Author Topic: So, negative voltages, what exactly are they for? (and v power supply question)  (Read 4086 times)

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Offline XOIIO

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Alright so I'm seeing this sort of stuff more and more, devices and circuits that use negative voltages, the main thing that I am wondering is... why? I have some negative voltage regulators (7905 and the like), but what are negative voltages useful for? Can you easily make a negative voltage power supply or convert a positive one to negative (or convert the positive voltage to negative)? It just seems like a somewhat odd concept to me, maybe it makes a lot of sense when you know what it is for, but it's just strange to me.

Also I was wondering, since there are loads of variable power supplies, usually fairly decent ones for cheap on ebay that can do 5 amps 30 volts, are there any of those style power supplies that can do negative voltages as well? Would it need to be a separate output completely, or are there perhaps some that let you go either way from 0 volts?

Anyways, I don't really think negative voltages are something that would be useful to a hobbyist, unless you are like me, someone who found a neat useful part, only to be unable to use it because it needs +15 and -15 volts. (still haven't found a way to supply -15 volts)

Offline Simon

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a power supply cannot got from positive to negative usually and if it can it's a dual one anyway. what is kore common is to have a dual outout supply often they are "tracked" so that one control will change both voltages together and keep them the same. Negative voltages are reiquired often especially in amplifiers and really it's just about where you take a reference from. I have two 3-15V power supply's, I can connect them in series so that I have +6-30V across them in total or I can use the middle connection as my reference and call one the negative rail and one the positive rail getting me +/- 3-15V.

op-amp circuits often use negative voltages but your not aware of it. Because it is easier to work with a single voltage a "virtual ground" is created with a potential divider so say the supply is 12V it will make a "fake ground" at +6V from the negative of the supply so you now have +/-6V but it's just an illusion that exists within a circuit.

Power amplifiers need negative voltages because if you take a speaker you need one polarity to push it one way and the oposiste polarity to push it the other way to make full use of it's range. Having a split supply in power amplifiers is a very good idea as it save on having less efficient circuitry that creates a false center point that is deemed as a new ground.
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Offline T3sl4co1l

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A couple of reasons.  The most fundamental is that, many components depend on the applied voltage difference (or current flow), including direction.

Alternating polarities can easily be generated by AC-coupling the signal with a capacitor, inductor or transformer.  These types of applications don't usually benefit from negative supplies anyway.  For example, audio amplifiers -- for much of history -- have been AC coupled, single supply.  The DC is turned on and off to varying degrees, and the DC filtered out to get nice bipolar AC at the output.

AC applications sometimes benefit from bipolar supplies.  Startup transients (the establishment of those DC offsets!) are often undesirable.  Early, "second generation" I guess you could say, transistor audio amplifiers were constructed as a power output stage (usually a quasi-complementary emitter follower), biased at half the supply voltage, with a series coupling capacitor connecting the output, to the speaker, to ground.  When power is turned on, the capacitor starts at 0V, so the speaker gets a whack of VCC/2, and similarly when turned off (the pulse being negative instead).  The input has the same problem, where the input amplifier might be biased anywhere from a few volts above ground, to VCC/2 or something like that, and that needs a coupling capacitor that has to charge and discharge as well.

Noise is also a consideration.  Power supply noise is introduced into the signal path by those bias-setting voltage dividers and such.  To some extent, split power supplies can be faked using capacitive dividers; the entire circuit, from input to output, is referenced to the capacitors' midpoint.  The circuit no longer has to be AC coupled, but it would be a good idea, since drawing too much current at a time from one supply or the other causes the capacitor divider voltage to drift (sausage effect, a persistent positive load might drag a +/-12V supply over to +2/-22V, causing time-dependent clipping).

The most important use is for relatively sensitive, DC coupled circuits, where ground is used as a voltage reference.  This saves the trouble of building a differential sensing amplifier -- assuming ground is nice and stable and "ground", of course.

Components that need many voltages and references can be hard to work with, and a compromise is usually chosen.  Take a basic electrostatic CRT for example: if the cathode is at 0V, the first grid will be 0 to -100, the first anode (focus) around 300V, final anode 2kV (+/- 100V for astigmatism), deflection plates 2kV with +/- 100V differential (for deflection).

So, intensity control (grid-cathode voltage) is referenced to the cathode end, and deflection is referenced to the anode end.

