Monday, March 13, 2017

Blocking Oscillator operation basics

I've reverse-engineered enough of the calculator's power supply to identify it as a blocking oscillator. Any electrical engineer would recognize this from intoductory classes, but I'm not a EE and had to poke around the 'net a bit to learn about its operation.

I often start with Wikipedia, but in this case the wiki article is terrible. It goes into great detail without first giving a basic overview of the circuit operation. Looking elsewhere I found a pretty decent description on the Integrated Publishing website starting here. The good stuff start several pages in, but I'll summarize below.

Blocking Oscillator circuit diagram
Here's the basic blocking oscillator circuit. At the start of the cycle, current flows through the resistor R1, turning the transistor Q1 on causing current to begin to flow through the transformer winding L1. Since L1 is an inductor it impedes the change in current, and the current through L1 ramps up over time. In a transformer L1 and L2 are coupled, so the change in current in L1 induces a proportional change in current L2, charging capacitor C1. This supplements the current through the base of Q1, keeping it fully turned on. This is a positive feedback loop.

Blocking Oscillator idealized waveforms
This positive feedback continues until the transformer core saturates, after which there is no further change in current through L1. Since it's the change in current in L1 that induces a current in L2, and there is no longer a change in current in L1, the voltage across L2 drops. This causes C1 to discharge, drawing current away from the base of Q1 and turning it off. The current through L1 stops, and the change in current in L1 again induces a proportional change in the current in L2. Except this time the current flows in the opposite direction, pulling more current from the base of Q1 keeping it off until we reach the state we started in.

It's interesting to compare the idealized waveforms to this one from the calculator. The yellow (bottom) trace is the transistor collector waveform, and the red trace above it is the transistor base. These are quite close to the idealized waveforms.

In the calculator circuit Vcc is normally around 9VDC, though for my tests I'm keeping it at 7.25 volts for reasons I'll explain later. R1 is 1,500 ohms and C1 is 4.7 nF (0.0047 μF, if you prefer). There is also a 100 ohm resistor between L2 and C1 that isn't shown in the schematic above; the green trace at the top of the scope screenshot is the L2 side of this resistor and the blue trace is the C1 side. I've also done some 'scope magic to display the voltage drop across this 100 ohm resistor (the pinkish trace), thus showing the current through the resistor on a scale of 20 mA per division. It's a bit hard to see on the screenshot, but I'm seeing a peak of +25 mA when the transistor starts to conduct and -40 mA when the transistor turns off. That's more than enough to overwhelm the basic bias current through R1 of 4.5 to 6 mA.

This is not the entirety of the power circuit, though. If the AC mains voltage is high enough to drive the unregulated Vcc voltage above about 7.7 volts another circuit grounds the base of Q1 for periods, causing the blocking oscillator to run in bursts. This appears to be a form of PWM voltage regulation to limit the output from the transformer. I haven't finished analyzing this part of the circuit enough to describe it further.

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