Bench Power Supply Confusion

[Mechatrommer]

i've been browsing bench power supply. one thing bugging me is why there's 3 terminal for each output (-,g,+) instead of 2 (-,+) or (g,+)? i was assuming the negative is to supply negative voltage, but no. the manual indicated the connection is -ve to +ve terminal, so why "gnd" terminal is there? dont ask me why i'm so stupid, its just i never own this kind of thing.

ps: i'm looking for psu that can supply +ve votage and -ve voltage, but since there's no mentioning about -ve voltage in the spec, only like (0-30V) or (0-15V) spec, makes me confuse. any suggestion on bench power supply that can give -12V to +12V supply? at least 2 output, CV and CC capable, maybe around 3-10A, good value for money stuff? and if possible, linear mode (not switch mode), i dont want the supply output terminal tied/connected directly to main's earth ground, ie: full isolation. thanx.

[Bored@Work]

It is a convenience thing. The green terminal is connected to earth, not to any electronics. The red and black terminals are the outputs of the power supply, fully isolated. That is, neither the red or the black terminal is connected to earth (unless some fly-by-night One Hung Low Asian company cheated).

Now, if you want that the output is referenced to earth you just plug in a test lead with stackable 4 mm connectors to earth and either red or black and have your earth referenced output. Or if you want any other part of your circuit referenced to earth you just use the green terminal and a test lead. Instead of, for example, prodding PE in a wall socket.

[Simon]

yes the green terminal will be the psu chassis which in turn should be the system earth

[Kiriakos-GR]

Some pictures of my own Kenwood .

There is an metallic bridge , that you can turn sideways ( no contact with earth ),
or  with contact to earth  ( last picture + default setting ) .
But this metallic bridge is made by an such way that you cannot totally remove it from the banana plug.
( Probably helps to not be lost by accident )

My earth banana looks blue , but in our case the color is not our subject.

About the usability , I have never needed to remove the connection with earth from the minus plug in all this years.
And the days that I was working with car & marine type transmitters ( CB/VHF) , I thought that it was an nice thing to have.
The metallic housing of it,  becomes also as shield against RF,
that in large amounts it can cause issues on the PSU .



  



  

[Kiriakos-GR]

I found accidentally one information about the Ground ( In a technical document) ..

It looks that it can influence low voltages that are floating
relative to earth ground. 

Well If some one more knowledgeable, can in-light us, with one example it will be nice .

[Simon]

yes, theoretically negative is at ground potential but if it is floating and not connected there can be anything from a few mV to a few V difference. For examp,e the neutral in the mains is supposed to be at ground potential, try measuring it: you will find it is a few volts as it is tied to ground miles away in the substation

[Kiriakos-GR]

Well I would be interested to read about DC low voltages that are floating
relative to earth ground.

About AC , thanks God , I have some awareness 

[Simon]

well it was just an example.

Consider this: you have a physical ground and a power supply, usually the output is off the secondary of a transformer so the output is totally floating, that means you could have 1000V from "negative" to ground for all the psu cares as it is independant, until you connect them together. Usually you want to guarantee that negative is at ground potential when you are interfacing different circuits and they all have the have the same reference even if they have different power supplies

[Kiriakos-GR]

i've been browsing bench power supply.

Since when you browsing Agilent PSUs that cost 750$ - 1000$  

(Photo No1)

[tecman]

I am by no means an expert but there should be ideally no voltage between the output of a properly isolated PSU and earth ground. In reality there are transformer parsitics "capacitive coupling" primary to secondary. Y-type caps are also comonly placed across the isolation barrier for EMI these provide a high impeadance path as well. I was supriesed my isolation trany had 25 volts with respect to earth ground; the voltage reduces with loading. I did some research and found it was due to primary to secondary capacitive coupling.

I read a while back something about using earth ground as the return for the secondary DC but it made no sense to me. Maybe someone good enlighten us.

Most good supplies will have a transformer with an electrostatic shield.  This is a layer of copper foil wound between the primary and secondary coils, and grounded to the chassis.  This prevents capacitive coupling between the windings, for the reason you sited.

paul

[Zero999]

Even with a shield there will be some capacitive coupling between the shield and secondary which will cause a small voltage to be measured between the earth and secondary.

Yes, the green terminal is just chassis ground which can also be useful for protection against ESD.

Having a floating DC supply is good because it can be connected to another power supply which is connected to earth and it also means there won't be a short circuit if the power supply comes into contact with earth, for example a scope's chassis.

[Mechatrommer]

ok. thanx ALOT for clarifying that the green terminal (topmost picture) is indeed the earth and the black and red terminals are isolated, or are they in "switchmode" power supply?

the reason that make me to look for a proper power supply is two reasons:
1) my current PC ATX switchmode psu (that i used for any project before for a long time) cannot be un-tied from earth, recently i got issue with that.
2) i want negative supply (-3.3V, -5V, -12V, -30V, -3.14159V? etc) and constant current (CC) capable, but CC is standard issue i can see that.
maybe not a single sentence can describe it better than the cartoon sketch below.

problem 1:
a) i want my circuit is independent (floating awaaaay) off the other measuring tool DSO / FG / DMM etc.
b) i dont want to power up my circuit using +ve PSU only and DSO probe as ground. do you know what i mean? -ve psu is not connected at all! but the circuit is powered up! magic!
c) when my gnd (-ve) dso probe touching anywhere in the circuit that is not ground, my circuit is screwed!

problem 2:
my PC ATX psu got -5V and -12V allright, but that is just darn useless. i want the real -ve supply. where can i get -ve supply? without DIY?

