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Author Topic: How to get -12V ?  (Read 1469 times)

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Offline craigdudeTopic starter

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How to get -12V ?
« on: August 05, 2012, 05:22:04 AM »
Oh hi guys  :D.
I am hoping you can help me. I am (attempting) building a buffer amplifier for an oscilloscope software so I can use my computer as an oscilloscope. Unfortunately I dont have much electronics experience. The circuit ( I will post a link to it below) requires -12V. And I have had alot of trouble trying to get this voltage. I know I could use a center tap transformer and bridge rectifier but  I would prefer to use my computer power supply to power this thing and not the mains. I have cut into the -12V blue wire on my computer and fed my circuit off that ( I know, bad bad bad). Now the computer wont start I think It has blown a fuse or something. In the documenattion with the circuit the author says this :

" A -12V power connector may not be readily available in your computer.
The standard disk drive power connectors are +5 and +12.  I added an
alternate power connector in my system by tapping my power supplies'
-12, GND, and +12 wires.  Be careful not to get these mixed up!"

Its quite vauge what he means and I havent been able to get in touch with him to ask but I thought It would be OK to simply cut and feed off the -12V. But something has now gone very wrong with my computer.

I have read an older thread on this site about getting -15V which was interesting but thought I would have more luck starting a new thread.







Thanks Guys, hopefully someone can guide me in the right direction  ;D
« Last Edit: August 05, 2012, 05:33:52 AM by craigdude »

Offline waltr

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Re: How to get -12V ?
« Reply #1 on: August 05, 2012, 09:06:42 AM »
I have used voltage invert chips to get a negative voltage to power op-amps. Most of the time the current requirements of the op-amp is low so I do not need much negative voltage current.

The Intersil ICL7660 is a chip I keep in stock for this usage. There are a few other makers of 7660 chips.

In a pinch, I have used a MAX232 (RS232 level translator) for just obtaining a negative voltage (tie all the signal input pins high and leave the output pins unconnected).

The 7660 and MAX232 only need some external caps to work. These are ok if there is only an op-amp or two that requires a negative voltage.

Note: The op-amp may not need the full -12V. Check the op-amp's data sheet for how close to the -rail the signal (input & output) can be then add this to the signal level.


Linear Tech, TI, Analog Devices and others make inverting voltage regulator chips. Most of these require more than external caps and careful layout of a PCB but can supply much more current at a set regulated voltage. These work very well when built correctly.

Offline craigdudeTopic starter

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Re: How to get -12V ?
« Reply #2 on: August 05, 2012, 10:57:05 PM »
Thanks for your help. I've had a look at the 7660 datasheet and it looks fairly easy to wire:



I suppose the +12 and GND off my computers PS will do to create this. Luckily, I disconnected my "mod" from the system and the computer starts up fine now.  It just didn't like my circuit on that -12V wire. I don't know why. But I think this solution will solve everything.
Now I know this isnt your problem and everything but since youre experienced with them .... the op amp i'm using is a TL082 and my voltage in is +12 ... Do you think the 10uF caps are big enough? Would there be any harm in upping them to 100 uF?

Cheers craig

Offline Soeren

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Re: How to get -12V ?
« Reply #3 on: August 06, 2012, 03:16:44 PM »
Hi,

" A -12V power connector may not be readily available in your computer.
The standard disk drive power connectors are +5 and +12.  I added an
alternate power connector in my system by tapping my power supplies'
-12, GND, and +12 wires.  Be careful not to get these mixed up!"

Its quite vauge what he means and I havent been able to get in touch with him to ask but I thought It would be OK to simply cut and feed off the -12V. But something has now gone very wrong with my computer.
If you have the +/-12V already, using a voltage inverter is a stupid extra that can go wrong (and inverters are pretty inferior to a real +/- supply).

It's not at all vague what he means. Tapping into means to keep the circuit intact and jut adding a tap.

If you have done as you describe (cutting the -12V wire), you have disrupted the negative supply to whatever else needs it in your computer.
Just make a tap!

And then you have to filter the supplies, as they're nowhere near the clean supply you need for a 'scope buffer.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline waltr

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Re: How to get -12V ?
« Reply #4 on: August 06, 2012, 06:28:27 PM »
Quote
the op amp i'm using is a TL082 and my voltage in is +12 ... Do you think the 10uF caps are big enough? Would there be any harm in upping them to 100 uF?
If you mean bulk filtering of the power rails then that depends on many variables. For low current devices 10uF is usually enough. What can be more important is smaller de-coupling caps right near a device's power and ground pins. 100nF is a common value to start with.

If you mean the 10uF caps on the 7660 schematic then no they should be 10uF for the frequency that the 7660 runs at. Also, be sure to get the data sheet for the exact 7660 you buy and check the cap values recommended. There can be differences among 7660's made by different manufactures. 

By the way. I just noticed an error in the schematic in your first post. D8 in backwards and will always conduct.