Electrostatic deflection CRTs were used in TVs before magnetic deflection was introduced (this is like '30s era).  Because TV deflection signals are AC, coupling capacitors do a fine job.  Unfortunately, they also have to withstand several kilovolts, which gets expensive, and the HV has to be well filtered, otherwise that noise/ripple gets coupled into the display.  Video (which is DC coupled via DC restoration) is driven into the cathode, which is near ground (0-100V), with the grid grounded (to an adjustable voltage, for brightness control).

The alternative would be a negative supply, with the cathode at -2kV, the video coupled down to -2kV, and the deflection plates near 0V (probably 0-300V, being a typical tube amp voltage range).  The detected video signal is DC coupled, but it can be AC coupled -- the DC can be recovered using a DC restore circuit (little more than a diode).  That's just one more thing to have hanging around at high voltages, and having to build circuitry on top of a high voltage is something better avoided.

An oscilloscope has the inverse problem, that intensity isn't a big deal, but deflection (linearity and bandwidth, including DC coupling) is paramount.  It would be insane to build the entire deflection amplifiers up at +2kV and somehow still couple the signal into it, so instead, the cathode goes to -2kV.  Intensity modulation usually amounts to blanking, which has a low duty cycle, so the lack of DC coupling isn't a big deal.  (Scopes with a "Z" axis -- variable intensity -- have to address this anyway, and usually do it by providing a second complete HV supply, just to establish the cathode-to-driver voltage!  Having to couple the signal through all this circuitry costs money, space and potential bandwidth.)

Fancier CRTs can end up with even more supplies, for example a ("second generation"?) electrostatic CRT, typical of most Tektronix tubes, have something like -2.5kV cathode, deflection plates (normal plates, or distributed transmission line types) near 0V, and an accelerator potential of perhaps 5-10kV.  Fortunately, no circuitry is needed up at the UHV!

But then there's Trinitron CRTs, which have a pre-anode sort of thing at, say, 31kV, and the final at 33kV.  So a 2kV power supply is needed on top of the first 31 (or below the 33).  This was often a small flyback transformer (it doesn't need to deliver much power, a few watts) encased in an awful lot of potting (it has to do it with 30kV+ isolation!).

Occasionally, physics experiments get even more difficult, with, oh I don't know, maybe a pulsed electron or ion beam source at megavolts negative potential.  These sorts of situations tend to be inductively or optically coupled, the pulse signals themselves being coupled with fiber optics.  Putting circuitry at such potentials isn't as much of a problem, since the hardware is big and expensive anyway, so Faraday shields keep fields and noise off the stuff.

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Offline Richard Crowley

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Alright so I'm seeing this sort of stuff more and more, devices and circuits that use negative voltages, the main thing that I am wondering is... why?
1) Because of the way the circuit is designed, signals may need to go BELOW zero/ground (i.e.a NEGATIVE voltage)
the most common example is an op-amp which typically operates on  "bipolar" (equivalent positive and negative) power.
2) Because some devices (like PNP transistors) operate on "negative" voltages from the common POV.

Quote
I have some negative voltage regulators (7905 and the like), but what are negative voltages useful for?
Negative voltages are required for many (most?) analog/linear circuits, particularly audio, etc.  Because the circuit (like op-amp) needs to swing equally above ground (positive) and below ground (negative).

Quote
Can you easily make a negative voltage power supply or convert a positive one to negative (or convert the positive voltage to negative)?
Any voltage source can be "positive" or "negative".  For example if you take a flashlight ("torch") battery and put it with the tip pointing up, then you have a "positive" voltage source.  But if you turn that same battery upside-down (with the tip "down", then you have created a "negative" voltage source.  Voltage is voltage.  Whether we call it "positive" or "negative" depends of what reference we are viewing it from.

Quote
It just seems like a somewhat odd concept to me, maybe it makes a lot of sense when you know what it is for, but it's just strange to me.
This is not uncommon among new players. Modern circuits are overwhelmingly designed to be powered by positive voltages, and increasingly, even analog circuits are using newer semiconductors that can operate from a single positive power rail.  Much of this is driven from the Mobile Society where everything is powered from batteries and it is 2x as expensive (or worse) to provide both positive and negative power in a small battery-powered gadget.

