Circuit Corner - Issue 1
Hello and welcome to the first installment in my new Circuit Corner series. For those of you who are just arriving, Circuit Corner is a section of my website devoted to presenting novel circuits that you may not have seen before. This is especially likely since I tend to design them myself.Today I have two small, useful circuits for you. They are both quite simple to build, and are very forgiving with circuit tolerances, so they make a good first entry.
Sweep Generator
The Story So Far
I recently acquired a "single board computer" from ebay. Unfortunately, it was removed from a device that was intended to have a monitor. The monitor was a strange type, some sort of plasma display. Anyway, I didn't have one on hand. Discovering the pinout of the display connector was easy enough. Alas, the timings were strange, and made the VGA monitor do unnatural things. The timings looked to be about EGA, but I didn't have an EGA monitor on hand.Of course, I didn't want to give up a good board just because I didn't have an EGA monitor on hand. What fun would that be?
So I started thinking about what I could do, and I noticed the oscilloscope. It had an intensity channel, and a built-in timebase with an external trigger. I could feed the hsync into the trigger input, and use the built-in timebase to sweep horizontally, and feed the data into the intensity channel. The only problem was sweeping the Y axis. So I got out a post-it note, and started doodling.
The Circuit
The circuit is conceptually very simple. The two BC548's form a current mirror, sinking a constant current out of C1. The current is determined by:
This current is kept constant by the current mirror, making the capacitor act as an integrator. The slope is equal to:
The capacitor is periodically discharged by the BC558 acting as a common-emitter amplifier, feeding current into the capacitor through the diode. The circuit will likely work fine without the diode, but your mileage may vary. The value of the 330 ohm resistor is completely noncritical, acting only to limit the current into the transistor to something reasonable.
Current Limiter
Protect Yourself
Recently, I found myself designing a backplane-based machine. Now, I'm a big fan of overengineering things, especially when it comes to reliability. In this particular case, I noticed that I had a 12V power line running right next to a 5V power line. This made me nervous.My solution was to design a current limiter circuit out of a few transistors to use on the +5V line. The board is TTL-based, and can handle a slight drop from 5V, but not a large one. So I got out the notebook and started designing. This is the result.
Editor's Note: this circuit is sub-optimal. Try the better regulator in issue 4 instead. It does appear, however, that my old assessment about EBJ breakdown is incorrect, so this circuit should be safe to use.
The Circuit
None of the components are critical except for the sense resistor, which determines the cutoff current. I built one for testing using entirely 4.7k resistors, which worked within 5% of prediction. As long as your transistors have a high current gain, then there is no problem. The resistors are chosen to reduce the need for high-gain transistors, so if you use these resistor values, the circuit should work for almost any transistors.Note that all the current used goes through the 2N2222. Make sure that your chosen current is below the handling capacity of the transistor you use (1A for the 2N2222), and also check that the transistor can handle the power dissipation of a short to ground (the regulation ensures that the power will never exceed IL * Vcc, so plan accordingly).
