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!

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. 

Preliminary Experiments With BJTs

I've just begun to experiment with bipolar transistor amplifier configurations powered by a 9V battery and this circuit gave an interesting clipped sound. I also made one that gave a clean boost but that's not as interesting to listen to. The output of the circuit was fed into a Roland keyboard amplifier. Interestingly (and a little sadly) when I got out the guitar to test this it was the first time I had touched it in months. The next circuit I tried had high current through one branch and it fried a pnp device in about ten seconds! I'll have to put my experiments aside for a while because of a heavy school load, lots of boring school assignments to finish this week.

Setting Up Shop

These are my new lab partners: Sophie, Shazer, Shule and Sally.
They are helping me set up an electronics lab at the house. My only piece of test equipment is a handheld digital multimeter but I've started designing a simple DC Voltage source which I've been collecting some components for. After building that I will make a preamp for a ribbon mic I have.

I just got way too busy to keep up with the circuit of the day postings, in place of that I'll start updating how my projects are going. School has been busy, I've found out that layout can be very time consuming but also I've been distracted by planning my own side projects, which I'm getting very interested in. Here's to playing safely with electricity!

Day 9: ESD Protection Diodes

I took the weekend off but we're back today. My schedule may prevent me from posting daily but I'll keep putting things up when I have time. Today I look at something that I've seen quite a bit as an intern in the protection division at ON Semi.

Day 8: Ideal Op-Amp with Negative Feedback

For stability and to avoid riding the rails (clipping at the op-amp +-Vcc), the feedback path impedance must not be allowed to become too large.

Day 7: PMOS Current Mirror