# Negative Voltage

Discussion in 'General Electronics Discussion' started by PeteVee, Apr 21, 2017.

1. ### PeteVee

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My question is about the concept of negative voltage in general, although the context from which it arises is to do with vacuum tubes in a guitar amplifier, biasing tubes in particular. This page http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm states that "Bias is a negative voltage applied to a power tube's control grid, to set the amount of idle current the tube draws". At this juncture my confusion has less to do with the concept of biasing and more to do with what the term "negative voltage" means. My first inclination is to think "how can a voltage be negative?" From my understanding, voltage (through current in a loaded circuit) is the notion of electrons moving in a circuit to do some sort of useful work, like lighting up a light bulb for example. So what could the term "negative voltage" be possibly describing? The "unability" to do some sort of useful work? That is obviously not what it means, but you can see where my confusion begins. Many moons ago I came across this terminology in the context of digital electronics, where a transistor might be said to be "on" at +5V and "off" at -5V. The explanation I remember for that type of scenario was that the -5 was in reference to some benchmark, lets say 20 V, so that -5V actually means 15V and +5V means 25V. I could be way off on this and if so I would love to be set straight. Anyway, to make a long story short, if someone could help me understand what it means when a "negative voltage" is applied to a power tube's control grid, I would be right chuffed. Thanks in advance.

PV

PeteVee, Apr 21, 2017

2. ### davennModerator

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it's just that, a negative value voltage relative to the 0V rail.

Think of a split rail PSU that supplies - 12V, 0V and +12V

In AC the voltage is cycling between negative and positive values, above and below the 0V reference line

in digital it's usually 0V and +5V

yes, as I said earlier, usually the 0V rail

as a more negative value of voltage is applied to the grid, more of the electrons ( negative charge) emitted by the cathode are repelled by the grid and don't get through to the positive anode plate

so increasing or decreasing the negative voltage on the grid allows more or less electrons to get through to the plate

Dave

Last edited: Apr 21, 2017
davenn, Apr 21, 2017

3. ### Ratch

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I wish all questions were this easy. Voltage is always measured from a reference point. If the voltage to be measured has a higher value than the reference point, the voltage is positive. Otherwise negative or zero.

Voltage is not the ability to do work. That is the definition of energy. Voltage is the energy density of the unit charge. A negative voltage means there is less energy density per unit charge than the reference point.

Applying a negative voltage to the grid usually means the grid is negative with respect to the cathode. This inhibits the electrons from traveling to the plate.

Ratch

Ratch, Apr 21, 2017
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4. ### hevans1944Hop - AC8NS

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Voltage is always measured between two points. Think of a common dry-cell where one end is positive and the other end is negative. A typical alkaline cell produces 1.5 volts between its terminals. Whether you consider that voltage to be positive or negative, depends on where you decide to measure zero voltage. Clearly either end of the cell provides zero voltage with respect to anything else connected to that same end of the cell.

If the end you choose for your "zero reference" is the positive terminal of the cell, then all voltages the cell can produce will be negative voltages. Flip the cell around and it now produces positive voltages because your "zero reference" is now the negative terminal of the cell.

In vacuum tube circuits, the grid voltage is measured with respect to the cathode of the tube. The cathode may not be at "circuit ground" but the grid voltage is still measured between the cathode and the grid, and the grid voltage is usually negative with respect to the cathode.

hevans1944, Apr 21, 2017
5. ### AnalogKid

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The thing to remember is that there are very few absolutes in physics, and this ain't any of them. Positive and negative are 1) completely arbitrary terms, just like up and down; 2) they are relative terms, and are meaningless without a reference point. A voltage is not positive or negative, it is positive or negative with respect to the voltage somewhere else. That is why a volt meter has two leads. Connect them one way to a battery and it reads -9 V; reverse the connection and it reads +9 V. Nothing about the battery changed; you were right there, you would have seen it. What changed were the relationships among the meter, the reference connection, and the measurement connection.

To your specific example, a tube is biased into its linear operating region when the control grid is negative *with respect to the cathode*, neither of which are required to be connected to ground (the circuit's arbitrary "0 V" point). If a 12AT7 has the cathode at +999 V and the grid at +995 V, it's happy.

