When and What are Negative Voltages used for? - Page 1
Forum section: electronics / beginners

author: DeKu
author_rank: Newbie
timestamp: November 02, 2024, 07:00:02 pm
content: Hi People,i am a hobbyist using electronics and "mechanics" in some, if not most, of my Projects. But i am not very well versed in it. What i know, i know by learning it because i needed it for some kind of Project. And i have a lot of them.However, after watching a few Repair Videos, i became much more interested in "negative Voltages". And i don't mean the ones you measure by having the wrong Reference Point or measuring with flipped Probes. I did a little (very little) search but it did not seem satisfying.So i wanted to ask if someone could give a short, comprehenisve Summary where and what for negative Voltage Supplies are used for. Maybe, if enough people find this to be an interesting Topic, Dave could make a Video out of it.I know that the needed Info is very much likely available, but i was thinking if i was the only one who was interested in this Topic.Feel free toif its really just a useles Question.Greetings from Hamburg / Germany,DeKu

author: TimFox
author_rank: Super Contributor
timestamp: November 02, 2024, 07:31:44 pm
content: Back in the day, it was normal to run op amps from +15 and -15 V, so that the output could easily go between +10 and -10 V, when that was required.It is now fashionable to use only a positive supply voltage, but that requires an artificial voltage of some kind to allow an output to go both sides of zero, unless this is an audio application that allows a coupling capacitor.For example, you might want a DC motor to rotate in both directions. Either use an amplifier with bipolar power supply, or essentially two amplifiers in an “H”-bridge and connect the motor between their outputs, with somewhat more complicated circuitry.If you really need an output node voltage to go to literal zero, it probably must be capable of going further to a negative voltage.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 02, 2024, 07:37:36 pm
content: Well, think about audio and a speaker. When you apply no voltage, the speaker coil sits in the middle of its mechanical range. When you apply voltage one way, the coil moves one way. With opposite polarity, it moves the opposite way. If you now choose either one of the wire as your reference voltage level which you call "zero", then you need to apply positive and negative voltages to the other wire to make the speaker coil move both ways.

author: IanB
author_rank: Super Contributor
timestamp: November 02, 2024, 07:49:22 pm
content: Another way to think about negative voltages is "above" and "below". Suppose the circuit has some reference point, call it the "common rail" and say that it is 0 volts. Then some voltages might be positive (above) the rail, and current flows "down" from the positive levels towards the common rail. Some voltages might be negative (below) the rail, and in that case current flows out of the rail and "down" towards the more negative voltage levels.The reason for this is that some circuit components work best with increasing negative voltages (below the rail), and some components work best with increasing positive voltages (above the rail). Some components, like op amps, might need both, to allow positive and negative voltage swings around the zero point.

author: Andy Chee
author_rank: Super Contributor
timestamp: November 02, 2024, 09:02:15 pm
content: Bipolar supplies were also used for RS232 signalling. e.g. MAX232, MC1489, etc

