Day: September 15, 2018

Topics for today’s Squidwrench Electronics Workshop: Session 5 in a continuing series.

Having discussed transistors as current-controlled current sources, we can now select one as a victim use one as a switch, then add capacitors to learn about exponential charging, and introduce the oscilloscope as a vital tool.

So, we proceed:

Transistors as switches

Review graphical parameters

• saturation voltage for high Ic
• cutoff voltage for near-zero Ic
• resistive load line: VR = Vcc – Vc
• power dissipation hyperbola (at all Vc)
• secondary breakdown limit (at higher Vc)

Something like this, only drawn much larger and with actual numbers:

Reminder of linear vs. log scales converting hyperbolas into straight lines.

NPN transistor as “to ground” switch

• where to measure device voltages?
• passing mention of flyback diodes
• IB needed for saturation?
• Darlington transistors: beta multiplier, VBE adder

For example:

Without the LED, you get nice square waves:

An ancient green LED reduces Vc by a little over a volt:

Discuss PNP transistor as “from supply” switch

• why VCC must not exceed controller VDD
• kill microcontroller and logic gates

Wire up pulse gen to transistor

• function generator for base drive voltage
• collector resistor (then LED) as output
• how do you know what it’s doing?
• add oscilloscope to show voltages
• explanation of scope functions!

Capacitor as charge-storage devices

Useful ideas and equations

• C = Q/V
• so C = ΔQ/ΔV
• therefore i = C * Δv/Δt
• energy = 1/2 * C * V²

Charging capacitor from a voltage source through a resistor

• Exponential waveform: e^t/τ
• time constant τ=RC
• show 3τ = 5%
• and 5τ < 1%

Add cap to transistor switch with R to soften discharge path

• charge vs discharge paths
• calculate time constants
• wire it up
• verify with oscilloscope

The circuit will look like this:

Discussion of parts tolerance: 100 nF caps are really 78 nF

With one cap:

Add another cap for twice the time constant:

Let the scope calculate 10-90% rise time:

Useful relations:

• rise time = 2.2 τ (compare with calculations!)
• rise time = 0.34/BW

Do it on hard mode with the old Tek scope for pedagogic purposes.

That should soak up the better part of four hours!