Exams are coming up and I’ve been going over some analogue electronics in preparation, just to make sure I actually do understand it. Whilst going over some notes I found some instructions in the lecturers notes that proved to be especially useful: how to use SPICE.
SPICE is a circuit simulator and it’s fairly good. Like all good systems it makes mistakes every now and then but we can overlook that in most cases. So I tried to test my understanding of how a Zener diode works by designing a really simple shunt regulator. It’s not much use in the real world but it can supply 100mA at 5V (well, 4.7V because Zener ratings are strange) to some load.
Theory says that the Zener will stop acting like a regulator when no current flows through it, and this happens when the voltage across it fails to exceed its breakdown voltage. As shown in the above circuit diagram, R1 and R2 form a potential divider, hence the voltage across R2 is V1*R2/(R1+R2). The breakdown voltage is roughly 5V as mentioned earlier, therefore the device will stop conducting when R2/(R1+R2) <= 5/12.
Personally, I think this graph is pretty awesome, even if it is completely unlabelled. Up the left hand side is the output voltage (i.e. the voltage across R2) and along the bottom is the resistance of R2. The curve bends quite sharply at about 50 Ohms which works out to be exactly what we predicted earlier: 50/(70+50)=5/12.
I’m impressed with LTspice. This took like 5 minutes to do. Most of the tools we use are fairly complicated, but this is pretty easy. It draws graphs well and it’s easy to get the plots you actually want rather than plotting everything against time. The only downside is that it doesn’t support parametric sweeps in the GUI so you have to type it in manually, but overall it seems pretty cool.
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