How are dual rail bench psu built, and why earth gnu output?

Hello,

I have seen bench psu with positive and negative voltage output.

How are these two-voltage-units built?

I reckoned it could be done in many ways. Here are some:

The two voltages come from separate coils on the transformer, and is regulated to two different voltages with separate controllers and transistors.

Another way is to regulate the voltage once for one output, and re-regulate the same current for the second voltage.

Perhaps there are other ways.

How should it be done most effectively?

Second question:
I have seen some PSU have separate green earth ground output. Is this just paralelled to the earth ground of the device? Is this dangerous if there is a fault inside the psu? And why is it useful to have earth GND output for other than mains power?

A lot of questions, I know

Thanks!M

hi,
For dual supplies.

If the mains transformer has two separate secondary windings its possible to make two isolated outputs.
These outputs could be used as positive or negative supplies simply by connecting one output to the projects 0V reference.

The two outputs could be connected in series to give a wider output voltage range, either positive or negative.
The above assumes that both power supplies are identically rated.

If the mains transformer has single, centre tapped secondary, then a dual output power supply is possible, it will however share at least one common output terminal, usually referenced as 0V common.

On some power supplies its possible to have a 'ground' free output, which can be connected or not with a linking strap on the 0V terminal to the chassis.

king.oslo said:

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Another way is to regulate the voltage once for one output, and re-regulate the same current for the second voltage.

Click to expand...

Don't understand this. What do you mean " re-regulate the same current for the second voltage"? The currents for the two voltages are quite separate.

crutschow said:

Don't understand this. What do you mean " re-regulate the same current for the second voltage"? The currents for the two voltages are quite separate.

Click to expand...

Carl.

I 'think' he means can he add a second parallel regulator to a to the input of the first regulated supply in order to get a second regulated supply.????

Have two separate regulators connected to the common 'smoothed' DC input , in order to get two different regulated outputs.

ericgibbs said:

Carl.

I 'think' he means can he add a second parallel regulator to a to the input of the first regulated supply in order to get a second regulated supply.????

Have two separate regulators connected to the common 'smoothed' DC input , in order to get two different regulated outputs.

Click to expand...

Yes. What is causing the confusion. Is it my weird English or is my terminology to blame? Thanks! M

ericgibbs said:

Carl.

I 'think' he means can he add a second parallel regulator to a to the input of the first regulated supply in order to get a second regulated supply.????

Have two separate regulators connected to the common 'smoothed' DC input , in order to get two different regulated outputs.

Click to expand...

But that won't give a positive and negative supply as he wanted.

crutschow said:

But that won't give a positive and negative supply as he wanted.

Click to expand...

I agree, I would say a dual secondary transformer would give him maximum flexibility.