[Mechatrommer]

Since when you browsing Agilent PSUs that cost 750$ - 1000$  

my current top list probably is Atten APS3005S-3D 30V 5A (topmost colored terminal picture), but googling for PSU Guide, i found BK Precision's pdf (black white picture in 1st post). but if this Atten cannot supply negative voltage, maybe i'll just forget that and cope with my PC PSU or battery cells or something. Agilent? Where is Agilent?
fyi: i did browse eevblog for bench power supply suggestion, but they are all the same species as this 3 terminalled type.

[tekfan]

Some pictures of my own Kenwood .

Wow that's one of the nicest looking supplies I've seen in a long time!

[Kiriakos-GR]

Some pictures of my own Kenwood .

Wow that's one of the nicest looking supplies I've seen in a long time!

Yes it is from them ones called as Serious Gear .
But it has a problem ,
it weights 14 kilos ,
other than that its perfect  

Edit : click on the picture for larger image.

( I got it used , and so it haves few scratches , but nothing as major problem )  
The only crazy that it does , is when i set it to zero volts , the output gives an negative DC 0.009V

[tekfan]

The only crazy that it does , is when i set it to zero volts , the output gives an negative DC 0.009V

I notice that most supplies do this (sometimes much worse). Yours may have a offset adjustment inside for setting it to exactly 0V

[Kiriakos-GR]

Well the positive part is that only the Fluke 28II can read it ( lots of digits )  

The unit is very complicate inside, and I do not have any service manual.
And so I will just let it as is.
On a second thought, i can add in line with the 10 turns pot, an small resistor !!
So to adjust it from there.  

Thanks for the tip, I feel much better now, by knowing that and others PSU acts the same.  

[metalphreak]

If you need +/- rails for circuits, just get a dual output power supply that has series/parallel tracking.

My PS has a Master and Slave output (and 5V fixed). When you enable series tracking, Slave Positive and Master Negative are connected internally and these become your (0) Ground. Slave Negative becomes your (-) negative rail, Master Positive becomes you (+) rail. If you want your supply referenced to earth ground, just connect one of the two (0) Ground points to the middle earth terminal.

If you want asymmetric voltages (ie -5V and +12V), you'll need to set each output, and then manually bridge the Slave Positive and Master Negative.
Series tracking mode only responds to the master controls (which is very important for parallel mode :p) so you get symmetrical voltage rails.

Kiriakos-GR: Mine reads ~10mV when set to the lowest setting on the multiturn pot.


If anyone's interested, its a Zhaoxin RXN-3010D-II Dual 30V 10A + 5V 3A fixed. Got mine on ebay for ~$300AUD although I think it was a mistake as its now ~$450.

Oh, and this may just answer your actual question If you want a negative voltage, you just swap the positive and negative leads. The voltage is just a reference between the two outputs. If you connect your circuit ground to the + terminal, and the input voltage to the - terminal, you'll get a negative 12V (or whatever you set) difference.

[shadewind]

Would this earth ground terminal be a good place to connect an ESD strap? It would sure be convenient since it's a banana plug and all.

[Simon]

yea should do the job

[Mechatrommer]

ok thanx metalphreak. that confirmed what i learnt few hours ago, thanx. with this series tracking mode, can also be setup as constant/limiting current right? and at the same time the +ve and -ve rail be maintained symetric?

[Zero999]

Would this earth ground terminal be a good place to connect an ESD strap? It would sure be convenient since it's a banana plug and all.

Yes but it's a good idea to connect a high value resistor (1M to 10M) in series with the wrist strap to protect against electric shock.

[metalphreak]

ok thanx metalphreak. that confirmed what i learnt few hours ago, thanx. with this series tracking mode, can also be setup as constant/limiting current right? and at the same time the +ve and -ve rail be maintained symetric?

Yeah, I believe the Master channel current limiter knob also controls both channels in series/parallel tracking modes. All the power supplies do is reduce the voltage until the current is at the set limit. Quite handy for testing LEDs and forward voltages. Set it to 20mA limit with a multimeter, turn the voltage dial up so it stays in C.C (current controlled) mode. Stick your LED in and you get the forward voltage on the readout

I have to remind myself that its a 600w supply and to be careful when testing low power stuff like LEDs etc xD Unless you need the higher voltages I'd take Dave's recommendation and buy a nice 0-15V 3A type supply. I have however found it quite handy a few times to test various motors and such (37V motors out of printers etc). 

[Ronnie]

If anyone's interested, its a Zhaoxin RXN-3010D-II Dual 30V 10A + 5V 3A fixed.

At 10 A maximum current output is it a switch mode PS?