multiple voltages on tap

*Begin infomercial voice*

Are you sick of your family not being able to agree on a single voltage?...

Want to convert your single channel power supply into a poor man's multi channel unit?...

Would  you like to save a little time breadboarding when you need multiple voltages?...

... Then build yourself a variable voltage multi-tap!

*End infomercial voice* (unless you really don't want to)

Ok, so what exactly is this? It's a multiple output (4 in this case) linear voltage source that share a common ground among them, intended to be used with a regulated power supply at the input. The output from this multi-tap is really only as regulated as the power supply that it's connected to, other than the ability to reduce transients to some extent as will be described a little later. So how does one know what the output voltages actually are? After all, there's no gauges or fancy displays. Well, that's what your multimeter is for! While I'll likely use this exclusively with a variable bench power supply, this could in theory be used with batteries, "wall warts" or other fixed voltage power supplies. Beware: this has some significant technical limitations so care and caution must be observed if you intend to make one! All responsibility lays on the user! 

Now lets take a peek inside and also take a look at the schematic which is drawn similar to how it's laid out.

This is the classic common collector (voltage follower) circuit, where the voltage between ground and the transistor's emitter terminal roughly equals the voltage applied to the base of the transistor, relative to ground. A variable voltage to be supplied to the base of the transistor is accomplished by using a potentiometer (R1 in drawing) as a voltage divider, where one end of it is connected to ground, and the other end connected to the V+ input voltage jack. A voltage anywhere between those two values can be supplied at the wiper output by turning the potentiometer's knob one way or the other. Again, since this is a voltage follower circuit, the voltage we'll get at the V+out jack will match the voltage at the output of our variable voltage divider. Technically, there will be a a slight drop, depending on the transistor used, which is inherent to the way transistors operate. Therefore, the maximum output at V+out will be roughly 1 volt less than V+ in.

Also attached to the base of the transistor is another resistor (R2) and a capacitor (C1). This creates a circuit known as a "capacitance multiplier", but I think is better seen as being a low-pass filter attached to a voltage buffer. It is this low pass filter which enables the output to filter out input transients as I mentioned earlier. To elaborate on this a bit more, basically the capacitor has one of its terminals connected to ground, while the other is exposed to a voltage above that determined by the output of the voltage divider. However, it takes time to charge/discharge the capacitor, which is in part determined by R2 (known as the RC time constant). Because it takes time to charge/discharge, the voltage between the capacitor's terminals must lag behind any change in voltage above a certain rate supplied in front of the low-pass filter (for simplicity's sake, this is at a point between  R1 wiper and R2). Furthermore, since the capacitor's positive terminal is attached to the base of the transistor, and the transistor's output voltage (emitter in this case) "follows" the base voltage, the output also lags behind any changes in voltage at the transistor's input, effectively reducing input voltage transients above frequencies determined by the values of R2 and C1. In the case of the circuit above, this is roughly 2.5Hz.

It needs to be said that this circuit is very simple, and does little else than supply a voltage at a lower impedance than the voltage divider. There's NO protection circuitry, so it will gladly release magic smoke if you neglect to operate it within its component's limitations (max input voltage, current, and power dissipation  ratings). In other words, this is a "dumb" circuit. Luckily, the circuit is so simple that it's easy to repair when you do fry something.

As for this specific build, you can see there's no PCB, and yet, despite its simplicity, was still somewhat tedious to assemble (unless I was willing to let it look more like a rat's nest). So in the future, any new variants will be on a PCB. The capacitors might look comically large for their value, and that's because these are rated for 400V. Of course, the rest of the circuit is not rated for anywhere close to that, but those were the only caps I had, and I forgot to order something more suitable when I purchased the other components. The transistors are actually NPN darlington pairs (BC517) rated for 30V between collector and emitter, 1A, and 625mW at 25 degrees C, and a minimum DC current gain of 30,000. The components were chosen for input voltages up to around 20V. It's not particularly suitable for a 20V input with significantly lower output voltages unless required currents are similarly low. I should probably plot some graphs and stick it to the box.

In the future, I might want to make a multi-tap that is both a variable source and sink, which could be handy when circuits/instruments use multiple op amps that require different rail voltages or biases. I'd probably also choose some more sophisticated and higher rated power transistors that have fancy features such as built-in short circuit and thermal protection.