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1 milliamp of current that is charging a 1,000µF capacitor will raise it’s voltage 1 volt every second. That was my goal with this PNP BJT current mirror circuit.
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The resistors on the collector side of the 2N3906 PNP BJT with the Base and Collector connected to each other, sets the current based on approx. 0.6V less than the 5V supply voltage (5V-0.6V = 4.4V) across the 2 resistors. 4.4V/4,400Ω would equal 1mA of current. So the total resistance of 3900Ω + 510Ω = 4,410Ω will be slightly lower than that.
- Capacitor charges all the way to the supply voltage (maybe slightly shy of it). I thought it would be approx. 0.6 volt short of the supply, like it is across the current setting resistors.
- 1,000µF wasn’t quite enough capacitance to get me to 1 volt per second for some reason. Current is probably slightly higher than expected, and the capacitor actual capacitance may be slightly lower than it’s rated value. Adding a parallel 100µF capacitor to the 1,000µF capacitor for a total of about 1,100µF got the voltage ramp a lot closer to 1V per second.
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