Quote
Also I was wondering, since there are loads of variable power supplies, usually fairly decent ones for cheap on ebay that can do 5 amps 30 volts, are there any of those style power supplies that can do negative voltages as well? Would it need to be a separate output completely, or are there perhaps some that let you go either way from 0 volts?
Most "bench supplies" have isolated outputs, which means that neither the positive nor the negative output is connected to "ground"  Note that they often have a red terminal (positive) and a black terminal (negative) and a green terminal (ground).  They are neither "positive" nor "negative" because you can wire them however you wish.  Many bench supplies are "dual-tracking" which means that they simultaneously put out both positive AND negative voltages of equal voltage.

Quote
Anyways, I don't really think negative voltages are something that would be useful to a hobbyist, unless you are like me, someone who found a neat useful part, only to be unable to use it because it needs +15 and -15 volts. (still haven't found a way to supply -15 volts)
How are you supplying +15V?  You simply need another one wired "backwards" for -15V.
Remember that +15V and -15V are both relative to ground.
Here is an example:  (showing 9V, but the principle is the same with 15V or whatever)

« Last Edit: April 19, 2014, 08:36:02 am by Richard Crowley »
 

Offline mrkev

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First of all, you should realise that voltage is always relative. If you have f.e. 5V supply, it's gonna be 5V relative to 0V pin, but at completely different voltage relative to f.e. earth ground. Easy way how to imagine this is when you hold your ruler in the air. You have 5cm from 0 mark to 5th mark, but also, it is the same distance from 5cm to 10cm. You can move your base and imagine 0 point anywhere on scale. When you choose 5cm mark as the base point, you get -cm to the left and +cm to the right. It is kind of same in voltage, you basically always get difference between two points.

Negative voltage is usefull when you need current to flow both ways (like in ac amplifiers).
Let's say that you would use only possitive voltage sources 7812 and 7806 with (chosen) reference ground point at 6V.

Even tho you'd technicaly get -6V, 0V and +6V (look at image), positive source is not (in most cases) designed for current to flow "into" (to "sink current"). It would work fine for negative wave (current i2 flowing from your 6V source to 0V), but it wouldnt work for possitive wave (current i1 flowing from 7812 to 7806) as 7806 is designed to work with voltage at IN pin higher than voltage at OUT pin (cannot stabilize "backwards").
 

Offline Porto

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Something simple like a steppermotor needs a negative voltage also to turn in the other way, right?
 

Online digsys

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Quote from: Porto
Something simple like a steppermotor needs a negative voltage also to turn in the other way, right? 
Yes and No :-)
You normally use a bridge circuit, which ITSELF operates on a fixed POSitive supply. It just cleverly changes how the +ve and -ve
are switched to the motor. This applies to any type of DC motor.
Hello <tap> <tap> .. is this thing on?
 

Offline Richard Crowley

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Something simple like a steppermotor needs a negative voltage also to turn in the other way, right?
Well, more accurately, motors depend on North and South pole magnetic fields to attract and repel.  Some stepper motors  require you to connect the coils one way or the other ("Positive" or "Negative") to create North or South pole magnetic fields, while other designs use windings of opposite direction to create the North or South magnetic fields from a single polarity voltage.
 

Offline xDR1TeK

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Alright so I'm seeing this sort of stuff more and more, devices and circuits that use negative voltages, the main thing that I am wondering is... why?
1) Because of the way the circuit is designed, signals may need to go BELOW zero/ground (i.e.a NEGATIVE voltage)
the most common example is an op-amp which typically operates on  "bipolar" (equivalent positive and negative) power.
2) Because some devices (like PNP transistors) operate on "negative" voltages from the common POV.

Quote
I have some negative voltage regulators (7905 and the like), but what are negative voltages useful for?
Negative voltages are required for many (most?) analog/linear circuits, particularly audio, etc.  Because the circuit (like op-amp) needs to swing equally above ground (positive) and below ground (negative).

Quote
Can you easily make a negative voltage power supply or convert a positive one to negative (or convert the positive voltage to negative)?
Any voltage source can be "positive" or "negative".  For example if you take a flashlight ("torch") battery and put it with the tip pointing up, then you have a "positive" voltage source.  But if you turn that same battery upside-down (with the tip "down", then you have created a "negative" voltage source.  Voltage is voltage.  Whether we call it "positive" or "negative" depends of what reference we are viewing it from.

