Stupid Dual DC PSU Question

I just picked up an HP dual DC for my workbench and I'm stumped over what seems to be a super basic thing, so I'm hoping you folks can help me out...  :-[


When checking the dual DC's voltage meter reading against my multimeter, they agree when I measure from positive to negative... so if the meter says 10v, and the black prod is on the negative terminal and the red on the positive I also see a 10v DC reading on my mulitmeter. So far so good.


Now when I measure the positive terminal against the ground terminal I get nothing... a short spike then it fades to nothing.


Shouldn't the ground terminal on the PSU be a 0v reference point and thus when measuring a positive DC voltage against a 0V point, shouldn't a I get a readout?  Shouldn't this be equivalent to reading a positive rail against the chassis on a piece of gear?


What am I missing?

Many bench power supplies are not internally refernced to ground to allow you do do things like float them above ground, or make the postive ground if you want a negative supply, or helps avoid ground loops where the piece of gear you supplying is earthed elsewhere.  In a nutshell you make any earth connections yourself.  It just make the PSU more flexible.

Rob Flinn said:

Many bench power supplies are not internally refernced to ground to allow you do do things like float them above ground, or make the postive ground if you want a negative supply, or helps avoid ground loops where the piece of gear you supplying is earthed elsewhere.  In a nutshell you make any earth connections yourself.  It just make the PSU more flexible.

Click to expand...

Thanks Rob.  That makes sense...  so connecting + to earth would give me a negative supply... and conversely, connecting - to earth would give me a variable positive supply.  Correct?

Looking at the supply, I was under the assumption that the + and - terminals would each output the same voltage on the meter readout... so if I wanted to supply power to a preamp that needed +/- 15v, I would simply ramp the voltage up to 15v on the meter (with appropriate amps) and run the negative PSU terminals to the - supply rail and positive PSU terminal to the + supply rail, and use the PSU earth as a ground.  What you're saying is that it won't work this way...

Instead, I'd need to use both sides of the dual DC power supply ... one to generate the positive voltage and one to generate the negative voltage.  Correct?

So then where do I find a ground reference relative to the supply... do I just use the chassis that I'm building in?

You have to ground a - from one supply and a + from another supply. The opposite + and - then form your positive and negative voltages to get +/- supply.
Many lab psu's have a separate banana jack for gnd/chassis. So three plugs for a single supply. Otherwise you can use some part of the case.

Arno said:

You have to ground a - from one supply and a + from another supply. The opposite + and - then form your positive and negative voltages to get +/- supply.
Many lab psu's have a separate banana jack for gnd/chassis. So three plugs for a single supply. Otherwise you can use some part of the case.

Click to expand...

Thanks!  I understand this...

So basic question... what is the - and + grounding to vs the banana jack for gnd/chassis?

The way to look at this is that each PSU has a + terminal (+ volts - RED)) and a 0 volts line (Black) and is floating with respect to the ground terminal which is the safety earth and normmaly connected to the PSU case chassis.  This is to avoid earth loops when you connect to other equipment.

Usually, you do not have to connect the 0 Volts line to ground except for safety reasons.

To make a balanced power supply, you connect the two power supplies in series.  You use one of the + connections as the Positive supply to your circuit.  The 0 Volts of THAT supply is connected to the + supply of the second units and becomes the circuit's 0 Volts line.  The 0 Volts of the second supply then becomes the - supply to the circuit.

Hope this helps

Mike

madswitcher said:

The way to look at this is that each PSU has a + terminal (+ volts - RED)) and a 0 volts line (Black) and is floating with respect to the ground terminal which is the safety earth and normmaly connected to the PSU case chassis.  This is to avoid earth loops when you connect to other equipment.

Usually, you do not have to connect the 0 Volts line to ground except for safety reasons.

To make a balanced power supply, you connect the two power supplies in series.  You use one of the + connections as the Positive supply to your circuit.  The 0 Volts of THAT supply is connected to the + supply of the second units and becomes the circuit's 0 Volts line.   The 0 Volts of the second supply then becomes the - supply to the circuit.

Hope this helps

Mike

Click to expand...

Mike,

Yes... this all makes sense.  I can see how 0v line which floats in respect to ground allows the PSU to be more flexible... I'm so used to using JLM PSU units which are only built do one thing that the more flexible HP bench unit kind of muddled up my concepts...

There is a ground connector on the BACK of the HP unit which I assume is that chassis ground... correct? (It's not a banana plug, but a nub that you might connect an alligator clip to).

So in practical terms, if I'm working on a project should I connect the HP chassis ground to the chassis of the unit I'm working on (with an alligator clip)?  What is the best practice here from experience?

Regards,

Damon

Hi Damon,

this is a bit of a sticky one since it involves safety aspects and I would not like to give advice as I have no control over the situation.

Usually, I float the zero volts lines from mains ground OR earth them at one point when I am testing on the bench - usually the scope chassis - sorry to be elusive.  Whatever happens do not disconnect the safety earth from the chassis or the mains plug - it is there for a reason.