Measuring volts on an unloaded PSU secondary

fyzygy

Member
2010-07-11 12:51 pm
I have started assembling a PSU out of some scrap.

My unloaded power transformer delivers approx. 390-0-390 volts AC at the secondary.

After diode rectification, I obtain around 358 Vdc (again, unloaded).

However, if I connect a power supply filtering/smoothing circuit (50uF/2.2K/50uF) to the output, the voltage at C1 and/or C2 jumps to over 500 Vdc - again, this is without any additional circuitry to load down the transformer.

Is this normal? Would B+ return to safer levels under load? Can I damage my input capacitor (rated for a mere 500V) by running the PSU unloaded?

I would like to build another RH84, wonder how I'd go about settling on or around 300 Vdc under load with this particular transformer?

Why does the voltage increase after C1, C2?

Thanks,
 

fyzygy

Member
2010-07-11 12:51 pm
I'd have to draw it & scan. But it's textbook stuff really. Full wave rectification via a pair of UF4007 diodes > pi-shaped filter section comprising 50uF shunt capacitor/2K2 series resistor/50uF shunt capacitor. That's it. I haven't hooked it up to any load or "circuit" as such, which may be the problem.
 
After you rectify it you will get approxomately 390v X .9= 351V.
When you add the filter caps you will get 390 X 1.142 = 551Vdc.

Here is an excerpt taken from page 111 of the ARRL 1978 handbook.

Enjoy !!

jer :)
 

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quikie22

Member
2009-09-24 1:53 pm
use higher voltage caps....... the voltages are correct...
as to how much the voltage will drop when the system is loaded will depend on the transformer your are using - ie the internal resistance of the transformer and how much current it is designed to provide and how much current is being drawn...without these parameters/details, impossible to tell how much the expected drop will be
 

quikie22

Member
2009-09-24 1:53 pm
use higher voltage caps....... the voltages are correct...
as to how much the voltage will drop when the system is loaded will depend on the transformer your are using - ie the internal resistance of the transformer and how much current it is designed to provide and how much current is being drawn...without these parameters/details, impossible to tell how much the expected drop will be