[Milsurplus] Any Tips on How to Drop the AC voltage to the Primary of a Power Transformer?

[Jim Whartenby]

If you buck the input 120VAC to the power transformer then all of the transformer secondaries will also be reduced by about the same 30% or so.  This is not a problem if the high voltage secondary is the only one used in this application.  But bucking the input will do nothing for the high voltage regulation so there will be a compromise between the receiver and transmitter B+ voltage levels.  Assuming you are still going to use just one power supply for TX and RX.

If you change the power supply from a capacitor input filter to a choke input filter, you will reduce the high voltage output to about what you want and gain some voltage regulation to boot.

You are going to add iron to the power supply anyway so why not add a 10H choke instead of another transformer and get the added benefit of the voltage regulation?
Jim
--- On Thu, 11/26/09, Mack <mccormick_mack at hotmail.com> wrote:

> To all who posted to use a bucking
> transformer that is a most excellent idea 
> and the one I'll adopt. Thank you everyone for your very
> rapid responses. 
> Happy Thanksgiving!
> 
> 73,
> 
> Mack de WB4MAK
> www.nfarl.org
> www.sedxc.org
> 
> --------------------------------------------------
> From: "W RICHARDS" <wrichar2 at rochester.rr.com>
> > Use a BUCK BOOST transformer. There should be info on
> the internet on how 
> > to do it.
> >
> >
> > Mack wrote:
> >> I need to drop the AC input voltage to a power
> transformer to get the 
> >> output voltage where I'd like it to be. I need to
> drop 120VAC to 85 VAC 
> >> at .75A under no load conditions. I'm currently
> using a variable 
> >> transformer to regulate the input voltage but
> would prefer a solution 
> >> that is MUCH more compact so it would fit on the
> power supply chassis. I 
> >> tried a household fan and incandescent controller
> but the high inductance 
> >> of the transformer primary causes the fuse to
> always blow using this 
> >> technique.
> >>
> >> I've considered using an approx 46 ohm 40W
> resistor but I'm not sure 
> >> about the voltage regulation under intermittent
> (CW) load. Are there any 
> >> better techniques than those I've described here?
> >>
> >> In case you're wondering about the project: It's
> my current ARC-5 
> >> receiver, transmitter, modulator restoration. I'm
> using one power supply 
> >> for all three pieces. The only HV transformer I
> had it my junk box 
> >> produces 800 VDC at 120VAC input. I'd like to run
> the output plates on 
> >> the transmitter at 550VDC. The power supply output
> also has taps for the 
> >> screens, receiver, and oscillator (which is
> regulated with a 200V 50W 
> >> zener). This is a VERY stiff HV transformer and is
> rated at 1A output. 
> >> I'm also using filtered 12.6VDC for the tube
> filaments as suggested.
> >> Thanks in advance for any ideas.
> >> 73,
> >>
> >> Mack de WB4MAK