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Archive for September 15th, 2018

Squidwrench Electronics Workshop: Session 5

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:

NPN switch - LED

NPN switch – LED

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!

 

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