Offline craigdudeTopic starter

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Re: How to get -12V ?
« Reply #5 on: August 06, 2012, 11:21:23 PM »
Hi,


If you have the +/-12V already, using a voltage inverter is a stupid extra that can go wrong (and inverters are pretty inferior to a real +/- supply).

It's not at all vague what he means. Tapping into means to keep the circuit intact and jut adding a tap.

If you have done as you describe (cutting the -12V wire), you have disrupted the negative supply to whatever else needs it in your computer.
Just make a tap!

And then you have to filter the supplies, as they're nowhere near the clean supply you need for a 'scope buffer.

I cut the -12V wire and put in another wire (leading to my buffer), and soldered all 3 back together. So the original -12V circuit is still intact. Perhaps there just a fault in my buffer that is causing the computer not to switch on when it is hooked into it?

Also filter, please excuse my ignorance,  do you mean I should put some caps in before the buffer circuit? Isn't the power supply dc totally flat?




If you mean bulk filtering of the power rails then that depends on many variables. For low current devices 10uF is usually enough. What can be more important is smaller de-coupling caps right near a device's power and ground pins. 100nF is a common value to start with.

If you mean the 10uF caps on the 7660 schematic then no they should be 10uF for the frequency that the 7660 runs at. Also, be sure to get the data sheet for the exact 7660 you buy and check the cap values recommended. There can be differences among 7660's made by different manufactures. 

By the way. I just noticed an error in the schematic in your first post. D8 in backwards and will always conduct.

D3 ? Are you sure? That could be the cause of all my problems then (maybe). So the anode goes to the -12..? Also, do you think the TL082 would have been damaged by me trying to start the computer with the diode backwards?



Offline waltr

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Re: How to get -12V ?
« Reply #6 on: August 07, 2012, 01:54:49 AM »
D3 ? Are you sure?

I am mistaken after looking at the circuit again. I first thought D3 was to clamp input voltages from exceeding the negative rail.

The diode D3 is in series with the 4.7k Ohm resistor. This will have (12-.7)/4700 =  2.4mA of current.
I'm not really sure of the purpose of doing this.

The circuit is adapted from the book The "Art of Electronics" by Horowitz & Hill. Can you find a copy to look up the original circuit? Maybe at your library.

Offline Soeren

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Re: How to get -12V ?
« Reply #7 on: August 07, 2012, 04:31:15 PM »
Hi,

I am mistaken after looking at the circuit again. I first thought D3 was to clamp input voltages from exceeding the negative rail.
You're not mistaken, that is the very purpose.


The diode D3 is in series with the 4.7k Ohm resistor. This will have (12-.7)/4700 =  2.4mA of current.
I'm not really sure of the purpose of doing this.
This is to establish a -12V (~-11.3V) clamping rail and keep a (very) good symmetry, although you may argue that it's a wee bit overkill, as the signal will be clipped anyway, so who cares about symmetry at that point (but that's very HorrorWiz & Hillbilly).


For a sound card 'scope front-end I wouldn't go to any of these lengths, when a single supply buffer (w. virtual earth) will still do better than the sound card and the overvoltage protection can be made simpler as well - nobody in their right mind will be probing anything connected to the mains with it (I hope), without galvanic isolation of either the PC, the circuit or preferably both.

Further, all the good intentions about load etc. was shot when it was "adapted" with a 10nF cap on the input.
-3dB low cut off is ~15.5Hz (the -3dB high cut off is >360kHz, far beyond the capabilities of even the best sound card) - I'd change this to 100nF or more, depending on the lowest frequency of interest (or keep it DC).

I have attached the original amp to show how amputated the suggested cicuit is, relative to the original - and I rush to repeat that, for a sound card scope, both are over the top.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline waltr

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Re: How to get -12V ?
« Reply #8 on: August 07, 2012, 08:26:47 PM »
Thanks Soeren for going over that circuit and showing the original circuit.

Offline craigdudeTopic starter

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Re: How to get -12V ?
« Reply #9 on: August 09, 2012, 03:07:49 AM »

This is to establish a -12V (~-11.3V) clamping rail and keep a (very) good symmetry, although you may argue that it's a wee bit overkill, as the signal will be clipped anyway, so who cares about symmetry at that point (but that's very HorrorWiz & Hillbilly).


For a sound card 'scope front-end I wouldn't go to any of these lengths, when a single supply buffer (w. virtual earth) will still do better than the sound card and the overvoltage protection can be made simpler as well - nobody in their right mind will be probing anything connected to the mains with it (I hope), without galvanic isolation of either the PC, the circuit or preferably both.

Further, all the good intentions about load etc. was shot when it was "adapted" with a 10nF cap on the input.
-3dB low cut off is ~15.5Hz (the -3dB high cut off is >360kHz, far beyond the capabilities of even the best sound card) - I'd change this to 100nF or more, depending on the lowest frequency of interest (or keep it DC).

