Hi I have built plenty of circuits in school but I always had a +/- 12V supply. Now that I want to build circuits at home I do not have a supply that goes negative (I do have a 24V DC supply) Can I use it to power an OPAMP that wants +/-12V? by giving it +24 Vcc and 0V Vee or must I come up with a -12V supply. How can I get the -12V from my current supply?

any advise will be appreciated

 

Yes, you can use the 24VDC as a +/-12V to the op amp, IF you do it right. What you have to do is bias the points of the opamp circuit that you would normally call "ground" to +12V. This can be a bit tricky, but once you get the hang of it, its not hard.

For example, consider a simple inverting amplifier. In the attached diagram we see the classic textbook configuration above and the version modified for 24V below. In this example, the amp only works for AC signals, not for DC. The reason for this is because I have blocked the DC path at input and output using capacitors. As a result, we can only feed an AC to the input and get an AC voltage at the load resistor. The op amp is dumb. It only knows that it must push its output voltage to insure that the voltage difference between + and - inputs is zero. Since the + input is biassed to 12V at DC, then the op amp will make its output bias point DC equal to 12 volts. No dc current flows through Rf so the - input will therefore also be 12 V. There are other ways than this. But that's the idea.

Attached Images

op amp 1.JPG (27.2 KB, 24 views)

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RadioRon

 

I have both types of opamp circuits here. Change the power supply voltage to +24V if you want but most opamps work fine with only 12V.

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opamps-2.gif (17.9 KB, 27 views)

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Uncle $crooge

 

yes you can do that or simply create -12 v...
try this link www.uoguelph.ca/~antoon/circ/741-ps.html

 

Use an old PC power supply, they usually have token -5 and -12v lines that can supply a few hundred ma's. I think it's a requirement of the ATX specification.

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hi barry,
Regarding the student who wanted to convert his +24v dc bench power supply
to a general purpose +/-12 volt supply.

Download the diagram, *.jpg format, it uses a -12v reg [7912] to give +/-12v.
The technique is sometimes referred to as a phantom ground. If the current drawn from the
-12v is less than 100mA, the 7912 will not require any heat sinking.

The cost of a 7912 reg, is around $2.

The IMPORTANT detail is that the 24v bench psu outputs must be floating.
Most bench psu's are, so that they can be used, as a negative voltage with the
positive terminal connected to your equipment ground.

The same method can be used on battry supplies, provided that the battery
is not already connected to ground.

The +12v side will always be +12v, provided that the +24v supply output is greater
than +15v. The nominal -12v will however, be the difference between 24v psu's actual
output and 12 volt.

Example;
1. If the 24v is 24v then you will get +12v and -12v.
2. if the 24v is actually 25v the +12v is +12v and the nom -12v becomes -13v
3. if the 24v is actually 23v the +12v is +12v and the nom -12v becomes -11v.

This negative voltage change when using op amps has little effect on the
op amp, as they design to be tolerant regarding voltage supplys [within limits].
A point to remember is that if you are using a +/- voltage rail divider for your
op amp experiments, use a zener reference diode in the divider supply, especially
the -v rail.

The psu could be configured using a +v reg [7812], in the 24v positive line but
the +12v would become nominal. I prefer to have the +12v supply stable.
You could connect a +5v reg between the +12v rail and ground to provide a +5v
voltage for any TTL i/c's devices you may use in your experiments.
Don't exceed the maximum current rating of the 24v supply.

Keep us posted.

Regards
EricG

Attached Images

Phantpsu1.jpg (32.0 KB, 20 views)

 

Thanks for the information I will try using a single supply and if that doesn't work I will try the phantom ground. Here is a copy of the circuit I am trying to put together. I want a Variable frequency drive for a single phase AC motor. I realize the output will be a square wave rather than sine wave but I will work on the shape later. At this point I just wanted to get it working. I am using a light dimmer as my control POT for varying the DC voltage and I am hoping I can sense the voltage thru a divider network to my square wave oscillator. This way I only have 1 knob to turn to vary the voltage and frequency together. Any comments are welcome.

Barry

Attached Images

VFD.jpg (20.9 KB, 15 views)

 

I don't think that will work, single phase AC motors are frequency sensitive, a lower frequency will cause core saturation, you could reduce the duty cycle of the modified sinewave but you still have the problem that the phase shifting mechanism probably won't work at the lower frequency, also the harmonics in the squarewave tend to be reverse clocked and try to drive the motor in reverse which causes additional heating.

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Thanks for your reply Hero999. I understand single phase AC motors are freq. sensitive. I am trying to take advantage of that fact so that I can vary the speed of my motor RPM= 120*Freq. / No. of poles. I am using a dimmer inline with my supply to lower the DC voltage before it is chopped into A/C and at the same time set the frequency. I will have some tuning to do to coordinate both frequency and voltage ie: at 60 hz and above I will have about 115Vrms and at 30 hz I should have about 56Vrms. I will probably not want to go lower than 30hz for my application and I am hoping my trigger source will still function. Also I am not overly concerned with the output waveform but I will try to re-shape it as best I can with a capacitor/inductor network.

I am hoping that what I will end up with is a cheap and nasty but effective AC motor speed control. With a little tuning I think I can tailor it to a specific motor/load combination.

 

Eric, you got the 0V and -12V terminals swapped. See below. Also, see the comment I added to the schematic.

Attached Images

Phantpsu3.PNG (31.3 KB, 18 views)

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Ron (aka Rube)

 

hi ron,
thanks for the reply, the -12v and 0v are reversed. Just checked my working prototype just to be sure. Ive got a right index typing finger which suffers from ADS!.
If you have the bits in your scrap box, knock it up and try it using a bench psu as a source. As you say he will have to keep an eye on the +/- loading balance, but its fairly tolerant with what he trying to do with his op amps.
If you do try it, let me know.
As with most 'fixes' it a compromise, with the best option a dual +/- psu.

Hope we chat again.

Regards
EricG

Sorry barry for duff info.