From: robin_listas@es.invalid   
      
   On 2024-03-26 14:27, Paul wrote:   
   > On 3/26/2024 7:50 AM, Carlos E.R. wrote:   
   >> On 2024-03-26 01:29, Paul wrote:   
   >>> On 3/25/2024 11:51 AM, Carlos E.R. wrote:   
   >>>   
   >>>> Same here,   
   >>>>   
   >>>> It is an Epson Perfection 1650.   
   >>>   
   >>> Someone here got a pattern in a scan, and it was   
   >>> related to the 24V power supply. An interesting   
   >>> question would be, why a scanner needs a 24V power   
   >>> supply, but I suppose that is handy for driving   
   >>> the motor.   
   >>   
   >> For the fluorescent tube.   
   >>   
   >>> Since scanners can be made for $50 or less, there cannot be   
   >>> a lot of money in making those chips. It's the "Intel $5 CPU problem",   
   >>> not a lucrative business to be in. When you pay more for a scanner,   
   >>> the money goes into a better scan head, or a transport with   
   >>> tighter dimensional control. The all seem to like the rubber   
   >>> belts with teeth, for transportation.   
   >>   
   >> And scanners using a camera are quite expensive.   
   >   
   > CCFL tubes run from high voltage, and it MUST be a pure sine power source.   
   > if there's any DC on the waveform at all, it accelerates the degradation   
   > of the CCFL electrodes. Ignition voltage is 1000VAC. The operating voltage   
   > after it starts to conduct, might be around 700VAC. This requires an   
   > inverter, to make the sine power. CCFL tube "power" is 3 watts, but   
   > it's delivered as 1000VAC and 3mA, and a sine wave.   
   >   
   > The sine wave can be at 25KHz (above human hearing range). Since the   
   > inverter operates at a high frequency, you're not supposed to be able   
   > to hear it.   
   >   
   > To control the intensity (your 1650 has intensity level control!),   
   > you can PWM the inverter at 200Hz. In effect it kind of runs   
   > in burst mode. Bursts of 25KHz high voltage. By using PWM   
   > modulation, the CCFL tube achieves a wider range of intensities.   
      
   I have not seen an intensity control in xsane. And I have never used it   
   in Windows.   
      
      
   > In the old days, intensity control was set "with a knob", and   
   > this was a simple resistive circuit. But the intensity range   
   > was small, and only a tiny reduction in light level could be   
   > achieved. Whereas the PWM method has a wider range than that.   
   >   
   > It turns out the light source, isn't as simple as you might think :-)   
   >   
   > Now mine does not modulate the intensity level, and runs at   
   > a fixed level. My scanner also "overscans" the glass. The scan   
   > head scans a "white patch" just before the glass begins, and   
   > that sets the "white level" for the scan. It takes up to   
   > 20 minutes for a CCFL to reach "stable intensity", and since   
   > many scans are taken while the CCFL is not warm, the scanner   
   > calibrates what it finds, by scanning a white patch just before   
   > it scans the paper right next to it.   
      
   Ah, that could explain why the colour of the same paper section is off   
   between two scans.   
      
   >   
   > And it's not really all that good of a scanner, but the   
   > marketing people "spared no effort" :-)   
      
   Well, I did not know :-)   
      
   --   
   Cheers, Carlos.   
      
   --- SoupGate-Win32 v1.05   
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