# Op Amp Power Question

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#### What

##### New Member
I was trying to learn about op amps but I am confused obout something. the sites I looked at say I need a -12V supply.
Now I have a power supply an AC adaptor for something. the ouput is 12v. Is what I have +12 volts and a ground? or is the ground actually -12v?

Also during my search to learn about opamps many of the circuits schematics Showed a sine wave on the input and how it changed on the output. This isnt an actual AC sine wave is it.

#### Nigel Goodwin

##### Super Moderator
What said:
I was trying to learn about op amps but I am confused obout something. the sites I looked at say I need a -12V supply.
Now I have a power supply an AC adaptor for something. the ouput is 12v. Is what I have +12 volts and a ground? or is the ground actually -12v?

You need a split supply, +12V/0V/-12V.

Also during my search to learn about opamps many of the circuits schematics Showed a sine wave on the input and how it changed on the output. This isnt an actual AC sine wave is it.

Yes, why wouldn't it be?.

#### ericgibbs

##### Well-Known Member
hi what

The psu you have is a +12V and ground [0V].

You will require another PSU for -12V and ground [0V]

Regards
EricG

#### audioguru

##### Well-Known Member
Opamps don't need a dual polarity supply.
If you bias their inputs properly at half the supply voltage then a single polarity supply is fine for all opamps.
Look at their datasheets to see the range of supply voltage. Most opamps work fine from a 7V to a 36V total supply voltage.

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#### Hero999

##### Banned
You can probably convert the op-amp to work from a single 12V supply. If you really need a +/-12V supply then use a 12V adaptor with an AC output and a diode+capacitor voltage doubler to get +/-12VDC - it's a failr common technique.

##### Banned
You can also use a PC power supply. They have -5 +5 -12 +12 voltages. Bit noisy but they can be filtered if you need low noise.

#### What

##### New Member
Wow Thanks everyone for the help.

##### Well-Known Member
What said:
Now I have a power supply an AC adaptor for something. the ouput is 12v. Is what I have +12 volts and a ground? or is the ground actually -12v?
.

I didn't see anyone else answer your question so: The adapter is providing a potential difference of 12V between two conductors. It is up to you to choose which conductor is going to be connected to your circuit ground. If you connect the lower potential wire to your circuit ground then the other wire will be +12V. If you connect the higher potential wire to your circuit ground then the other wire will be -12V. In other words, you are getting 12V and you can use it as +12 or -12 as you see fit.

#### Nigel Goodwin

##### Super Moderator
I didn't see anyone else answer your question so: The adapter is providing a potential difference of 12V between two conductors. It is up to you to choose which conductor is going to be connected to your circuit ground. If you connect the lower potential wire to your circuit ground then the other wire will be +12V. If you connect the higher potential wire to your circuit ground then the other wire will be -12V. In other words, you are getting 12V and you can use it as +12 or -12 as you see fit.

I think we all answered his question, and you've just confused him!.

##### Well-Known Member
Nigel Goodwin said:
I think we all answered his question, and you've just confused him!.

Oops. Well i didn't get the impression that it was properly explained, but I fear you are correct. I reread my posting and almost got confused myself!

Edit: I took exception to Ericgibbs comments and felt a better explanation was necessary.

#### What

##### New Member
Nigel Goodwin said:
I think we all answered his question, and you've just confused him!.

Your right About the confused part thats for sure. But I think i understand what Ron was trying to tell me. The same princple applies when hooking up my multimeter. If attach the leads in reverse it displays -12 volts. I am right. Man Op-Amps suck I am not really sure how to make one do anything yet.

##### Well-Known Member
What said:
Your right About the confused part thats for sure. But I think i understand what Ron was trying to tell me. The same princple applies when hooking up my multimeter. If attach the leads in reverse it displays -12 volts. I am right. Man Op-Amps suck I am not really sure how to make one do anything yet.

Hooray, you get it!

#### eng1

##### New Member
I haven't understood if you're using an AC to DC adaptor or an AC to AC adaptor? If you're not sure about that, you might use two 9 batteries in series.

#### What

##### New Member
I have 2 AC to DC Adaptors
one is 12v the other is 5v

#### ericgibbs

##### Well-Known Member
hi Ron,
Sorry to hear you took exception to my post.

But the OP did say he was working on opa's and he did require a -12V supply as he already has a +12v supply.

So his original request still stands, unless he goes as audioguru suggests to single psu opa operation, he still needs a second 12V supply connected to give him -12V.

EricG

##### Well-Known Member
ericgibbs said:
hi Ron,
Sorry to hear you took exception to my post.

But the OP did say he was working on opa's and he did require a -12V supply as he already has a +12v supply.

So his original request still stands, unless he goes as audioguru suggests to single psu opa operation, he still needs a second 12V supply connected to give him -12V.

EricG

Of course you are right. At the time, it seemed like the OP did not understand that there is no difference between a +12V and a -12V wall wart other than how you hook up the wires, and your post almost implied that he needed a "-12V" wall wart. At least that was what went through my mind. Hence my confusing post.

I try to put myself in the place of someone who knows nothing about electronics (and find it all too easy!) and sometimes see an odd point of view compared to other posters.

#### ericgibbs

##### Well-Known Member
hi Ron,

I agree, I should have explained it a little more.

I often find 'novices' get confused between the terms 0V and Gnd and Common.

Trouble is if we 'dumb it down' when responding to a more experienced guy, he gets up tight.

Regards
EricG

##### Banned
Are all wallwarts floating though? I've seen a lot more switch mode wallwarts lately, and I don't know if those are true floating supplies. I know all common cheap wallwarts are because of the transformers, but it's a good idea to at least mention the possibility that a wallwart might be ground refrenced. That's not something you want to make a mistake about.

#### Nigel Goodwin

##### Super Moderator
All the ones I've seen just float - in fact the earth pin on the plug is only made of plastic, and is just there to open the shutter on the socket.

##### Banned
Are all wallwarts floating though? I've seen a lot more switch mode wallwarts lately, and I don't know if those are true floating supplies. I know all common cheap wallwarts are because of the transformers, but it's a good idea to at least mention the possibility that a wallwart might be ground refrenced. That's not something you want to make a mistake about.

Safety regs state that there must be 3-5KV isolation between the user and the line. In the case of the switcher that is transformer. They can be made smaller because of the frequency of the switcher.

A 20VA 60Hz wall transformer is about 2"x2.5"x1.5" and can safely supply 10W.

A 100W 200KHz switching transformer only needs to be 2"x2.5"x0.5", and it still supplies safety isolation. That might be slightly exaggerated as I have not looked close enough at the winding dissipation to know for sure.

D.

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