The New Hotness

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.

NPN Switch - protoboard — NPN Switch – protoboard

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:

Transistor characteristics - saturation and cutoff - load line — Transistor characteristics – saturation and cutoff – load line

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:

NPN - 100 Hz - 2.2k - no cap - Vc — NPN – 100 Hz – 2.2k – no cap – Vc

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

NPN - 100 Hz - 2.2k green LED - no cap - Vc — NPN – 100 Hz – 2.2k green LED – no cap – Vc

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:

NPN switch - Cap charge-discharge — NPN switch – Cap charge-discharge

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

With one cap:

NPN - 100 Hz - 2.2k 2.2k 78nF - Vc Vcap — NPN – 100 Hz – 2.2k 2.2k 78nF – Vc Vcap

Add another cap for twice the time constant:

NPN - 100 Hz - 2.2k 2.2k 2x78nF - Vc Vcap — NPN – 100 Hz – 2.2k 2.2k 2x78nF – Vc Vcap

Let the scope calculate 10-90% rise time:

NPN - 100 Hz - 2.2k 2.2k 2x78nF - Vc Vcap - rise fall times — NPN – 100 Hz – 2.2k 2.2k 2x78nF – Vc Vcap – rise fall times

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!