[AMRadio] Transformer help

[Gary Schafer]

Donald Chester wrote:

> >From: Gary Schafer <gschafer at mediaone.net>
> >I seem to remember from the early days that a swinging choke was
> >not recommended as power supply dynamic regulation would suffer badly
>
> Gary,
>
> It depends on how much bleeder resistor and filter capacity you use.  The
> bleeder resistance in ohms should be no more than 1000 times the filter
> choke inductance at no load (other than the bleeder resistor). This might
> require some trial and error.  Try different values of bleeder resistance
> until the DC voltage drops to about 0.9 times the rms voltage of the
> transformer winding (one side of CT if you are using full wave CT circuit).
> Reducing the resistance below the critical value should cause relatively
> little additional voltage drop. If you key and unkey the final (as in CW
> operation) with the power supply on, the value at full  load should drop to
> no less than 90% of the no-load value.  You can use less resistance, but you
> start using up current capability.
>
> Use as much filter capacitance as you can muster, without blowing a fuse
> when you key the power supply.  I use about 30 mfd. in mine.  That maintains
> good dynamic regulation with the swinging choke.  In the old  days, filter
> capacitance was hard to come by, so most power supplies used 1 to 4 mfd.
> You could get the ripple down to an acceptable value with multiple filter
> sections, but the dynamic regulation was terrible.  It is better to use as
> large a capacitor as practicable, with one section of filter using a
> swinging choke.  I have found that with more than 50 mfd, you have to use
> some kind of step start circuit to avoid popping fuses when you key up.
>
> Don K4KYV
>
>

Hi Don,

I had assumed that the critical value of bleeder would be used to hold the
voltage at the .9 value of the rms. But what I had heard and some still claim
that the transient response is not good with a changing load like ssb presents.
Maybe this myth (?) is a carryover from the days when high filter capacitance
values were not generally available. Or maybe some did not meet the critical
value of load current which of course would allow the voltage to swing much
farther.

I wonder if you have ever looked at your hv supply voltage with a scope when
keying the transmitter on cw? That would be a vary wide swing in load current.
According to Rich Measures you get a momentary dip in the output (then it comes
back up) when applying the load. (poor transient response because of the choke)
There must be some critical value of filter cap needed to compensate.

73
Gary  K4FMX