# Question about wiring from a power supply

1. Feb 21, 2009

### atlbraves49

I'm considering using this power supply:
http://salestores.com/pr30trouposu2.html

now.. if i wanted to get a voltage value of -5V into a circuit (such as http://www.tekscan.com/images/flexi-circuit-new.jpg" [Broken] ), could i just put a wire from the (-) terminal of the power supply, to the Vt? And then what.. would i need the ground terminal of the power supply wired somewhere, and if so.. where?

Thanks

Last edited by a moderator: May 4, 2017
2. Feb 21, 2009

### MATLABdude

Yes, these sorts of power supplies normally have floating outputs, and you can treat them as if each side was a separate battery. The green ground terminal is connected to the chassis ground which is connected to your local earth ground (so, in undergraduate labs, it's not employed more frequently than it is).

If you were looking to generate, say, +/- 9 V (to power something like an Op-Amp) you would set both sides to (positive) 12, and then connect the negative terminal of one side to the positive terminal of the other side, using this node as your circuit ground (which you may, or may not, connect with another jumper or the metal clip thingie that's on the banana plugs).

The easier way of doing the above (without the jumpers) is to press the 'Series' button which (internally) does the same.

Note that the (fixed) 5V supply may or may not be connected to earth ground, and in any case, doesn't offer any current limiting!

EDIT: You can use the (fixed) 5V supply in a similar fashion to the above, thus using all $300-$500 worth of your bench power supply!

Last edited by a moderator: May 4, 2017
3. Feb 21, 2009

### atlbraves49

my idea for using this power supply with the above circuit was as follows:

For the left supply (set to 5V):
(-) terminal to the -5 Vt
(+) unused
(Ground) to the ground of the circuit

For the right supply (set to 9V):
(-) terminal to the -9V of the op-amp
(+) terminal to the +9V of the op-amp
(Ground) to the ground of the circuit

Would that work?
(note that the fixed 5V wouldnt be used)

4. Feb 21, 2009

### MATLABdude

This wouldn't work. As I mentioned in the previous post, the ground terminal isn't connected to anything! Note that when they say +/- 9V, they mean that you need 18V across the two terminals, tapped in the middle to give you a ground point. Reread the previous post, and then do the following:

1) Put the power supply into series mode
2) Set both sides to 9V
3) Use the positive terminal of the left supply as +9, the negative terminal of the right supply as -9, and either of the other two terminals as ground. DO NOT USE THE 'GROUND' TERMINAL AS GROUND!
4) Connect the positive terminal of the 5V fixed to either of the terminals in 3, and the negative terminal as your -5V supply.

EDIT: And then verify the voltages, relative to the negative terminal of the left or positive terminal of the right using your DMM.

5. Mar 12, 2009

### atlbraves49

alright i tested and got it figured out for the above example, but the power supply we have to use doesnt have a negative terminal for the "6V supply". It has the 6V (+), the +/-25V, and the COM. I can set the 6V supply to 5V, and if i inverse the multimeter leads, it will read it as -5V, but how do i get it to actually supply -5v to the circuit? Wont it actually be supplying the +5V?

6. Mar 12, 2009

### MATLABdude

EDIT: I reread your post, and it appears I was attempting to answer the wrong question. Is this power supply not the one you linked to above? If, as you say, there are a grand total of 4 terminals (+/-25V, +5V, COM) then you're probably stuck, as you don't have independent outputs. I'd ask your lab instructor if you can get a better triple output power supply with independent outputs.

Last edited: Mar 12, 2009
7. Mar 14, 2009

### atlbraves49

here is the power supply we were allowed to borrow:

hp 6236B

http://cgi.ebay.com/HP-6236B-Triple...th-manual_W0QQitemZ170309876636QQcmdZViewItem

so we cant get -5V from this?

if not, i guess im back to needing to buy this:
http://salestores.com/pr30trouposu2.html

unless someone knows a cheaper alternative to get -9V, 9V, and -5V

8. Mar 15, 2009

### MATLABdude

If you have an old ATX power supply, the following may be of use to you:
http://www.techpowerup.com/articles/other/22

ATX v2.0 connector pin-outs:
http://en.wikipedia.org/wiki/Atx#ATX12V_2.0

Be careful... These things often don't have great output regulation, stability, or nice features like current limit / short circuit protection. What do you expect for something that costs $20-$50? You could try using a 7809 and/or 7909 to regulate the +/-12V down to +/-9V with a little better regulation.

For future reference, please refrain from PMing people that you've updated a thread; most people don't have PM notification, but DO get notified whenever a thread they're subscribed to gets a new post. That and we can see something's floated up to the top of the page again.

EDIT: Before you start hacking, make sure the sticker on top of the power supply says that there is a -5V output. You could also just use a 7905 on the -12V rail to regulate this down to -5V instead.

Also, how to convert a computer ATX power supply to a bench power supply:
http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

Last edited: Mar 15, 2009
9. Mar 16, 2009

### atlbraves49

so according to my professor, the power supply without the negative terminal for the +6V output CAN still give -5V instead of just +5V....

Sadly he wouldnt tell me how...

he said "if I showed you with a meter on my desk - it can be done on the bench."

(when you use a multimeter and reverse the leads, it shows -5V)

so any ideas?

again, this is what it looks like:
http://www.eqrentals.com/sale/details/HTE0506.html [Broken]

Last edited by a moderator: May 4, 2017
10. Mar 16, 2009

### MATLABdude

Well, you can, if that's all you wanted. You can't if you want +/- 9V, since all of these outputs share a common ground, and a common banana jack. Now, if you happen to have another power supply with which to supply +/-9V, you could use this one for a dedicated -5V supply.

Last edited by a moderator: May 4, 2017