Early Ampex video recorders had video processing circuits powered by +300 V and -300 V (with respect to ground). The output video signal was centered at ground, created by two vacuum tubes connected in series just like a totem-pole output stage in an audio power amp quasi-complimentary output stage. The output point was the cathode of the upper tube connected directly to the plate of the lower tube. Linear as all get-out, great bandwidth, and designed by a college kid - Ray Dolby.

ak

AnalogKid, Apr 21, 2017
6. ### BobK

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Actually, the ability to do negative work. Work can also be negative. When a weight is lifted in the earth's gravitational field, it requires positive work. When it is lowered, requires negative work, which is to say that it does work instead of requiring work done on it. Charges (electrons for example) behave the same way with respect to voltages. If a positive charge goes from a lower voltage to a higher voltage, you must push it, doing work on it. If it goes from a higher voltage to a lower voltage, work is done by the charge, possibly heating up a resistor or wire.

bob

BobK, Apr 21, 2017
7. ### PeteVee

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So does saying the grid has a negative voltage with respect to the cathode simply mean that the cathode has a higher voltage than the grid? In other words, both the grid and cathode have positive voltages with respect to chassis ground, but the cathode has a higher voltage with respect to ground than the grid does, and if you measure voltage between grid and cathode you would get a negative reading as a result?

PeteVee, Apr 28, 2017
8. ### hevans1944Hop - AC8NS

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This is essentially correct, although I would question your use of the word higher when you probably should use the words more positive. Everyone knows what high voltage means... sufficient voltage to cause grievous harm to people or equipment... but the adjective "high" simply refers to magnitude without regard to sign or polarity. If the cathode is more positive than the grid, then the grid is more negative than the cathode.

hevans1944, Apr 28, 2017
9. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Except for the statement above, yes.

It's more like saying that the passengers in the top deck of an A380 are at a higher altitude than those in the lower deck.

That is true if the plane is on the ground, 40,000 feet in the air, or parked somewhere below sea level.

In your case, the anode could be at chassis potential, or the cathode, or the grid, or none of them, or the chassis could be floating.

The grid will be negative with respect to the cathode, IF you plane the meter's black lead on the cathode (which means you're taking measurements with respect to the cathode) AND if your meter has a high enough impedance compared to the voltage applied to the grid. The later point is less of an issue today, but certainly may be an issue if you decide to use an analogue meter.

(*steve*), Apr 28, 2017
10. ### PeteVee

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Man, you're killing me here ;-) Would it be more accurate to say "greater positive voltage"? For a layman like me it seems logical that if both grid and cathode have positive voltage with respect to ground but the cathode has a greater positive voltage (e.g. Cathode is +13V with respect to ground and grid is +10V with respect to ground) then we can safely say that the cathode has a greater/higher/more positive voltage than the grid. I mean, isn't context everything? In this context, wouldn't it be clear what I meant by "higher" voltage? It's hard for me to simply accept something as "just the way it is" without actually fully understanding. Anyway, I appreciate the explanations.

PeteVee, Apr 29, 2017
11. ### davennModerator

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no, it is more correct to use hevans1944 's term more positive.

davenn, Apr 29, 2017
12. ### PeteVee

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Darn. OK, you win

PeteVee, Apr 29, 2017
13. ### hevans1944Hop - AC8NS

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It isn't a contest. If you truly understand the concept of negative numbers, then positive and negative voltages should make sense as well.

Many years ago while attending grammar school I recall being introduced to negative numbers. We had already been introduced to positive integers by the simple example of the "number line" which included zero at one end and a series of successively increasing integer "whole" numbers along the line thus: 0, 1, 2, 3, 4, ... and so on without end. Then we were introduced to addition and subtraction of integers on the number line, things like 5 minus 2 or 17 minus 13. No problemo! You just moved a distance away from zero to the first number and then turned around and moved back toward zero a distance equal to the subtrahend. But what if the subtrahend was bigger than the first number? What did 13 minus 17 mean? Well, it meant we had to extend our number line in the opposite direction from zero and mark off negative integer values. Same rule as before for subtraction: move out on the positive half of the number line to the first value, then turn around and go back the distance represented by the subtrahend. So, for the example of 13 minus 17, after reaching 13 on the positive half of the number line we turn around and move a distance of 17 toward zero. Clearly this will result in having to move a distance 4 past zero to reach a distance of 17. Since we are moving in the negative direction on the number line, when we reach zero and continue, the distances we are moving become negative. So 13 minus 17 becomes a negative 4 or -4.

At this point most of the class was totally confused. One typical question (similar to your question about "negative voltage") was: How can you have a negative length? A tape measure or ordinary ruler measured positive lengths, right? What does is mean to measure negative lengths? And where do you find objects with negative lengths to measure? About this time the teacher would trot out the ordinary outdoor thermometer with its glass tube filled with a (usually) red liquid and point out that it measured both positive (above zero) and negative (below zero) temperatures. That satisfied most of the class, and no one ever questioned how the zero position on the thermometer was determined. Fortunately this took up most of the school year to get the idea of negative numbers embedded in our young and pliable minds. Just in time to learn about rational and irrational numbers.

hevans1944, Apr 29, 2017
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14. ### PeteVee

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Of course its not a contest, that was merely my weak attempt at humour. As I said previously, the concept is making much more sense to me now thanks to all the explanations, I do appreciate it.

