Understanding AC rectification and voltage effects

[surfsup]

Okay so I am struggling to understand this circuit. I've labeled points A thru D. At each node here are my readings:

A - 335 VAC
B - 293 VAC (42V drop across resistor)
C - 133 VAC (160V drop across one UF4007)
D - 272 VDC (~2*C)

Two questions:

1) Why a 160V drop across one UF4007?
2) Shouldn't I get 1.4x voltage at node B after rectification?

[ToneMerc]

Okay so I am struggling to understand this circuit. I've labeled points A thru D. At each node here are my readings:

A - 335 VAC
B - 293 VAC (42V drop across resistor)
C - 133 VAC (160V drop across one UF4007)
D - 272 VDC (~2*C)

Two questions:

1) Why a 160V drop across one UF4007?
2) Shouldn't I get 1.4x voltage at node B after rectification?

[img:632:432]http://chicagocadcam.com/ChrisHahn/schem/Rectifier.jpg[/img]

I was going to try to explain it, but a picture is worth a thousand words. Also, concerning question #2 was your intent to say "C or D" instead of "B"?

TM
http://www.allaboutcircuits.com/vol_3/chpt_3/4.html

[surfsup]

Oops sorry I meant to say C instead of B. The V at D is the rectified voltage = 272VDC. As I understand it, rectified voltage is peak AC voltage, AC voltage being RMS which is 0.7 of the peak. So if I'm getting 133 V at C, shouldn't I get 133*1.414 at D = 188VDC?

I don't understand why there is a 160V drop across one diode.
I also don't understand how I can read 133 VAC and get 272VDC.

I appreciate your willingness to help.

[ToneMerc]

Oops sorry I meant to say C instead of B. The V at D is the rectified voltage = 272VDC. As I understand it, rectified voltage is peak AC voltage, AC voltage being RMS which is 0.7 of the peak. So if I'm getting 133 V at C, shouldn't I get 133*1.414 at D = 188VDC?

I don't understand why there is a 160V drop across one diode.
I also don't understand how I can read 133 VAC and get 272VDC.

I appreciate your willingness to help.

Look at the link again, middle of the text, "Full wave rectifier center tap"

[XgamerGt03]

Sounds like a bad diode to me.

I've seen diodes act very strangely when they are bad and they haven't gone fully open or shorted. I've seen them show no effects when current draw is very small and drop lots of voltage when on... and I've seen them do the opposite.

If you haven't tried already, I would replace the diode and see if that fixes your problem.

[martin manning]

Between the diodes you are seeing half-wave rectification, so you are only measuring the AC from the positive half of the wave and you read about half the AC voltage of the full wave. It's also clipped by the voltage drop across the series resistor. The clipping is caused by current flowing only for short intervals at the peak of the positive swing, and there is only a voltage drop when current flows. The reservoir cap will charge up to the clipped voltage peak, and that's what you see on the other side of the second diode (less the two diode drops). The 1.41 relationship is only good for sine waves.

[XgamerGt03]

Between the diodes you are seeing half-wave rectification, so you are only measuring the AC from the positive half of the wave and you read about half the AC voltage of the full wave. It's also clipped by the voltage drop across the series resistor. The clipping is caused by current flowing only for short intervals at the peak of the positive swing, and there is only a voltage drop when current flows. The reservoir cap will charge up to the clipped voltage peak, and that's what you see on the other side of the second diode (less the two diode drops). The 1.41 relationship is only good for sine waves.

This is another really good possibility that I didn't even think about. I doubt it is the diode anymore.

[surfsup]

Xgamer,

This is another really good possibility that I didn't even think about. I doubt it is the diode anymore.

Both sides of the secondaries read the same so unless each side's diode went bad, its prob not the diode.

Martin, So if what you say is true, my original post would be the proper question. At node B, the reading of the clipped AC voltage is 293.

So if the cap charges to the clipped voltage peak, and 293 is the clipped voltage RMS value, wouldn't the cap be at least a voltage value of 293 or higher? hence my original question node B*1.4 should render 410 for a full sine wave, but here it would be somewhere between 293 and 410, but I'm getting 272.

[martin manning]

If your meter is a true RMS type, then you may have a pretty good value for that, but the peak won't be 1.41 x Vrms unless you are dealing with a sine wave, which you are not. The DC is also a little less than the peak because of ripple.

[LOUDthud]

Because there are no caps to charge up to the peak when the Standby switch is open, you will measure the average voltage at points C and D. The average voltage of a FULL WAVE rectified sine wave (point D) is 0.636 times the peak. The average voltage of a Half Wave rectified sine wave (point C) is half that. When the standby switch is closed, there is a cap connected that will charge up the the peak but any load will drop that voltage because of losses in the transformer, the 1K resistors and ripple at the cap.

[diagrammatiks]

ya measure it with the cap in like everyone's been saying.

without the cap you'll only get .7 times the secondary.

[martin manning]

ya measure it with the cap in like everyone's been saying. without the cap you'll only get .7 times the secondary.

The schematic shown is incomplete. The voltages quoted are measured with a cap and a load present.

[surfsup]

Sorry about that, I updated it to be more complete below (you might have to hit F5 or refresh to get the image to update). with standby open, I get 460VDC at D, with it closed I get 272VDC:

nodeA - 335 VAC
nodeB - 293 VAC
nodeC - 133 VAC
nodeD - 272 VDC

[img:589:281]http://chicagocadcam.com/ChrisHahn/schem/Rectifier.jpg[/img]

[d95err]

With standby switch open, no current is flowing, so there is no load on the transformer. This results in maximum DC voltage. With the standby switch closed (assuming there are tubes and other things not shown in the schematic) current will be flowing and the transformer gets loaded down. Also, you get a voltage drop over the 1k resistors (1k seem very high for a typical tube amp - is this value correct?).