Tuesday, December 30, 2014

Human Touch Temperature

A lot times, we use our finger to judge how hot a device is and whether it has adequate heat sink. The late Bob Pease describes a 5-second rule in his well-known book "Troubleshooting Analog Circuits":  "A good rule of thumb is the 5-second rule: If you can hold your finger on a hot device for 5 seconds, the heat sink is about right, and the case temperature is about 85°C."

NASA Johnson Space Center actually conducted a study on human touch temperature, "A New Approach to Defining Human Touch Temperature Standards" by Ungar and Stroud.  The onset of pain threshold is about 44°C.  The touch temperature is material dependent.  For Aluminum, 1 second touch corresponds to about 70°C, and 5-second touch corresponds to about 55°C.  Bob was probably talking about plastic case, 5 seconds is about 85°C, then 1 second is about 140°C, 2 seconds is about 110°C.


Tuesday, December 23, 2014

Temperature dependency

All electronic components have some temperature dependency.
  • Resistance: 
    • Copper wire resistance has a positive temperature coefficient of 3900ppm/°C; for 100°C, the copper resistance changes 40%. 
    • Carbon film resistor has a negative temp co of -200ppm/°C and is worse for larger value resistors.
    • Metal film/Thin film/Thick film resistor has a temp co of +/-10ppm/°C to +/-100ppm/°C.  Testings show that the temp co is valid to -150°C.
  • Capacitance:
    • C0G or NP0 ceramic capacitors have very good temperature co, +/-30ppm/°C.  
    • X7R capacitors have +/-15% variation over the range -55°C to +125°C.  The capacitance typically peaks at the room temperature and falls off at both ends, but it could have very nonlinear temperature dependence.
    • Aluminum electrolytic capacitors have considerable temperature dependence.  The capacitance falls off significantly at low temperature because the viscosity of the electrolyte increases.
    • Solid tantalum capacitors have much less temperature dependence in comparison with aluminum electrolytic capacitors.
  • Inductance:
    • Core materials have nonlinear temperature dependence. 
    • The permeability of a ferrite core is usually maximized around room temperature, but falls off at the extreme temperatures.  At -180°C, the inductance can drop as much as 70%.
    • The powder core holds up well at the lower temperature. 
  • Diode:
    • Silicon PN junction forward voltage decreases with temperature, usually -2.1mV/°C.
    • Silicon PN junction avalanche breakdown voltage increases with temperature.
      • High voltage Zener diode is actually avalanche diode.  1N5245 15V Zener has a temp co of +820ppm/°C and 1N5271 100V +1100ppm/°C.  MMSZ5245B is tested to -150C and has a fairly constant temp co about 1000ppm/°C over temperature.
    • Silicon PN junction Zener voltage decreases with temperature.
      • 1N5221 2.4V Zener diode has a temp co of -850ppm/°C.
      • At around 5V Zener and avalanche balance out.  1N5231 5.1V Zener has a temp co of +/-300ppm/°C.
      • Temperature compensated reference Zener diode can have very low temp co, e.g. 5ppm/°C for 1N829 6.2V.  The temperature compensation is achieved with a forward diode in series.  At 5.6V, the temp co is about 2 mV/°C, similar to that of the forward diode voltage.
    • Schottky junction forward decrease with temperature.
      • -1.2mV/°C (ref 1)
  • Transistor:
    • Bipolar beta
      • Beta increases with temperature.
    • MOSFET threshold
      • Threshold decreases with temperature.
    • MOSFET Kp
      • Kp decreases with temperature.
    • MOSFET breakdown voltage
      • Breakdown voltage increases with temperature.

Reference:
1. Zetex, Temperature Effects On Silicon Semiconductor Devices, June 1995. (http://www.diodes.com/_files/design_note_pdfs/zetex/dn4.pdf) .

Sunday, December 21, 2014

Linux Wine

I'm pleased that some of my old applications actually work.  
What works
  • LTspice IV
    • This is very good. Now I have a superb Spice simulator running on Linux.
  • Adobe Acrobat 6.0 Standard
    • I can edit PDF files, but Linux solution may also exist.
  • Merrian-Webster
    • Works only after installing msvcirt.dll.
    • Matter of convenience with excellent pronunciation. Online dictionaries exist.
  • Viewmate 11.2
    • Good free Gerber viewer.
    • Most parts work except it cannot load directory.  Can load zip file.
  • ExpressPCB
    • An easy to use commercial PCB design tool for simple boards. Can import netlist from LTspice.
What does not
  • Protel 99 SE
    • Fails to start with error message: Unhandled exception 0x0eedfade ... It would be nice if this works. But I can always run it on a virtual machine.
  • Altium Designer Viewer
    • Installs fine, but fails in the middle of starting up.
  • Visio 2014
    • Cannot even install.  Linux alternatives are Dia and xfig.