Weird Voltage readings after rectifying

[superR]

I was planning on building a GC but when I measured my transformer, I seamed to get some weird measurements and i was wondering what could be the problem;

The transformer is wired properly since it is still in it's original case (of a dead amp). The output gives 3 wires, 2 red and one black.

When powered,
red-black = 32 V AC (both tested) 0 V dc and between
red-red = 64 V AC. 0 V dc

After connecting the two red wires to the 4 diodes as displayed in;

I measured some weird voltages between the + and - output:
when having the switch in off-mode; 1.7 V AC @ 100 hz and 5.8 V DC. After switching on the voltages are; 1.6 V AC @ 100 hz and 57.7 V dc.

The voltages between the + of the rectifier and the black wire give the following voltages:

when having the switch in off-mode; 6 V AC @ 50 hz and 3 V DC. After switching on, the voltages become; 3.8 V AC @ 50 hz and 29.3 V DC.

Measurements between the - of the rectifier and the black wire give simular measurements (only DC is negative now!)

So here are my questions. How is it possible that when the power is switched off, the transformer gives both AC and DC? How come the rectified voltages output a (quite large) AC? How to solve this voltage issue? (if there is an issue!)

Thanks in advance

[Nordic]

What bridge or diodes are you using...

Is the transformer disconnected from everything else, or still in the dead amp... that would explain charge being stored on capacitors and availability after power down, I would say the tranny is fine...

[AndrewT]

Hi,
I think you have a spark suppressor across the mains switch.
It is feeding a tiny current into the transformer and you see the low AC voltages on the output.

I suggest you take the transformer out and connect each of the wires into separate terminals of a terminal strip.

Then connect up a light bulb tester and feed the live in through that to your transformer primary.

Measure carefully, we want to hear from you!

By the way, if you want to measure real DC after the rectifier you need to add some smoothing to hold the DC steady while the meter takes a reading.

[superR]

it is disconnetec from the original circuit sice I took that out. There are no capacitors left within the amp. I used the original 4 diodes ( in5402 )and connected them like in the picture I posted above.

[teemuk]

Originally posted by superR

When powered,
red-black = 32 V AC (both tested) 0 V dc and between
red-red = 64 V AC. 0 V dc

Looks ok to me, black is the center tap.

Originally posted by superR
How is it possible that when the power is switched off, the transformer gives both AC and DC? How come the rectified voltages output a (quite large) AC? How to solve this voltage issue? (if there is an issue!)
[/B]

When the voltages are switched off (and the secondary is not connected to filter capacitors) there should be no voltage readings whatsoever.

Rectified voltages will have a very large AC component unless you filter them.

If you want to test that transformer correctly remove the rectifier and set meter to AC. Then test the DC supply by connecting both rectifier and filter caps and measure DC with meter set to DC. The AC test for DC is wortheless with most multimeters since they are set to show RMS value of AC sine wave. Ripple is not sine wave.

[I_Forgot]

Small capacitances between wires, windings in transformers, rectifier junctions, and etc. all couple some voltage to your meter. Your meter will show some voltage even if you just wave the probes around in the air. It doesn't mean anything except that the meter has a very high input resistance (typically 10 M Ohms or more).

Put 10K or even 100K resistor across the probes and check again. You should see those readings drop to very nearly (but not quite) zero.

I_F

[Greg Erskine]

hi superR

If you think about the waveform after the rectifier, it is just an sine curve with the negative part of the wave flipped to the positive side. It's not really AC or smooth DC. Some multimeters have difficulty giving a reliable reading.

As Andrew has said, just put a capactior across the output to measure smopthed DC.

EDIT: I just re-read a couple of the posts and realise others have given the answer in different words. Sorry.

regards

[AndrewT]

Hi,
are you using the original mains switch to fed the transformer.
Then check again to see if there is spark suppression.

When the probes are connected across a winding they are connected to a low impedance. This low impedance WILL attenuate any stray fields down to just about unmeasurable with a normal DMM.

[DJ Exprice]

Sounds like weird rectifiers to me...

I'd say you try one of those 4-pin jobs. You know, the square ones (or circle ones).

[superR]

After a weekend of not having acces the my stuff, I got back to it and put two 6800 uF 50V caps between the rails and the AC has gone to less then 1 mV AC and the DC is better @ +43 V and - 43 V. When the amp is switched off, the voltages are 0 V both AC/DC.

So i guess that's fixed. Next job; trying to find someone that can help me make a lm4780 pcb...