Bench power supply bipolar connection

Thread Starter

kiltro

Joined Oct 24, 2011
50
I have a linear bench power supply with two isolated 0-30V outputs.

It has a button to switch between "independent, series, parallel" internal connection (obviously you can even do it externally with wires)

On the manual I read
Before the series connects. you need to check if the negative terminals of both master and slave outputs are
connect to the GND terminal. if they are. must be disconnected. otherwise, short circuit will be caused in the slave output
when the two outputs are connected in series.
I'm not completely sure what this means

Suppose that I set it to series.
If for example I want to see ripple of the positive rail using an oscope, can I connect the common of the probe (ground) to the + of CH1 (or - of CH2)?
 

MrChips

Joined Oct 2, 2009
30,706
This is a question about common GND.
Show us photos of the AC outlet, your oscilloscope, and the bench power supply.

1673199230275.png

Does your AC connection include a proper ground connection?
In older homes there might be only two prongs in the plug, with no safety ground.

Firstly, oscilloscopes are usually grounded, that is, voltage measurements are taken with reference to earth ground. Check for continuity with an ohmmeter.

The oscilloscope has two or three grounding points,
1) GND pin on the AC plug,
2) GND connection on the face of the oscilloscope,
3) GND clip on the oscilloscope probe.

Check that there is continuity with all three grounding points.

1673192851348.png

Dual output PSU might have connections that look like this:

1673193093312.png

Check for continuity with GND and the GND pin on the AC cable.
Check for continuity with GND on the PSU and GND on the oscilloscope (when both are plugged into AC and power is OFF).

Check for open circuit between all + and - terminals and GND.

After you have done all of the above we can talk about "floating" power supplies and how to use the oscilloscope without causing a short circuit.
 

dl324

Joined Mar 30, 2015
16,839
I'm not completely sure what this means

"Before the series connects. you need to check if the negative terminals of both master and slave outputs are
connect to the GND terminal. if they are. must be disconnected. otherwise, short circuit will be caused in the slave output when the two outputs are connected in series."
I interpret it to mean if you have negative outputs from both outputs connected together, you have to disconnect them before using the series option. Otherwise, you will short one of the outputs.

It could also mean that you have the negative outputs connected to earth ground.
 

Thread Starter

kiltro

Joined Oct 24, 2011
50
I interpret it to mean if you have negative outputs from both outputs connected together, you have to disconnect them before using the series option. Otherwise, you will short one of the outputs.

It could also mean that you have the negative outputs connected to earth ground.
Yeah put in this way make sense indeed
 

ericgibbs

Joined Jan 29, 2010
18,766
hi kiltro,
As you posted, with two isolated 0-30V outputs.

The AC ground connection has no relevance to your query when connecting in Parallel or Series.

E
 

Thread Starter

kiltro

Joined Oct 24, 2011
50
hi kiltro,
As you posted, with two isolated 0-30V outputs.

The AC ground connection has no relevance to your query when connecting in Parallel or Series.

E
Yeah it is that sentence that looked confusing

It says negative of both master and slave channel must be not connected to ground

But if in series
- slave + - master +

Connecting the ground from oscope probe to that central connection the negative of master will go to ground

But it is probably how @dl324 suggests
 

KeithWalker

Joined Jul 10, 2017
3,063
The ground connection is very important when you connect the two supplies in series.
If both supply negative terminals are connected to ground when the supplies are connected in series, both output terminals of the negative supply will be grounded, shorting out that supply.
When they are connected in series, you can externally connect the ground to any one of the output terminals.
 
Last edited:

MrChips

Joined Oct 2, 2009
30,706
Here are things to note.

1) The oscilloscope probe makes voltage measurements with reference to earth ground. Thus the device or circuit must also be referenced to earth ground at some point. There are various ways to do this which will be explained further on in this post.

2) Since the oscilloscope is already referenced to ground, you do not need to use the ground clip on the probe for low frequency measurements.

3) If you want to examine AC voltage ripple on a DC supply rail, set the oscilloscope vertical input to AC.

Now we discuss "floating" power supplies.

A 9V battery is a floating power supply, i.e. there is no reference to ground.

1673197579575.png

A bench PSU is also a floating power supply.

1673197667947.png

To create a positive voltage referenced to ground, connect the - terminal to GND.
To create a negative voltage referenced to ground, connect the + terminal to GND.

To create a bipolar PSU with two supplies, connect the two in series, with the interconnection going to GND.
One PSU with the - terminal at GND becomes the +ve supply.
The PSU with the + terminal at GND becomes the -ve supply.

1673198057252.png

Thus, as stated before, there is no need to connect the oscilloscope ground clip.
You connect the oscilloscope ground clip to the circuit in order to reduce noise artifacts or if your circuit under test is not already grounded.
In other words, if your circuit is floating, i.e. there is not connection to ground, you can ground it (i.e. it must be grounded) to the oscilloscope ground.
 

ericgibbs

Joined Jan 29, 2010
18,766
1) The oscilloscope probe makes voltage measurements with reference to earth ground. Thus the device or circuit must also be referenced to earth ground at some point
Hi,
This statement is misleading, the device under measurement need not be self connected to Earth ground, it will be connected to ground when the Scope ground clip is attached, at the Node chosen by the scope user.

E
 

MrChips

Joined Oct 2, 2009
30,706
Set your PSU to series connection.
Use the oscilloscope to measure the voltage at all + and - terminals.
Do not connect the ground clip of the oscilloscope.

This will tell you which terminals are +ve, 0V, and -ve.
 

MrChips

Joined Oct 2, 2009
30,706
Hi,
This statement is misleading, the device under measurement need not be self connected to Earth ground, it will be connected to ground when the Scope ground clip is attached, at the Node chosen by the scope user.

E
I think I made it perfectly clear.
The oscilloscope is already grounded.
The circuit under test must be grounded. It can be grounded via its own ground.

Only after it is confirmed to be floating should you use the oscilloscope ground clip.
If you use the oscilloscope ground clip it must be connected to a known ground node in the circuit.
 

ericgibbs

Joined Jan 29, 2010
18,766
Hi Chips.
If the two PSU outputs are isolated, how will the Scope display a signal on the Pos and/or Neg terminals, with the scope Common/Earth not connected.?
E
 

MrChips

Joined Oct 2, 2009
30,706
Hi Chips.
If the two PSU outputs are isolated, how will the Scope display a signal on the Pos and/or Neg terminals, with the scope Common/Earth not connected.?
E
If the PSU is isolated you have to make it not isolated.
The way to do that is to connect one of its terminals to GND.

We both understand each other. It is a matter of clarifying the language so that newcomers don't create a short circuit.
 

ericgibbs

Joined Jan 29, 2010
18,766
If the PSU is isolated you have to make it not isolated.
The way to do that is to connect one of its terminals to GND.
hi.
As I said earlier, by connecting the Scope Ground/Common clip to the PSU will automatically ground it at that Node.
E
 

MrChips

Joined Oct 2, 2009
30,706
Note
What we are trying to avoid is a newcomer who thinks that they can connect the oscilloscope ground clip to any place they choose. Not so if you want to avoid seeing sparks.
 
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