PeteVee, Apr 29, 2017
15. ### PeteVee

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If I can revive this discussion, I'm looking at the schematic for a Fender Twin with the AB763 circuit. There is a wire that is connected to the middle (output) lug of the bias trim pot that is labeled -52 V. This appears to be the negative voltage applied to the grid of the power tubes. But on the schematic it says "voltages read to ground". That doesn't seem to jibe with the explanation that the negative voltage we've been discussing is with respect to the cathode. And if ground is 0V then I'm back to square one wondering how on earth (no pun intended) a voltage with respect to ground can be negative.

PeteVee, May 1, 2017
16. ### AnalogKid

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In the physics of the situation, a point (thing, node, pin, whatever) can have a measured voltage that is "negative" with respect to ground if it has lotsa electrons, because an electron has a "negative" charge. In this sense, a "positive" voltage is a place where there are fewer electrons than somewhere else.

Notice the quotation marks. You have to remember that all terminology is arbitrary. Positive negative up down left right black white - all completely arbitrary, and all meaningless without reference points everyone agrees on. To demonstrate this, use a DC voltmeter to measure an AA battery, then reverse the leads and measure it again. Without the markings on the case and the bump on one end (both are international standards), there is no way to tell if you have a positive battery or a negative battery.

A negative voltage in a circuit with positive voltages is a higher step in complexity, but for real conceptual eye-crossing consider alternating current, a single point that alternates between positive and negative potentials.

ak

AnalogKid, May 1, 2017
17. ### hevans1944Hop - AC8NS

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Mmmm, A positive battery or a negative battery, eh? I have never heard of batteries having the characteristic of being positive or negative. Batteries are all just little containers of electro-chemical energy with a positive pole and a negative pole, the poles always coming in pairs. Similar to a capacitor, but with a different energy storage mechanism. I've never heard of capacitors being positive or negative either. I agree the markings on the case and (in the case of commercial cells) the bump on one end may help to identify which pole is positive (or negative) with respect to the other pole, but the definitive test is to use a voltmeter to determine polarity of the poles.

If you examine the schematic a little closer, you will see that the cathodes of the power output tubes are all connected to ground. Further inspection will reveal that the "-52 V" from the bias adjust potentiometer is a result of negative voltage half-wave rectified from a tap on the secondary winding of the power transformer. The same winding produces a positive voltage "+260 V" full-wave rectified from the center-tapped secondary winding. As the Analog KId pointed out in post #16, both -52 V and +260 V are possible because there is AC on the secondary winding and AC is characterized by having alternating positive and negative polarities with zero in between. The diodes in the power supply select which alternation (positive or negative) will be used to produce a positive or negative DC output with respect to circuit ground.

So, if you truly cannot grasp the concept of a negative voltage, you are going to find it impossible to grasp the concept of alternating current. Rotsa ruck with your new-found journey in electronics.

hevans1944, May 2, 2017
18. ### PeteVee

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Good explanation, I was actually following it. Disappointed with the last paragraph though. As it happens I have no problem at all with understanding alternating current, never have. My issue was with negative voltage in a DC context. The concept of positive and negative voltages as the rectified portions of an original AC wave makes sense as does you explanation about the cathode being connected to ground. Too bad you couldn't have had just a little more patience before adding a condescending comment. Anyway, I appreciate all the help just the same.

PeteVee, May 2, 2017
19. ### davennModerator

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I cant believe this thread is still going !
the answer has been given several times and am not sure why you haven't understood it yet ?

don't confuse the use of the word ground .... in this situation ground and 0V are the same
has NOTHING to do with the earth / ground outside your house

why cant it ? what is it about negative numbers you don't understand ... they are used in every day devices eg a thermometer
a temperature less than the stated 0 temperature point is a negative temperature ..... -10 deg C (f)

Do you have a problem with that ?
if not why do you have a problem with a voltage less than zero when related to the zero point ?

take these examples I have drawn

the way the voltages are read is all related to where you state the zero volt point to be

take battery A, out of brackets voltages as usually seen, + terminal is 9V and - terminal is zero volts
We can just as easily state that the + terminal will be the zero volt rail in the circuit and then we can
supply the circuit with -9V from the negative rail

Batteries B and C are in series, we nominate the point between the 2 batteries ( where the + of one connects to the - of the other)

We can have 3 main options
1) state that the - of battery C is going to me out 0V rail, then we can have a +9V tap in the middle or a +18V tap off the top of B
2) we could do as what I have shown out of brackets .... +9V off the + of Bat B. 0V in the middle and -9V off the bottom of Bat C

this is called a split rail supply .... used for op-amps and all sorts of other uses

3) we could nominate, as shown in the brackets, that the + terminal of Bat B is 0V
the centre connection would then be -9V and the lower connection, the - of Bat C would then be -18V

I don''t know how to make it more clearer than that ..... it's really just that simple

Dave

davenn, May 2, 2017

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