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.

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) .

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.