From: nospam@needed.invalid   
      
   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.   
      
   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.   
      
   And it's not really all that good of a scanner, but the   
   marketing people "spared no effort" :-)   
      
      Paul   
      
   --- SoupGate-Win32 v1.05   
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