author: SteveThackery
author_rank: Super Contributor
timestamp: November 02, 2024, 09:49:02 pm
content: All of the above.The basic idea is this: all voltages are relative to something else; there is no such thing as an "absolute" voltage. Take the terminals of a 1.5V battery: using the negative terminal as a reference, the positive terminal is at +1.5V. Using the positive terminal as a reference, the negative terminal is at -1.5V. There is nothing special about either terminal, so whether the voltage is negative or positive depends solely on which terminal you use as a reference.Most electronic circuits use a pair of supply rails with a voltage difference between them; in digital circuits it is typically a difference of 5V or 3.3V. Analogue circuits tend to use higher voltages: 9V, 12V, or more. It is normal to nominate one of the supply rails as the reference rail, and the reference rail is traditionally called the ground rail ('GND'). And traditionally it is the negative rail that we use as the ground reference. In that case, the other rail is positive with respect to ground, and all the voltages in the circuit are positive with respect to ground.HOWEVER, we can just as easily nominate the positive rail as the ground. In that case the other rail isnegativewith respect to ground, and all the voltages in the circuit arenegativewith respect to ground.Note that the circuit is unchanged, but simply by changing which supply rail we call the reference ground, we can make all the voltages negative or positive. That is the key takeaway: whether a voltage between two wires is positive or negative depends solely on which wire you decide to use as the reference.Some analogue circuitry uses three supply rails: let's call them A, B and C and assume a 12V difference between them. All three of the following statements are true:Using A as the reference, B is +12V and C is +24VUsing B as the reference, A is -12V and C is +12VUsing C as the reference, A is -24V and B is -12VSo again, there is nothing special or different about negative voltages; they appear (or not) according to which supply rail you use as your reference. In all three cases above, nothing changes in terms of the wiring or power supply - the identical circuit can have all positive voltages, a mix of positive and negative voltages, or all negative voltages, depending on which rail we use as the reference.I'm sure that has hammered it home! In closing, when we have a situation with three supply rails, which is a popular arrangement for op-amp circuits, it is traditional to use the middle voltage as the reference ground, so you have a -12V rail, a GND rail, and a +12V rail.

author: Harry_22
author_rank: Frequent Contributor
timestamp: November 02, 2024, 10:14:11 pm
content: Hi DeKu!I'll tell you as a "mechanic" to a "mechanic".You looked it straight in the eye when you called the negative voltage source The Supplier.In fact, electrons run from minus to plus. There are no more charges in the conductor that can move. And these things run very conditionally, or rather crawl slower than a snail (8 cm per hour). So it is the minus that gives lifeblood to electronics. (We are also a little biologists)You don't have to go far. Near Hamburg is DESY Research Center, where these electrons are accelerated to almost the speed of light. This is where you will find a cool negative voltage supplier!

author: TimFox
author_rank: Super Contributor
timestamp: November 02, 2024, 10:26:50 pm
content: Normally, "positive" and "negative" voltages are with respect to "ground".However, "ground" can be an ill-defined thing.In many systems, ground means the metal case or chassis that contains the rest of the stuff, and then the assignments are obvious.In other systems, there may not be such a case, nor a connection to "PE" or "protective earth", the third prong on the mains power cord.However, most systems are "single ended", meaning the relevant signals are single nodes measured with respect to some sort of "common" node, often the outer shell of coaxial signal connectors.The power supply or supplies for these systems may be only positive (or only negative) with respect to that node, depending on what's more convenient.In my youth, one had to be careful with automotive electrical wiring, since some cars connected the negative end of the storage battery to the car chassis, but others connected the positive end to the chassis ("ground").If you misconnected jumper cables to charge the battery, you could get a nasty spark."Positive ground" autos were discontinued after 1971.

author: golden_labels
author_rank: Super Contributor
timestamp: November 02, 2024, 11:32:01 pm
content: Voltage is a difference in electric potential. If it is positive or negative depends only on what we choose as the reference. I know that sounds like repeating of trivial basics, but getting too used to a convention makes one blind to ground truth. Please bear with me, at least until the third paragraph.With circuits using two power rails, e.g. 5 volts apart, we conventionally name those rails 0 V (GND) and +5 V. But calling the upper voltage 0 V and the lower one -5 V is perfectly valid and changes literally nothing. Same with +2.5 V and -2.5 V. It is convenient to build circuits around a single reference point. A kind of a stable foundation. And so a convention is chosen to use the lower rail.(1)What you are really asking about is not negative voltages, butmultiplevoltages. And why sometimes we choose the reference point to be one of the middle rails. Most commonly: because signals are symmetric. They go both positive and negative relative to their average value. So it’s very convenient to describe the entire circuit using that average value as the center and have wide margins on the sides. Since this central voltage usually also has to be stable and provide low impedance return path, it’s an even more natural choice. And so have GND (0 V) around the signal center, inevitably leading to both positive and negative voltages for higher/lower power lines. Another common issue is that the logic circuitry works between GND and some positive voltage, but occasionally voltages lower than that are needed. Again, leading to them being below 0 V, that is negative.(1)It’s not completely arbitrary. Holes and electrons in semiconductors aren’t symmetrical. Neither we can have a vacuum tube which emits positive charges. So there is some rationale to choose that convention for a number of circuits.

