Building a small power supply

[W4RFM]

I have done this countless time in the last 50 years, BUT here goes.  I have a simple little transformer to build a power supply for my VFO.  I found the center tap and grounded it, the two outer windings show 645 v AC on the meter.  When I connect one lead to a good diode (properly oriented), I get 105 volts D C!  What am I missing here?

[w4bfs]

connect a small value (.1 ufd) 1 kv from the cathode to ground ... it should charge up to nearly 460 V, if I understand whatcher doin' .... this is only a test circuit, not a practical application .... 73...John

[stevef]

Doesn't it need a full wave rectifier if center-tap grounded?

[W4RFM]

Steve, I think you've got something there, pardon my ignorance.  I wanted to draw this out, but have never been able to master posting pictures on this page.

Thanks
BM

[WA1GFZ]

you need a filter after the rectifier. your meter is trying to integrate the 1/2 wave waveform coming out of the diode.

[W8ACR]

Hi Bob,

I would go ahead a build the power supply - filter, bleeder, and all. If you are getting 645VAC on the secondary the transformer is apparently working. If you use a choke input filter, I would expect somewhere around 300-325VDC at the top of the bleeder, depending on the values of the chokes and the bleeder. You will get a higher DC voltage with a capacitor input filter.

Ron

[KM1H]

A choke input will deliver about .9 of the applied AC and be reasonably stable under a varying load with a good bleeder.

A capacitor input will deliver 1.414 of the AC but will drop with a load dependent on the input C value, the chioke and the bleeder. The output C will have little effect on voltage but a big effect on removing ripple.

Carl

[N4LTA]

As an adder to the above, a choke input will look like a capacitor input unless the critical curent is drawn via a bleeder or other constant load -  it will deliver approx 1.414 x V until the critical current is drawn.

I critical is approx V out/L in MA if I remember correctly and often results in a hefty bleeder  - A 400 volt supply with a 10H choke would have a critical I of approx  40 MA

That is why swinging choke worked so well - they had high L at low currents

[kb5zxm]

I have been saving old power supplies, from commercial FM stations for parts <mostly Micors,Motracs and GE Master II and Pro's,the Ge stuff seems to have less current ability>.
Recently a friend suggested looking at transformers from Micro wave ovens? They seem to be hefty.
Can't seem to throw any thing away, "One Mans junk is another Mans Junk"