Triple Output Power Supply Question

[DROBNJAK]

I've saved a bit for Christmas and I would like to buy myself a super-duper triple output supply, with bipolar / split rail. I am looking for something around £1,000 ($1,600) or less, for something in 120-150W range. I definitely want to have positive and negative 25-30V outputs, with one 6-8V output. Basically I need that for op-amps.

Now I found this Aim-TTi QL355TP Programmable supply: http://www.tti-test.com/products-tti/pdf-brochure/psu-ql2-6p.pdf and I compared it to several others, like: KEITHLEY - 2230-30-1 and RIGOL DP1308A and TTi came on top.

But one thing I do not understand is, although TTi has two 30V outputs it doesn't say that one output can be positive, while other output is negative. I checked with their sales, and they told me that I can't have negative. Now I do not understand why in the world one would want to make supply with two positive 30V outputs? Aren't all triple output supplies always made with one negative, one positive and one digital 5V output.

[tom66]

The supplies are floating on a good power supply, what this means is you can tie the - and + of two supplies together. This puts them in series, giving you a split +/-30V, if you take the ground as being between - and +, or a 60V supply total (on the outer + and -.)

[DROBNJAK]

Thanks.

Does that mean that I can provide -30V, Gnd and +30V to the circuit?

The only thing that really worries me is that I can connect ground from oscilloscope probe to the floating ground of the power supply and burn the oscilloscope. If I connect it as you described, am I safe in connecting scope's ground to the -30/0/+30 arrangement?

This is what sales person replied to me:

"The QL355TP cannot be configured as a bipolar supply as such I’m afraid.  If you require a power supply which continuously changes both positive and negative polarities passing through zero, you will need an dedicated Bi-polar power supply. (also referred to as a four quadrant power supply).  Unfortunately we do not have any bipolar power supplies in our product range."

[shailesh3t]

 I have used QL 355 TP before its a nice power supply and works well , but am not in a position to test the symmetrical supply as i have left my previous work , i have only used the positive supply of it most of time. can say its very practical  had no problem using it.
 else i think u can buy  2 smaller units that have 0 - 30 volts  and then u can  make it symetrical supply i think.

[DaveW]

I've bought a few of these, and the outputs are definitely floating with respect to mains ground, so you can configure it as a split rail PSU. I presume they mean you can't make a single pair of terminals change polarity

[DROBNJAK]

I have used QL 355 TP before its a nice power supply and works well , but am not in a position to test the symmetrical supply as i have left my previous work , i have only used the positive supply of it most of time. can say its very practical  had no problem using it.
 else i think u can buy  2 smaller units that have 0 - 30 volts  and then u can  make it symetrical supply i think.

That is great idea. Are you saying that one can buy 2 similar power supplies and than connect them in such a way that one gets +30/Gnd/-30V?

There is this Chinease Maynuo M8811 Programmable DC Single Output Power Supply, which is actually the best all around value for money in terms of accuracy v price. It comes with very good free software. So you are saying, I can buy two of these and connect them to get +/-30V?

[metalphreak]

Try to think of isolated DC supplies as batteries. If they are 30V batteries, you can:

Stack them in series for 60V using the outer terminals.
The above for split -30V and +30V supplies by using the centre connection as ground
Put them in parallel for twice the current


Because they are isolated, you can connect that middle terminal to mains earth ground, so the entire circuit is connected to mains ground, but this won't have any effect if none of your circuit actually interacts with anything else that is mains earth referenced!

The downfall occurs when a supply isn't isolated, and people connect the positive of one rail to the negative of another rail (the series connections above). The negative rail of the other rail is mains earth referenced, as is the first rail, which means the negative of both is internally connected, so you have just shorted out the first supply rail!

[DROBNJAK]

Is isolated the same as floating?

[quarros]

Is isolated the same as floating?

It means that the device is galvanicaly isolated from any external potential level... therefore Floating.

[codeboy2k]

I've always preferred multiple independent (thus also isolated) single output supplies.  You can still connect them together to get  a bipolar (+15V/-15V) output and if one power supply craps out, it doesn't take out 3 others that are embedded with it (i.e. a quad supply)

You can often get 3 or 4 really good single output supplies for less than the price of a triple or quad output supply.

What you lose in this configuration is the gimmicky stuff, like tracking (I hardly ever use that -- TTi calls it link mode in their brochure on the QL355TP), all outputs on/off, over-voltage controls all outputs or linked outputs, etc.)