author: Andy Chee
author_rank: Super Contributor
timestamp: November 03, 2024, 02:38:19 am
content: Carlson has a small explainer and demonstration of negative voltages in this video @24:40https://youtube.com/watch?v=mEFdADrU9MA&t=24m40s

author: schmitt trigger
author_rank: Super Contributor
timestamp: November 03, 2024, 03:20:39 am
content: Negative drive voltages are used to ensure a robust and reliable turn-off for IGBTs.

author: David Hess
author_rank: Super Contributor
timestamp: November 03, 2024, 03:50:22 pm
content: In the past the convention of negative voltage was more common in solid state designs because germanium transistors were more commonly PNP, but negative voltages also apply to ECL logic because of how it uses silicon NPN transistors. Below are examples of each.The Harrison power supply is shown with a negative input and output because it is drawn with a germanium PNP emitter follower pass element. The output is floating so negative and positive at the output can be swapped, like any other floating power supply, but by convention it produces a regulated negative output.Logic circuits reference their outputs to common or ground, however with ECL logic, common or ground is the positive supply. This is because the positive power supply rejection is greater than the negative power supply rejection. The current outputs from the differential pair do not care what the positive supply voltage is, so the outputs across the collector load resistors follow the positive supply. The ratio of the 4.98 kilohm and 907 ohm resistances means that the negative supply has less effect than the positive supply on the internal reference voltage, so the logic levels are referenced to the positive supply.With TTL (transistors transistor logic), the inputs and outputs are referenced to the negative supply with a low logic level determined by transistor Vbe to the negative supply, so by convention TTL operates off of a positive supply.

author: tooki
author_rank: Super Contributor
timestamp: November 03, 2024, 09:46:09 pm
content: Quote from: David Hess on November 03, 2024, 03:50:22 pmThe other day at work, I was looking at an old book I found from the 1960s, a compendium of transistor circuits. And while skimming through it, I quickly realized that even familiar circuits looked a bit odd (why negative voltages everywhere??) — then I realized that every single transistor used is PNP.

author: CaptDon
author_rank: Super Contributor
timestamp: November 03, 2024, 10:10:38 pm
content: No one mentioned vacuum tubes. Think about this, a 'ground' reference is applied to the heated cathode electron emitter and a positive voltage applied to the plate will cause a current flow. This is a one way deal. Reversing the polarities causes no current flow, thus we have a vacuum tube rectifier or diode. now with the voltages applied to cause current to flow we insert something looking like a wire screen and call it the grid. Voltage that is more negative than the cathode applied to the grid will repel electrons causing less current flow. Voltages applied to the grid that are more positive than the cathode will cause more current flow (and also grid current to flow). There is your basic triode vacuum tube amplifier. Since it takes very tiny amounts of grid change (either voltage or current) to make large changes in plate current or voltage we get a power gain. Negative grid voltage when referenced to the cathode is seen in the majority of vacuum tube circuits.

author: TimFox
author_rank: Super Contributor
timestamp: November 03, 2024, 10:25:10 pm
content: Most tube circuits (e.g., simple receivers and amplifiers) achieve negative grid-cathode voltage by biasing the cathode positive, usually with a resistor bypassed by a large capacitor, to avoid an extra supply.Fancier audio power amplifiers often had a low-current negative supply for adjustable grid bias.

author: PGPG
author_rank: Frequent Contributor
timestamp: November 03, 2024, 10:38:25 pm
content: Quote from: DeKu on November 02, 2024, 07:00:02 pmWhen in 1982 I was building an oscilloscope (to at last have one) the oscilloscope tube needed anode-cathode voltage to be 1200V and anode should be at the medium voltage of deflector plates. I set them at about +100V (my final amplifier stage was supplied by +175V) so I had to supply tube cathode with -1100V. This is one example of what for negative supply can be used for.I have also to use negative supply for my X and Y amplifiers (to process input signal that can be positive or negative you need (+) supply being higher than signal maximums and (-) supply smaller than signal minimums.

