Re: message repost

At 10:41 PM 9/4/97 -0700, you wrote:
>If you look at the P314 schematic and "remove" the pre-drive transistors,
>zeners, and resistors, you can see that this effectively "disconnects" the
>unregulated power from the base of the power transistors. So all I did was
>put my LV PCB in the "gap". Now for a visual...
>Being inspired by your fine ascii art, Zonn...here
>is my attempt at it. My LV PCB is wired up basically as follows:
>
>(+33V unregulated) ---------*----------------------+
> | /
> | | / Q102
> | |/ 2N3716
> +--[LM317]---------| (NPN)
> | |\
> | | \
> | V---- to + HV and
> | + deflection BJTs
> [adjust parts] adjust to +25V
>
>
>(-33V unregulated) ---------*----------------------+
> | /
> | | / Q103
> | |/ 2N3792
> +--[LM337]---------| (PNP)
> | |^
> | | \
> | \---- to - HV and
> | - deflection BJTs
> [adjust parts] adjust to -25V
>
>So as you can see, the regulators keep the base voltage constant, thus
>causing the power transistors to "follow" the output of the regulators.
>The output voltage of the power transistors goes to the HV and to the
>deflection BJTs, and is only one diode drop less than the output voltage
>of the regulators.
>I designed the adjust circuitry for maximum ripple rejection (80db if you
>read the data sheet). So the DC power generated at the emmiters of the
>power transistors is *very* clean/flat, and constant. And, since the base
>current is approximately two orders of magnitude less than the output
>current, the three-terminal regulators "don't get overworked".
>
>An area of improvement that has cropped up in this discussion is to add
>some form of current limiting to the unregulated power -- i.e., limiting
>the current into the collector of the power transistors.

What you have done is what WG *tried* to do using a zener diode as the
reference into the base of the power followers.

Obviously using the LM3x7 is a much nicer way to go! The only nitpicky
thing I see is that your not actually regulating the output voltage. By
regulating the base of the transistor you leave the voltage drop across the
BE unregulated. My understanding is that while this is right around .6v it
can vary based on temperature and current draw, though in this case it
should be pretty insignificant. (What's a few millivolts compared to 27 volts?)

A bigger problem could be adding current limiting to the design. I haven't
really thought it through for the above design, but when placing the pass
transistor inside the regulation loop it's a simple matter of adding a
resistor and another transistor to allow for current limiting.

(Nice ASCII art!)

-Zonn
Received on Fri Sep 5 15:07:45 1997