Text 257, 53 rader
Skriven 2005-02-22 09:13:26 av MIKE ROSS (1:167/133.0)
    Kommentar till text 256 av Roy J. Tellason (1:270/615.0)
Ärende: FAN OF TRUE MULTITASKING
================================
"Roy J. Tellason" bravely wrote to "MIKE ROSS" (21 Feb 05  20:06:53)
 --- on the heady topic of "FAN OF TRUE MULTITASKING"

 RJT> MIKE ROSS wrote in a message to Roy J. Tellason:
 MR>  Anyways there is usually a small
 MR> auxiliary ac transformer to supply the PWM regulator startup and
 MR> this means the PSU wouldn't even try to start if the voltage got
 MR> too low.

 RJT> In the designs that I've managed to find schematics for,  if there's a
 RJT> second transformer it's used for regulator feedback,  not what you're
 RJT> staying here. Some units will use an optoisolator instead for that
 RJT> purpose.

I have found optoisolators used for the feedback is very common too.
The problem is if the regulator is left on the AC live side then all
the circuitry is live. It is much preferable to have as much of the
circuitry as possible on the line isolated side of the switching
transformer because less mechanical isolation is then required. It's
the same idea why your electric drill is double isolated and it costs
more to build it that way.


 MR> However, I've seen supplies set to 220vac try to work with
 MR> a 120vac input. As expected they regulated rather poorly but
 MR> amazingly they almost worked.

 RJT> I wonder what it would take to make one work?

It would actually be pretty straightforward but cost more to make. All
that is required is to design the supply input for 120vac to 240vac
then use switching transistors that can handle the largest current and
highest voltage respectively. Instead of using 300 volts 3 amp
transistors it would require 600 volts 3 amps transistors.

That's about the same voltage as horizontal output transistors but
much higher frequency than these, i.e. expensive. Similarly it would
require 400wvdc supply filters instead of 200wvdc. The bridge
rectifier would need to be upgraded from 200v to 400v +. Most of the
rest can remain the same.

However it could be done another way without the voltage doubler by
using lower voltage devices but with higher current handling. This
would require a larger switching transformer and double the supply
capacitance, etc. Probably increase the cost about the same either
way. It boils down to either pay for the higher voltage semiconductors
or pay for more iron and higher current devices. Take your pick.

 M*i*k*e

... As I suspected, you're a rank sentimentalist! --Cpt. Louis Renault
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