[NEWBIE] Daft question, again, on Bench Power Supplies...

I see Bench Power Supplies with variable voltage all the time
One thing I can't quite gather...
How do you identifier whether you can get a bipolar supply out of them

I see that there are +/- terminals - is that an indication of a bipolar supply and if so how to you "do" the 0v
and if you want just +24v, ov how do you do it??

[quote author="uk03878"]
How do you identifier whether you can get a bipolar supply out of them
[/quote]

Normally a power supply capable of a bipolar output will be described as a 'dual' supply.

+/- terminals probably only indicate that the supply is isolated, i.e. not referenced to ground. There may be a third terminal for ground, so you can connect that to one of the outputs to create +/0 or 0/- supplies, but a single supply can't create a +/0/- from this.

I have a Thurlby Thandar PL series power supply that I'm very happy with. Other manufacturers are likely to offer a very similar range of models:

http://www.tti-test.com/products-tti/psu/pl-series.htm

These are available as single, dual, or triple supplies. Mine is a triple, which has the same two full-featured outputs as the dual, and adds a fixed 5V output for logic.

On the dual (and triple) supplies, the two main outputs are isolated and independent. You have a choice of four ways to use them:

1) Two completely independent supplies.
2) Tracking - for a symmetric bipolar output.
3) Series - for higher voltages, or asymettric output.
4) Parallel - for more current.

The data sheet PDF on the web site provides a good overview of the modes of operation, and how they can be used.

Hope this helps.
Steve.

Ahhh.. many thanks
Starts to make sense now

So if I had two very cheap single 15v bench Power supplies.
Each with a +/- out terminals I could wire the first one to +15v and ground
How could I wore the second one to add a -15v?
If that is possible at all....

Anybody please have a look at this....
Is this bipolar?

[quote author="uk03878"]
So if I had two very cheap single 15v bench Power supplies.
Each with a +/- out terminals I could wire the first one to +15v and ground
How could I wore the second one to add a -15v?
If that is possible at all....[/quote]

The first thing to do is to check that your supplies really do have floating outputs. i.e. they do not have one of the outputs (probably '-') connected to ground internally. If one of your outputs is grounded then what I am about to describe will not work. At best, the supply will self protect, at worst, there will be smoke.

You can think of a single floating/isolated power supply output as being like a battery. You can stack them just like you can stack batteries to get a higher voltage.

e.g. With 15 V suppliess:

0V--PSU--15V 0V--PSU-15V

Connect them together:

0V--PSU--15V--PSU--30V

But, the voltages are all relative, so that's exactly the same as:

-15V--PSU--0V--PSU--15V

Viola - a bipolar supply. Most circuits will be perfectly happy with this. If it becomes important that 0V on this supply is the same as 0V on another supply, then you can connect all the 0Vs together, or to a common reference. A third GND terminal on a single power supply is probably connected to the safety earth, so is often a convenient reference point.

Consider the case where two single supplies are not floating, but are referenced to ground:

GND--PSU1--15V GND--PSU2--15V

If you connect the two middle terminals together PSU1 is trying to hold that point at 15V above GND, but it's already connected directly to GND by PSU2. PSU1 sees this as a short circuit, and shuts down (or blows a fuse, or a regulator, or something).

Steve.

[quote author="uk03878"]Anybody please have a look at this....
Is this bipolar?
[/quote]

That looks like a single supply to me.

My guess it that you have floating outputs (+ and -) and a ground reference point between.

If you unplug from the wall, but switch the front panel power switch ON (in case it affects internal connections), you should be able to measure a low resistance from G to the earth pin on the plug, and high resistances between - and G and between + and G. This will indicate that the G pin is connected to safety earth, but is not connected internally to either of the outputs.

Steve.

[quote author="smorphet"]
At best, the supply will self protect, at worst, there will be smoke.
Steve.[/quote]
Steve - thank you very much...
I think I nderstood it...
To remind me I have utilised your quote as my post signature

The best units probably have the word tracking on the front panel.
AND
... even though they may say it we have had a few that just didn't track properly.

Simple bench supplies for the sorts of projects we do often may just be a couple of spare supplies from actual projects.
I still like testing stuff with that battery box of mine.

Anyone know anythign about htis PS?

http://store.yahoo.com/webtronics/sioubeposu3a3.html

I have a Bench supply schematic and board layout and parts list if you're interested. It'll crank out 4 to 20V, pos. & neg. with short protection. I mentioned this a while back but there wasn't any interest so I didn't bother organizing the files to a PDF.

You could buy a fancy one for probably cheaper than building this one, but it might be good as an educational exercise.

Here are some pics:
http://www.groupdiy.com/forum/ethanstuff/3.jpg
http://www.groupdiy.com/forum/ethanstuff/4.jpg

[quote author="Ethan"]I have a Bench supply schematic and board layout and parts list if you're interested. It'll crank out 4 to 20V, pos. & neg. with short protection. I mentioned this a while back but there wasn't any interest so I didn't bother organizing the files to a PDF.

You could buy a fancy one for probably cheaper than building this one, but it might be good as an educational exercise.

Here are some pics:
http://www.groupdiy.com/forum/ethanstuff/3.jpg
http://www.groupdiy.com/forum/ethanstuff/4.jpg[/quote]
Ethan
Absolutely!!!! - I must have missed that post...

Hi Ethan!

I'm also really interested in power supply....I was about to plump down £200 for a Digimess bench supply but this may save a few pennies for now.

How much current does your box put out per rail? Also how much do you think it roughly cost to build in terms of $$$ and time?

Thanks for offering to share your work...

Keep rocking!
Tom

Hi Guys,
I've organized the files into PDFs.

A few notes:
-If you use a standard 6X6" board you could make 2 and just cut it in half. I made the traces and pads really BIG so it would be easy to etch at home.
-Ground the chassis to Earth (AC ground plug) but leave the PS reference floating.
-The Power transformers are available at your local radio shack (you can also order them online) they're about $11. The reason for using 2 is because I couldn't find a 48V,CT 2A but if you can by all means use it.
-You could attach meters, but it is unnecessary and that would of course make this project more expensive. Using a DVM to adjust the voltage output works just fine.

If something doesn't make sense, let me know. *Disclaimer: Although this project is intended for beginners, AC voltages are lethal. Please do not attempt this project if you are not familiar with basic precautions for dealing with electricity.*

Download 220K ZIP files here

Ethan - many thanks
Printed and filed....

I forgot to mention.

I DO NOT recommend using the enclosure than is listed in the BOM (the one in my pictures.) The spacing is tight, and your wiring and placement has to be pretty meticulous if you intend to keep the power transformers inside the chassis like I did. If you try to mount them on the outside on the back, the chassis will tip backwards.

Also, the inlet and fuse holder in the pictures is not the same one listed in the BOM. The one in the BOM is a fused inlet, which I like better, but I used the other one since I had many of those lying around.

Cheers Ethan,

I'll scan it over now.

Regards Tom

Know this is a very old post but found it in the meta. The link to the files are not working anymore...
Has anyone still the files available?