I have attached the original amp to show how amputated the suggested cicuit is, relative to the original - and I rush to repeat that, for a sound card scope, both are over the top.

Thanks for digging up the original amp Soeren. Most of that is beyond my understanding. I dont even know what the amplifier does :-[ I just built it because on the Zelscope software site they say you need it if you want to measure more than 1V into your soundcard. I thought it reduced the  size of the signal amplitude, like the opposite of an audio amp. For example if I wanted to probe 10V the amp would reduce it down to 1V but the same waveshape. But I know its not that simple.  Nevertheless thank you for the help and education, at least I know now that it was the amp that was at fault for powering off the computer and not my power supply wiring. It's a step in the right direction.
Cheers

Offline waltr

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Re: How to get -12V ?
« Reply #10 on: August 09, 2012, 06:24:55 PM »
For prototyping and testing 2- 9V batteries will power the op-amp. They don't need the full 12V to run.
A lower voltage just lowers the maximum amplitude of the input and output signals (see the data sheet charts).

An advantage with using batteries is the power circuit is totally isolated from the computer's power supply.
I have a feeling that you have something wired wrong and it was shorting the computer PS.

Offline Soeren

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Re: How to get -12V ?
« Reply #11 on: August 10, 2012, 09:47:51 PM »
Hi,

I just built it because on the Zelscope software site they say you need it if you want to measure more than 1V into your soundcard. I thought it reduced the  size of the signal amplitude, like the opposite of an audio amp. For example if I wanted to probe 10V the amp would reduce it down to 1V but the same waveshape. But I know its not that simple.
Quite the opposite.
With the switch open, it's just a buffer with a gain of 1 (i.e. neither gain nor attenuation) and with the switch closed, it has a gain of 10 (100mVin => 1Vout), assuming the potentiometer on the output is turned all the way up. It's to enable you to see lower amplitude signals.
(The gain falls off as the power rails are approached of course, so 10V in on the x10 range cannot (obviously) produce eg. 100V out when fed from a +/-12V supply, it would just clip to close to the rails (depending on op-amp used).

The potentiometer can be used to reduce the signal, but if all you want is attenuation, a passive attenuator is a much simple thing to use, as each step is known (3 resistors and a 3 position switch could give you extremely close to 1:1, 10:1 and 100:1 with resistors like 1k, 9.1k and 91k.

A buffer amplifier with a gain of 1 should be used to avoid loading the circuit under test, and a simple protection to overvoltages would be a good idea - mind you though, that setting it to 1:1 and applying a higher voltage will still feed a higher voltage to the sound card.

To properly protect the sound card against overvoltages, a couple of anti-parallel signal diodes (like 1N4148 or 1N914) closest to the sound card (i.e. after any amplification and attenuation) and limiting the signal to perhaps 0.5Vp-p would be safe - if the sound card can handle up to 1.5'V safely, 2x2 diodes will let you use up to 1Vp-p without problems.

I would have thought the net was filled with such circuits, but strangely, a (very) quick spin in Google yeilded nothing but the circuit you posted and a lot of requests for such circuits - I know that the magazine EPE made a fairly large circuit for this some 6..10 years ago though (or perhaps it was Elektor...).


As waltr stated, using two 9V batteries is a very good idea for experiments, as you then hve the cleanest supply without risk to your PC's supply - a 100µF or larger cap on each rail will be needed and remember that the grounds must be common (just through the 3.5mm connector though). It will work as intended all the way to down to when the 9V batteries are totally flat (5.4V) and even lower.


Perhaps I should bang up a circuit with proper protection - just for the heck of it :) Anyone got requests for needed ranges?

The same circuit could be used as a front end for a smart-phone, but as I haven't gotten around to study Android programming (Yes, my sweetheart thought I could use a smart-phone and gave me an SII a while back - almost 3 times the size of my old phone, but it's nice nevertheless :)), so I either have to bite the bullit, or find and team up with a hardcore graphical Android programmer, before several apps I have planned will be realized - Oh, the plans I have... World domination is small potatoes compared ;D
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline craigdudeTopic starter

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Re: How to get -12V ?
« Reply #12 on: August 15, 2012, 03:39:32 AM »

Perhaps I should bang up a circuit with proper protection - just for the heck of it :) Anyone got requests for needed ranges?



That would be much appreciated. As for myself,  I am starting to understand what this op- amp does and think i might use a couple of signal diodes like you said, to protect the soundcard. And for gain and attenuation I'll use 3 - selector dials and the resistor values you mentioned. It will be a helluva lot simpler than the circuit I tried first, and once I put it inside a disk drive, it will fill the gaping hole left above the harddrive in my computer :D. I found the circuit on Elektor : http://www.elektor.com/products/pcbs/04xxxx-x/040479-1-ecg-via-de-geluidskaart.12240.lynkx but i'm not registered so i cant access the plans. However i'm starting to think all that is too much anyway, these diodes and switches should do what i need.  ;D

 


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