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Old 23rd April 2011, 03:45 PM   #1
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Default what if common ground not zero volt

Is it possible common ground not equal to zero volt? let's say more(+) or less(-) than zero volt.
Does it cause any problem?
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Old 23rd April 2011, 04:23 PM   #2
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Originally Posted by cent88 View Post
Is it possible common ground not equal to zero volt? let's say more(+) or less(-) than zero volt.
Does it cause any problem?
Hello,
Just donít hold common in one hand and earth in the other.

You can reference common to any voltage you want just do it on purpose not by accident.
DT
All just for fun!
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Old 24th April 2011, 05:10 AM   #3
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If secondary voltage of transformer +12.1volts and -11.9volts. common ground voltage = +0.2volts and not zero?
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Old 24th April 2011, 06:00 AM   #4
benb is offline benb  United States
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So you're saying you've got a power supply with +12V and -12V outputs, and the actual voltages are +12.1 and -11.9. Ground is still 0V and these voltages are still measured with respect to ground. It does not matter if one voltage is a little more than the other. The average of these two voltages may not be exactly 0V, but that's not important. This does not cause a problem.
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Old 24th April 2011, 01:38 PM   #5
AndrewT is online now AndrewT  Scotland
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Now, I finally understand the original question.

You assumed the common voltage was Zero Volts and attached your Black Voltmeter probe to that point.
You measured +12.1Vdc to one line out and measured -11.9Vdc to the other line out.

You have three points (ends/lines out) which relative to each other are +12.1, 0, -11.9

This is exactly the same as 0, -12.1, -24 and is also exactly the same as +24, +11.9, 0

It all depends ONLY on where you decide to call Zero Volts.

You could have a second dual polarity supply giving +80, +65, 0Vdc
You can now connect these two isolated PSU to a Chassis tapping.
Let's assume that the Chassis Tapping is Zero Volts.
Connect the +80 of the +80, +65, 0 to the chassis. You now have 0, -15Vdc, -80Vdc from the second PSU.

Now attach your original +12.1, 0, -11.9Vdc PSU. Let's connect the -11.9V line of that to the -15Vdc tapping.
You end up with 0V at the chassis connected tapping. You still have -15Vdc and -80Vdc. The new connection to the -15Vdc tapping is still -15Vdc relative to chassis. The other two tappings become (-15V) + (+12.1V) = -2.9V and (-15V) + (-11.9V) = -26.9V.

The 5 PSU tappings are 0, -2.9, -15, -26.9, -80Vdc.

Attach your black Voltmeter Probe to chassis and measure the voltage to each of the tappings.
Note there are two wires/lines out connected to the 15V tapping.
That makes a total of 6 PSU tappings from your two dual polarity supplies.
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Last edited by AndrewT; 24th April 2011 at 01:42 PM.
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Old 25th April 2011, 10:00 PM   #6
DF96 is offline DF96  England
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Voltage is always expressed in terms of voltage difference between two points, even if one of the points is not named (or even thought about). So +0.2V means +0.2V higher than what?

Take an isolated circuit which works perfectly. Disconnect its ground reference from 0V and attach it to +5V. The circuit continues to work perfectly. Internally, nothing has changed.
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Old 29th April 2011, 05:19 PM   #7
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Going to rebuild the psu, can I connect it in this way?
Click the image to open in full size.
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Old 29th April 2011, 06:17 PM   #8
AndrewT is online now AndrewT  Scotland
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No.
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Old 29th April 2011, 06:32 PM   #9
CBS240 is offline CBS240  United States
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No.

What is the purpose of this power supply? The caps need some DC voltage reference, like a resistor voltage divider or active device. A floating GND is a viable option for light loads but will not work well with low inpedance circuits such as power amplifer output stages. AC1 and AC2 should be perfectly equal, as in bifiler winding, in order to paralell them.
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Last edited by CBS240; 29th April 2011 at 06:36 PM.
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Old 17th May 2011, 06:22 AM   #10
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Quote:
Originally Posted by benb View Post
So you're saying you've got a power supply with +12V and -12V outputs, and the actual voltages are +12.1 and -11.9. Ground is still 0V and these voltages are still measured with respect to ground. It does not matter if one voltage is a little more than the other. The average of these two voltages may not be exactly 0V, but that's not important. This does not cause a problem.
No no. I mean I have dual secondary. A secondary and B secondary.
A secondary -neg out -11.9v and B secondary +pos out 12.1v.
If I parallel them(single bridge rectifier), then I will get +0.2v at the ground?
If I parallel them after dual bridge rectifier(imperfect diode 0.6-0.8v drop), then I will get more or less than 0.2v at ground?
Do this cause high offset to the input/output of amp with reference to the non-zero volt ground?

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Originally Posted by CBS240 View Post
No.

What is the purpose of this power supply? The caps need some DC voltage reference, like a resistor voltage divider or active device. A floating GND is a viable option for light loads but will not work well with low inpedance circuits such as power amplifer output stages. AC1 and AC2 should be perfectly equal, as in bifiler winding, in order to paralell them.
How to get near zero volt ground? Replace transformer?
Earth the ground? How if lightning struck?

Last edited by cent88; 17th May 2011 at 06:25 AM.
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