The AM Forum

If i have a center-tapped transformer that is marked with the secondaries as 240-0-240, does that meant that each lead is 240v and by putting a bride rectifier across the two 240 leads I would get 480v DC? So then if i wanted 240v from that same transformer, I would connect the rectifier between one of the secondary leads and the center tap? I'm using this as an example to explain my real question. What I'm trying to figure out is if my thinking is right on this, because for a project I'm thinking of building I need a 800v power supply, but want to be able to switch the supply between 800v and 400v. If I have an 850v center-tapped transformer, would the leads be marked 425-0-425 or 850-0-850? if it's 425-0-425, then could I get the two voltages by using the full secondary for the 800v, but then have a switch that switches one side of the rectifier from one leg of the secondary to the center tap to get the 400v, using only half of the secondary? Or would it be better to have a transformer that had dual primaries, one for 117v and one for 234v, then switch the 117v AC input between the 117v primary for full power and the 234v primary for half-power?

If your center tap has the right insulation you can hook a bridge to get 480 into a choke input filter. A cap input filter will give you a higher voltage up to 1.4 times the ac input under light load. The center tap will be at 1/2 voltage DC so there is no need to add diodes but you will need to filter it.

that would be for the 240-0-240 transformer, right? So using the full secondary would give me 480v, and using half of the secondary gives 240v. So it's exactly like the 240 volt AC wiring in a house? Using both legs together to feed something gives the full voltage, but using only one leg gives half the voltage.

only if you ground the center tap. Then you will be twice house voltage. House voltage is 120 -0- 120. You get 240 across the two hot leads.

When you use a FWB rectifier across a center tapped secondary you get half the B+ off the secondary CT lead.  Very simple,  example, 800VCT or 400-0-400,  a full wave bridge will give you the 800 volts into your filter choke.  The center tap of the xfmr will give you 400 volts into your filter choke.  You can make a supply that will deliver both voltages at the same time by using two sets of filter chokes and capacitors.  One from the bridge rectifiers and the other connected to the center tap.  You can load both or one at a time, the higher voltage or the lower one.

If you want to have a supply that can deliver the higher voltage or the lower voltage, but not both at the same time,  you can simply switch your filter choke from the B+ terminal of the bridge rectifiers to the center tap.  Very simple.

As for understanding the voltage ratings of xfmrs, it could be stated as voltage CT, like 500VCT which means 250-0-250.  Some xfmrs, like the Stancor 80 series plate xfmrs were marked with the DC voltage output after a two stage filter (LCLC).  The AC voltages were higher and NOT shown on the xfmr.  Although, most xfmrs are rated with the AC voltage of the winding.

Fred

What would a schematic of this look like?

There are some great manuals on line that have helped me -- try this site www.pmillet.com/technical_books_online.htm  and look for the The Radio Handbook or the Radio Amateur's Handbook/ ARRL manuals.  They are about half way down the page.  Lots of great tube stuff.

Alex

In reference to your original post, where do I get one of these? 

"putting a bride rectifier"

It would be a great gift for a friend of mine....

73DG

oops, I can't believe I missed that, but made sure I corrected all the other spelling mistakes.

Well, there's a way to fix the misspelling, go back, insert the "g", and finish it...

"putting a bride rectifier"

Now I understand where the 'choke' term cames from.


klc

usually I reread what I type because it usually takes me about 5 or 10 minutes to type a post and have it read what I want it to, and then I read it to fix mistakes and make sure that everyone else can understand it.


I had to laugh at that.

The circuit is nothing more than a FWB rectifiers connected in the normal way to the two HV secondary terminals of your xfmr.  I'm assuming you know what a FWB circuit looks like.  For voltages up to about 800-900 Vac (450-0-450) I would suggest you use two diodes in series in each of the four sections of the FWB (8 diodes total)  I use 1N5408 (3amp 1KV PIV).

To use the supply at the 800 volt level, connect the filter choke and caps to the B+ out of the FWB  To use the supply at the lower 400 volt level simply connect the filter choke and caps to the CT terminal of the xfmr.  DO NOT do anything to the FWB rectifiers they remain connected as normal, just connect the filter choke to the CT instead of the B+ out terminal of the FWB diodes.

Hope this helps,

Fred

Ok that's what I thought you meant but wasn't sure because that's the first time I've ever heard of doing it like that.

Great, glad you now understand the circuit.  To add to the understanding,  if you study the FWB diode circuit, you will see that when you draw power from the xfmr CT only two legs of the FWB are working.  The two legs that start from ground to the two HV terminals are working as a full wave rectifier (not a FWB) (the other two diode legs are not doing anything, the cathodes of the two unused diode legs are facing each other).  Current flows from ground through the diodes, the HV winding and out to the CT.  Current flows through one diode leg on each half cycle, just the same way as a full wave rectifier circuit. Because the diodes are placed from ground to the winding terminals, the CT becomes the B+ output rather than the ground.

I guess if we really get picky we should say that current (electrons) flows up from ground through the load and back into the B+ terminal of the PS.  Something to think about.

A few last things,  example, if you have a xfmr that is rated 400-0-400 at 200ma, if you use a FWB at the 800 volt level you can only draw half the current or 100 ma.  The rating of 200ma is for FW rectifier at the the 400 volt level.  The current rating may also depend on the filter you use.  Some xfmrs are current rated to be used with a cap input filter.  Many xfmrs are current rated to be used with a choke input filter.  There are differences to be considered. A xfmr that is current rated for choke input filter and you use a cap input filter to get higher voltage output, you must reduce the current load by about 30%.  OTOH if you have a xfmr that is current rated for cap input filter and you use a choke input filter, you can increase the current load by about 30%.

Most all HV plate xfmrs are rated for choke input filters.

Not all xfmrs can be used with a FWB rectifier because it puts the CT above ground.  Although, most small power xfmrs commonly used in the 300-400 volt range can be used with a FWB rectifier.

Fred

Thanks for schoolin' us Fred. I lernt sumpin' new today. :)

Ron W8ACR

It was a very nice refresher course. No new building projects lately, and the mind is a little rusty when it comes to the good old HV power supply.
Fred