Thanks a bunch! I had no idea those regulators existed; even though I searched the web for hours. I will get some of those IC's and later debrief the result. I love this forum. Once again, thank you!

 

[CocaCola]

CocaCola: But as I mentioned earlier, if the current is fixed: I consume power whenever I don't need it and Voltage drops when I do.

Not sure what you are saying, the voltage drop across just the diode is pretty consistent you are not fixing a current just dropping voltage...

However, the wireless module won't be always on and the current consumption varies greatly, whenever the current consumption drops, it ends up either as a voltage rise or being dissipated as heat by other resistors (Which makes me waste precious battery charge ); if the current consumption rises, the voltage drops.

With just the diode you won't see that swing, as it's minimal across the entire usable range of the diode and for most purposes can be assumed consistent except in extreme low current (a few 1000 µA or less) or huge current applications... Certainly consistent enough for this application where the devices won't need a perfect exact voltage and the current should all be well within a minimal shift in dropped forward voltage...