Quote
It just seems like a somewhat odd concept to me, maybe it makes a lot of sense when you know what it is for, but it's just strange to me.
This is not uncommon among new players. Modern circuits are overwhelmingly designed to be powered by positive voltages, and increasingly, even analog circuits are using newer semiconductors that can operate from a single positive power rail.  Much of this is driven from the Mobile Society where everything is powered from batteries and it is 2x as expensive (or worse) to provide both positive and negative power in a small battery-powered gadget.

Quote
Also I was wondering, since there are loads of variable power supplies, usually fairly decent ones for cheap on ebay that can do 5 amps 30 volts, are there any of those style power supplies that can do negative voltages as well? Would it need to be a separate output completely, or are there perhaps some that let you go either way from 0 volts?
Most "bench supplies" have isolated outputs, which means that neither the positive nor the negative output is connected to "ground"  Note that they often have a red terminal (positive) and a black terminal (negative) and a green terminal (ground).  They are neither "positive" nor "negative" because you can wire them however you wish.  Many bench supplies are "dual-tracking" which means that they simultaneously put out both positive AND negative voltages of equal voltage.

Quote
Anyways, I don't really think negative voltages are something that would be useful to a hobbyist, unless you are like me, someone who found a neat useful part, only to be unable to use it because it needs +15 and -15 volts. (still haven't found a way to supply -15 volts)
How are you supplying +15V?  You simply need another one wired "backwards" for -15V.
Remember that +15V and -15V are both relative to ground.
Here is an example:  (showing 9V, but the principle is the same with 15V or whatever)



Is that picture from Circuit Maker 2000?
 

Offline Richard Crowley

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Is that picture from Circuit Maker 2000?

Here is the source: http://www.generalguitargadgets.com/richardo/distortion/
You could ask them.
 

Offline xDR1TeK

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Alright so I'm seeing this sort of stuff more and more, devices and circuits that use negative voltages, the main thing that I am wondering is... why? I have some negative voltage regulators (7905 and the like), but what are negative voltages useful for? Can you easily make a negative voltage power supply or convert a positive one to negative (or convert the positive voltage to negative)? It just seems like a somewhat odd concept to me, maybe it makes a lot of sense when you know what it is for, but it's just strange to me.

Also I was wondering, since there are loads of variable power supplies, usually fairly decent ones for cheap on ebay that can do 5 amps 30 volts, are there any of those style power supplies that can do negative voltages as well? Would it need to be a separate output completely, or are there perhaps some that let you go either way from 0 volts?

Anyways, I don't really think negative voltages are something that would be useful to a hobbyist, unless you are like me, someone who found a neat useful part, only to be unable to use it because it needs +15 and -15 volts. (still haven't found a way to supply -15 volts)

Hi man, this is the most important set of questions you have asked here, on paper all is nice; however moving into the practical implementation, it is then where everything becomes messy.

Negative voltages can be part of the driving sourcing lines or more interestingly part of control signals.
Regarding your question, is it easy to create a negative rail, this is the tricky part. Depends on what you are using it for.

There are chips that can generate the negative rail, but might introduce jitters and have limited driving ability.
They make use of voltage doublers that do invert the voltage to the negative rail.

You can simply use a voltage divider to get middle reference point and call it your ground. Not a good example since depending on how much current you are dumping to this raised ground alters the resistance balancing in the circuit. This will be mess of a circuit to debug. Imagine a rubber band with a dot in the middle and trying to grab the dot up or down. A better example than the rubber band would be the voltage divider in the inverting operational amplifier. the output needs to keep the virtual ground equal to the noninverting input, the boundary values of the voltages around the voltage divider change but the virtual ground remains however slightly moving visible under magnification. Imagine that kind of voltage divider even not balanced how it alters the balancing of your circuit.

Best negative rails are made from an inverter with center tap in the transformer being your reference ground followed then by the rectifying bridge then filter then regulator. Only here, you can make use of the L7905 along with the L7805 (if the intention was to create a supply of 5V or variable supply of 7-12V with the L7X05 pkg).

and i wrote too much...
 

Offline xDR1TeK

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Is that picture from Circuit Maker 2000?

Here is the source: http://www.generalguitargadgets.com/richardo/distortion/
You could ask them.

LOL, end of page credit, mentions year 2000.
Man, the good old days.
I'm sure it's CM2000.
I still remember the keyboard shortcuts for the damn thing.
Good site you posted.
 


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