gnd

Toroid voltages - is this normal?
« on: January 18, 2007, 02:53:44 PM »
Hi all.

I have toroid power transformer (220v primary, and 2x19/5.79A secondary), and I'm getting some funny voltage readings at secondary, which I cannot understand. Pls help.

Secondary has 4 wires, two black and two blue. One black and one blue are labeled with (1) label, other two are not labeled.

There is sticker on trafo, saying:
(black1/blue1  19V/5.79A )
(black/blue  19V/5.79A )
So I concluded, that pair labeled with (1) is one secondary, and the other without label is another secondary. And I connected blue (1) with black (no label), and that was to be my middle, zero.

Measuring secondaries gives me 21.8V twice, so I got 2x 21.8V secondary, which is fine.

But measuring secondary zero against mains ground shows 59.5V AC. So my zero is shifted by 59.5V. And other two rails from secondary give me 42.5V and 76.5V.

Besides that zero is shifted, measuring secondaries against zero mains gives me 17V difference (shifted by 59.5V), while measuring secondaries against their own middle point gives me 21.8V on secondaries. Why is this?

Why are voltages shifted against mains ground? Is this normal? Or did I connect wires incorrectly? How to connect it properly, so that secondary middle is same as mains zero? Do I need to connect secondary middle to mains ground, to force it to ground's zero? Pls help.

thnx


keefaz

Toroid voltages - is this normal?
« Reply #1 on: January 18, 2007, 05:14:46 PM »
Quote
... I connected blue (1) with black (no label), and that was to be my middle, zero....

Try to measure voltage between black1 and blue, see if it is near to
40V AC (2 * 19 V) or a little more, then the connection is right

gnd

Toroid voltages - is this normal?
« Reply #2 on: January 18, 2007, 05:53:45 PM »
Quote from: "keefaz"
Quote
... I connected blue (1) with black (no label), and that was to be my middle, zero....

Try to measure voltage between black1 and blue, see if it is near to
40V AC (2 * 19 V) or a little more, then the connection is right


Yes, measurement across full secondary gives 43.7V AC (which is 2x21.8V), so this is correct.

What is puzzling me is 60V of AC offset comparing to mains (240V) ground.

How do I make secondary middle (signal ground) same potential as mains(240V) ground? Do I just connect them together at chasis? Or via some resistor ot condenser?

gnd

Toroid voltages - is this normal?
« Reply #3 on: January 19, 2007, 06:45:51 AM »
bump  :?

Pls help. How do I make secondary middle to be at mains ground? How do I connect those two?

I have 60V AC difference between mains ground and secondary middle. How it is best done? Via resistor, two diodes, capacitor, or all three is series? Or shall I just connect signal zero directly to power ground?

thnx

keefaz

Toroid voltages - is this normal?
« Reply #4 on: January 19, 2007, 08:00:49 AM »
You should be able to connect your center-tapped secondaries to
the mains power ground, but depending on your power supply
circuit, you may want to connect the ground at another place

On my DIY projects, the mains power ground is always connected to
chassis, and also the power supply ground at the same location

I would build the power supply and connect the transformer
(with secondaries connected as you mentionned), that should
work ok

gnd

Toroid voltages - is this normal?
« Reply #5 on: January 19, 2007, 09:17:22 AM »
What about that 60V AC offset? I thought centre of secondary should be near zero, like I guess voltage is swinging +/-, so middle should be near zero, is it? Where is this AC offset coming from? And if I connect it to mains ground, will I not short-circuit something, and possibly burn something?

Is mains 240V AC going +/- 240 (peak to peak) relative to mains ground? Or is it going from zero to +240? Because if it is offset by 120V, then maybe thats the reason for offset on secondary....

Has anybody some good explanation about where this 60V AC offset on trafo secondary is coming from?

thnx

SSLtech

Toroid voltages - is this normal?
« Reply #6 on: January 19, 2007, 11:02:40 AM »
Quote from: "gnd"
How do I make secondary middle (signal ground) same potential as mains(240V) ground? Do I just connect them together at chasis?

Yes, that's how you'd do it, but...

I recommend that you DON'T, or do it through an on-off switch. You'll stand a much better chance of being able to isolate hum loops later on. -Remember, you'll almost certainly be connecting it to other gear that has it linked elsewhere... and that's a loop right there.

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

gnd

Toroid voltages - is this normal?
« Reply #7 on: January 19, 2007, 08:23:38 PM »
So, I connected secondary middle to mains ground anyways, and all is fine. I think I will keep it connected, but I can still open the box and disconnect it later, if needed.

Regarding ground loops. I prefer solving them with cables having shield disconnected at one side. RANE company is giving a nice table of all possible connections, and it is shown where to disconnect shield in order to avoid ground loops.

thnx

SSLtech

Toroid voltages - is this normal?
« Reply #8 on: January 20, 2007, 09:27:22 AM »
Quote from: "RogerFoote"
Ummm, OK. Don't tie CT to gnd?

Yep.

Everyone's instinct is to measure to chassis ground and expect it to be 0V, but Neve don't do it. SSL don't do it... lots of very well informed companies don't do it... and in fact it's MUCH better NOT to do it.

That way, you can maintain a technical ground which is seperate from the required (and VERY sensible) power safety ground. Dirty power or power ground noise can therefore NEVER affect your audio ground. It utterly staggers me that people can spend so much on "power conditioners" and "clean power" then instinctively connect their clean audio ground to a potentially filthy power ground.

Very standard procedure. You CAN link them, but then on things like metal tip/sleeve ΒΌ" connectors, it becomes IMPOSSIBLE to cure ground loops and maintain the safety ground connection.

Last week I was troubleshooting an SSL power supply. If I put my black meter or scope probe on the chassis, I got NO readings on ANY pins. If I measure between '0V' and chassis, I measure an open circuit. That's how I can quickly check that nobody has defeated the original factory seperation of digital, analog and power grounds.

Shielding is effective when it is connected to mains ground irrespective of a connection to audio 0Volts, so why do they need to be linked? -They don't.

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

gnd

Toroid voltages - is this normal?
« Reply #9 on: January 20, 2007, 09:52:17 AM »
Quote
Everyone's instinct is to measure to chassis ground and expect it to be 0V, but Neve don't do it. SSL don't do it... lots of very well informed companies don't do it... and in fact it's MUCH better NOT to do it.



Thnx, SSLTech.
I would be perfectly happy with disconnecting power ground and audio ground, if they would be both near to 0 V, or at least similar within some milivolts. But in my case there is 60V AC difference. Wouldn't this huge voltage do something to signal? After all this means that my ground is AC, moving at 50Hz by 60volts. Isn't this a bit strange? Shouldn't ground be DC voltage? What is the voltage offset with Neve and SSL? Also tens of volts? Is it AC also?

I must be missing something. Could you maybe explain a bit more?

thnx


 

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