Saturday, January 18, 2014

How to Fix No Signal to Monitor

Sometimes when using an old computer the monitor won't connect to the tower. To resolve this follow these simple steps.
  1. Check the Power Cable and make sure its firmly connected
  2. Check the Monitor Cable and make sure its firmly connected
If the above is true then it is quite possible that the ram might need to be reset in order to do this:
  1. Remove the side cover of the tower
  2. Find the ram which will look something like what is shown below
  3. Gently push down the two clips holding the ram
  4. Remove the ram and clean it with a dry cloth
  5. Replace the ram in its initial position by pushing down firmly, the clips will snap the ram in place if successful

Monday, January 13, 2014

KP Fundamentals

Overview of Kelvin Probe Method
  • Background
    • Kelvin method invented by Lord Kelvin over a century ago
  • Work Function - energy required to remove an electron from material 
    • indicates surface conditions such as
      • absorbed, evaporated layers, surface reconstruction, surface charging, oxide layer imperfections, surface and bulk recombinations
  • Use a reference surface to study a sample electrode
  • the two are conductors that form a parallel plate capacitor at the tip
  • Capacitance - C = ε A/D
    • ε is the permittivity of the material in between plates, A is area connected as capacitor, D is distance between two conductors
  • C = Q/V
    • C is capacitance, Q is charge, v is voltage
  • Q = V ε A / D
    • As long as it is possible to determine voltage difference charge can be calculated, ε = permittivity of free-space if instrument is in vacuum
    • Null method - apply external voltage V1 to probe. If V1 is equal to Voltage on probe then V is 0, so no current is flowing to or from the probe

  • Current Nullification method
    • Probe is vibrated in sinusoid and is perpendicular to the surface
    • Distance between plates D = D0 + D1sin(ωt)
    • D1 is the amplitude of oscillation
  • Find new capacitance
    • C = ε A/[D0 + D1sin(ωt)]
  • Find Current generated
    • I = V dC/dt
    • I = V d (ε A/[D0 + D1sin(ωt)]) /dt
    • I = V ε A [D1ωsin(ωt)]/[D0 + D1sin(ωt)]2
    • Then we nullify this current by bringing Voltage to 0

  • This is the solution to noise difficulties that come from null field method
  • Distance D is maintained due to parallel movement of probe
    • Capacitance remains constant as permittivity and area doesn’t change
    • Therefore we are looking for the variations in surface potential/charge
    • Current I is generated I = (dV/dt) C
  • Drawbacks
    • This results in an averaged potential measurement
    • Given some potential function, the instrument will measure the enclosed charge averaged over the size of the measuring tip
  • Resultant Error
    • If diameter of measured potential > diameter of tip error results = 2 times the size of the tip
    • If diameter of measured potential = diameter of tip error results = 2 times the size of the measured potential
    • If diameter of measured potential < diameter of tip error results = 2 times the size of the measured potential
  • To get accurate measurements the probe must be both close to the surface and small in comparison to measured charge to minimize edge effects

Kelvin Probe Research ToC

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