What you gain is full isolation and each supply has it's own LCD display (most triple and quad output supplies don't give each output it's own display for voltage and current).

There are some single output supplies that have control outputs on the back to control another supply from the same family, so you can take 2 or more single output supplies and link them over an external cable and they can do tracking, all outputs on/off, etc, over-voltage protection on all outputs if any 1 single output is OverVoltage tripped, etc....

[Kryoclasm]

Thanks.

Does that mean that I can provide -30V, Gnd and +30V to the circuit?

The only thing that really worries me is that I can connect ground from oscilloscope probe to the floating ground of the power supply and burn the oscilloscope. If I connect it as you described, am I safe in connecting scope's ground to the -30/0/+30 arrangement?

I'm no expert but referring to the center pole of a series connected dual power supply should not be referred to as Ground unless it is tied to ground. If it is tied to ground then it isn't floating and could give a path to zap your scope... I think. 

[peteroakes]

I agree, referring to it as possibly 0V may be more accurate (Assuming both sides track accurately you could call it Mid Rail ?). at the end of the day you need to read the manual, if in doubt don't get it, and if you do then use a meter and measure the continuity between the posts and the chassis of the PSU (If its metal) or the ground pin of the PSU power lead. if it is from China you should measure anyway even If the manual claims isolated just to be safe and to avoid finding out the hard way.

A true floating PSU would mean you could use any post as a ground or 0V reference, the rest of the posts would have volts relative to that, so a 30-0-30 setup could easily be a 0 - 60V if the -30 is grounded or tied to a zero volt reference else where.

If you have a triple output supply where all the outputs are isolated from each other and from ground then you could series them up to close to or over a hundred volts (Depending on the max output of each channel) so be ware of what your doing and what your connecting it too, Smoke is a good sign of a problem

[GK]

Triple output? Bah! My ideal bench supply for general purpose (low power) electronics prototyping would make avaliable, simultaneously, multiple fixed output voltages of +24V, +15V, +12V, +5V, +3V3, -24V, - 15V, -12V -5V and perhaps a pair of additional  outputs variable from 0V to +/-40V.

Have never seen a commercial supply like this, though I recall many moons ago the now defunct Australian ETI designed a kitset along such lines called the "Versiply", or something like that. Dunno why the concept never took off.

[DROBNJAK]

   Yes, but nobody replied to my question: Does term "floating" means the same as term "isollated" in a context of power supply. I mean, technically battery is "floating".

[quarros]

  Yes, but nobody replied to my question: Does term "floating" means the same as term "isollated" in a context of power supply. I mean, technically battery is "floating".

I did but it seems you did not listen. Look... whatever power supply we talking about that has its positive and negative rails galvanicaly isolated from any reference voltage level is FLOATING the same way as a battery would. For example: If the power supply's negative line would be bundled with the mains earth, than it would be not floating. 

If somehow I misunderstand your problem than please excuse me.

[dr_p]

So you want to spend 1600 $ on a power supply but have a problem understanding what isolated means, even though you were told so. I take it you're really new to the world of electronics.

Therefore I think you should lower the price point to 1-200$ and use that PSU until you really know what you need. I can't possibly see how FOR YOU a 1600$ power supply would be better than a cheap 100$ one. Seriously, get a cheap one and spend money on what you will need along the way - tools, soldering equipment, parts, lab equipment etc. 1600$ is a lot of money, and you seem to agree, since you "saved a bit for Christmas".

[DROBNJAK]

actually, you might be right. But it is such a nice instrument.

[ju1ce]

actually, you might be right. But it is such a nice instrument.

Do you really need all the fancy features? You could have for example TTi EL302RT, which is a 300 quid 2x30V + 5V supply I happen to own, and use the rest of the money for something more important. Of course, having really nice instruments feels nice, so you should go for QL355TP if you really want to and can afford to 

http://uk.farnell.com/aim-tti-instruments/el302rt/power-supply-bench-2-x-30v-5v-2a/dp/1670782

[KJDS]

Triple output? Bah! My ideal bench supply for general purpose (low power) electronics prototyping would make avaliable, simultaneously, multiple fixed output voltages of +24V, +15V, +12V, +5V, +3V3, -24V, - 15V, -12V -5V and perhaps a pair of additional  outputs variable from 0V to +/-40V.

Have never seen a commercial supply like this, though I recall many moons ago the now defunct Australian ETI designed a kitset along such lines called the "Versiply", or something like that. Dunno why the concept never took off.

Agilent make a 16 output supply, however I think that control is only via GPIB