voltage-regulator
I'm using a AMS1117 fixed 3.3V linear regulator and I want to increase its output voltage to about 4V. The input voltage to the 1117 is in the range of 9-10V.
Wikipedia suggests that it's possible to put a zener or resistor on the ground terminal of the regulator to change its output voltage but it provides no citation or reference. Anyone know how this works?
Integrated DC-DC buck regulators for 12 Volts 1 Ampere do exist, from manufacturers like Recom, R-78C12-10. These are drop in replacements for the popular 7812 type linear regulators - thus, totally integrated packages, as desired.
Yes: If your load is pulling 1 Ampere at 12 Volts with 22 Volt source, the regulator is dissipating (22 - 12) * 1 = 10 Watts as heat.
If desired power quality is not met by the switching regulator chosen, a hybrid approach, such as using an adjustable integrated switching regulator set to 14 Volts or so, followed by a low drop-out linear regulator, would give the best of both worlds - low heat, yet low ripple.
If I understand you correctly, you are trying to adjust the output voltage of your 7805 regulator to output 5V - 10V by adjusting the voltage at it's ground pin between 0V and 5V.
If you are doing this, the pin needs to be able to sink current in order to maintain regulation, so whatever the power source used, it needs to be capable of this. Many supplies will only source current, and if this is the case what you are seeing will occur.
A simple resistor divider with potentiometer can be used, or an opamp to drive the 7805 ground pin is maybe the best solution, preferably with a output including its negative rail if you want to go all the way down to 5V. You will find many example adjustable supply circuits on the web, and in the datasheets of the regulators also.
Here are a couple of example circuits:
Pot adjust, from this page with more info:
Opamp adjust, from datasheet pg.24 (don't use a 741 if you can possibly avoid it, as they are obsolete):
Best Answer
Fairchild datasheets for their LM78XX series of linear regulators contain a circuit for increasing the output using a resistor to hold the reference above ground. I guess how well it works in practice will depend on how stable the quiescent current is over the operating range for the AMS1117.
The above image was from the Fairchild 7805 datasheet that may be found here:
http://www.fairchildsemi.com/ds/LM/LM7805.pdf