author: CaptDon
author_rank: Super Contributor
timestamp: November 03, 2024, 10:45:06 pm
content: Tim, I had a super nice guitar amplifier made by Earth Research. It had 4 X 6L6 output tubes. It had 'slide bias' where in the absence of audio input for around 30 seconds would cause a high negative grid voltage and cut off the 6L6's. You could actually come back from break and touch the outputs without getting burned!! It would of course cut off about the first 1/4 second of the first note you played coming out of the 'resting' mode but I usually just thumped a string before starting into the first song of the set. I am very sad that I sold that amplifier!! It really rocked with a quad of 6L6's and just to add being impressed, the basic amplifier chassis survived a bar room fire!! The cabinet and speakers were destroyed but the amplifier chassis just needed cleaned up, sprayed the controls and built an 'amplifier head' style cabinet that also housed my Nady wireless rig!

author: BillyO
author_rank: Super Contributor
timestamp: November 06, 2024, 12:48:29 am
content: Negative voltages were used for ECL logic up until about 30 years ago. Most of the fastest computers of that time used ECL.Not sure if it is used anymore.It was also used for early dynamic memory in personal computers and for RS232 communications.

author: David Hess
author_rank: Super Contributor
timestamp: November 06, 2024, 11:50:18 am
content: Quote from: BillyO on November 06, 2024, 12:48:29 amECL is still used where extremely high performance discrete logic is required. There is also PECL (positive ECL) using +5 volts, but it is the same circuits and its logic levels are referenced to the positive supply.Tektronix at various times used ECL with a TTL +5 volt supply, TTL with an ECL -5.2 volt supply, and TTL with a +5 volt supply and ECL with a -5.2 volt supply. In all cases level shifting was required between logic types.I do not remember ECL ever being used in personal computers or for RS-232.

author: SoQ
author_rank: Newbie
timestamp: November 07, 2024, 09:52:33 pm
content: Quote from: SteveThackery on November 02, 2024, 09:49:02 pmI think I understand some of OP's confusion.If I took two identical 12V AC transformers (attached to the same AC outlet) and stacked the secondary AC outputs as per your diagram, would it do what I want, or would there be a conniption-fit on the primary side?

author: Andy Chee
author_rank: Super Contributor
timestamp: November 07, 2024, 10:10:07 pm
content: Quote from: SoQ on November 07, 2024, 09:52:33 pmAC supplies don't use the same terminology as DC supplies (i.e. positive and negative), but there is an equivalent term called phase.So when one is connecting AC transformers together, it is important that the windings are properly phased in order to do the job correctly.

author: radiolistener
author_rank: Super Contributor
timestamp: November 08, 2024, 01:50:24 am
content: Quote from: DeKu on November 02, 2024, 07:00:02 pmIt appears you are referring to bipolar power supplies, which are commonly used in amplifiers that operate with signals centered around ground. For instance, a sine wave generator with an output that changes between -max and +max voltage relative to ground.Bipolar power supplies are frequently employed in audio power amplifiers and are also commonly found in circuits utilizing operational amplifiers.

author: BillyO
author_rank: Super Contributor
timestamp: November 09, 2024, 12:40:19 am
content: Quote from: David Hess on November 06, 2024, 11:50:18 amSorry for the apparent obfuscation, but the mention of PC and RS232 was to the negative voltages, not the ECL.Sometimes my mind gets ahead of my fingers and I forget clarifying points.

author: DeKu
author_rank: Newbie
timestamp: November 15, 2024, 12:40:15 pm
content: Well,i just wanted to thank all of you for your Answers. I do have a much "clearer" Picture now.Thank You Everyone.... just love this communityDeKu