Saturday, April 30, 2011

Voltage Division with coarse and fine adjust

In designing the Voltage source of the last post, I originally wanted to have 2 adjustment knobs, one coarse and one fine. I concidered designs using two linear potentiometers but the fine adjustment wasn't as I wanted it to be. I wanted it to always provide a uniform range +- around the coarse setting ( I call this +- range delta V). It didn't provide this, instead it would give a large delta V when the coarse was set at one extreme and zero delta V at the other. Circuits A and B below demonstrate this effect.
I then discovered that by using a standard linear pot for the coarse adjustment and a dual gang linear pot for the fine adjustment my desire could be accomplished as represented in Circuit C.

Equations for Vout/Vin for the three circuits and corresponding plots with R1=5k, R2=100k, R3=20k are shown below. I've introduced variables x and y to represent the fraction of R2 and R3 which are enabled by the pot knob and can take values from 0 to 1. The plots show the extremes of the fine pot (y=0, y=1) for all values of x. The size of delta V can be set primarily with the value of R3. I do like this design but didn't end up using it because I was impatient and didn't have immediate acces to 5k dual gang pot. Maybe next time!

Tuesday, April 19, 2011

Variable DC Voltage Source

I wanted to have a DC voltage source for home projects and I decided to make one. I cheated and used a "wall wart" transformer which I picked up at the Goodwill. This takes care of AC step down transformer and rectification, I ended up adding a number of components though to protect from voltage spikes, smooth out the voltage, reduce noise and buffer the output so the load impedance has minimal effect on the voltage setting. The 100k pot sets the voltage output but there is a switch which can select between a 0 to 20V output range and a 0 to 10V output range which gives a little more control in the lower voltages.
I breadboarded most of the circuit before soldering to perf board, it seems to be working well so far but I am waiting for the analog panel meter (one of those old ones with a needle indicator) and I still need to find a nice little box to enclose it in.
This is a low current supply, I may build another one very similar but with a heavy duty 25V transformer that I have sitting around. We'll see how this Goodwill special holds up.