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- Thread starter manv
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Perhaps you are used to the negative terminal being grounded. With that arrangement, the voltage of the negative terminal is held at 0 Volts, and the positive terminal supplies positive voltage.
In your case, when the positive terminal is grounded, the positive terminal is held at 0 Volts. Therefore, the negative terminal provides negative voltage, below 0 Volts. There is nothing unusual about having a negative voltage output. Again, if the positive terminal is grounded, then the other terminal is being used as a source of negative voltage.
For example, you can take a 9 volt transistor radio battery and connect the positive terminal to ground. Ground is always 0 Volts, and so the positive terminal is at 0 Volts. Therefore, the negative terminal is at -9 Volts. (Negative 9 Volts)
It is unusual for a negative power supply (grounded positive terminal) to be "alone." Usually, there is a second power supply with a grounded negative terminal, which provides a positive voltage on its positive terminal. Often, the two power supplies are contained within one cabinet.
I.e., the negative terminal of the first power supply is connected to ground, and the positive terminal of the second power supply is connected to ground, also. In this way, one supply provides negative voltage, and the other provides positive voltage, relative to ground.
It takes a special power supply to be able to connect either terminal to ground. The assumption is that neither terminal is connected to ground until you connect it to ground, yourself. Until then, the power supply’s output is isolated from ground, which is called "floating."
One terminal might have 15 volts more than the other. But if neither terminal is connected to ground, then you can't say what the voltage is - relative to ground - of either terminal, but only that there is a 15 volt difference between the terminals.
Such a floating power supply can be turned into either a positive power supply or a negative power supply by connecting the appropriate terminal to ground. By definition, the grounded terminal will be at 0 Volts, and the other terminal will be either a positive or negative voltage, relative to ground.
You might connect the negative terminal to ground. Then, the positive terminal of the supply is at +15 Volts, 15 Volts higher.
You might connect the positive terminal to ground. Then, the negative terminal of the supply is at -15 Volts, 15 Volts lower.
You might connect the negative terminal to -1000 Volts. Then, the positive terminal of the supply is at -985 Volts, 15 Volts higher.
But this is a game that you can only play with a floating power supply. A battery is an example of a floating power supply, until you connect one of its terminals to ground.
Many power supplies have an internal wire connecting their negative terminals to ground (the 3rd prong on the AC plug). Therefore, their negative terminal is always at True 0 Volts, and the other terminal is at some positive voltage. With this arrangement, you cannot make a negative-voltage power supply. You might try connecting the positive terminal to ground, but since the negative terminal is already connected internally to ground in this inflexibly designed power supply, you will in fact be causing both terminals to be connected to the same point, shorting out the power supply.
The reference for voltage is ground, absolutely 0 Volts. But if the output of the isolated power supply [either terminal] is not connected to ground [or a voltage that has been measured with respect to ground], then the "absolute" voltage of the power supply - relative to ground - cannot be assumed.
For instance, you can have a 9 Volt battery on which a joker has just deposited a tremendous static charge without telling you. If you measure the voltage from the battery to ground, you will see that it has 10,000 Volts on it. (+/- 9 volts on the other terminal.) This demonstrates that you can't assume that the negative terminal of a 9 Volt battery is at 0 Volts (relative to ground), nor that the voltage of the positive terminal is at 9 Volts (relative to ground). Only when one side of the battery is connected to ground can you say what the absolute, real voltages are of the two terminals. But while the battery is floating, you only know that the voltage difference between the two terminals is 9 volts, but you don’t know what their absolute voltages are, relative to ground.
Best luck,
-Neil-
In your case, when the positive terminal is grounded, the positive terminal is held at 0 Volts. Therefore, the negative terminal provides negative voltage, below 0 Volts. There is nothing unusual about having a negative voltage output. Again, if the positive terminal is grounded, then the other terminal is being used as a source of negative voltage.
For example, you can take a 9 volt transistor radio battery and connect the positive terminal to ground. Ground is always 0 Volts, and so the positive terminal is at 0 Volts. Therefore, the negative terminal is at -9 Volts. (Negative 9 Volts)
It is unusual for a negative power supply (grounded positive terminal) to be "alone." Usually, there is a second power supply with a grounded negative terminal, which provides a positive voltage on its positive terminal. Often, the two power supplies are contained within one cabinet.
I.e., the negative terminal of the first power supply is connected to ground, and the positive terminal of the second power supply is connected to ground, also. In this way, one supply provides negative voltage, and the other provides positive voltage, relative to ground.
It takes a special power supply to be able to connect either terminal to ground. The assumption is that neither terminal is connected to ground until you connect it to ground, yourself. Until then, the power supply’s output is isolated from ground, which is called "floating."
One terminal might have 15 volts more than the other. But if neither terminal is connected to ground, then you can't say what the voltage is - relative to ground - of either terminal, but only that there is a 15 volt difference between the terminals.
Such a floating power supply can be turned into either a positive power supply or a negative power supply by connecting the appropriate terminal to ground. By definition, the grounded terminal will be at 0 Volts, and the other terminal will be either a positive or negative voltage, relative to ground.
You might connect the negative terminal to ground. Then, the positive terminal of the supply is at +15 Volts, 15 Volts higher.
You might connect the positive terminal to ground. Then, the negative terminal of the supply is at -15 Volts, 15 Volts lower.
You might connect the negative terminal to -1000 Volts. Then, the positive terminal of the supply is at -985 Volts, 15 Volts higher.
But this is a game that you can only play with a floating power supply. A battery is an example of a floating power supply, until you connect one of its terminals to ground.
Many power supplies have an internal wire connecting their negative terminals to ground (the 3rd prong on the AC plug). Therefore, their negative terminal is always at True 0 Volts, and the other terminal is at some positive voltage. With this arrangement, you cannot make a negative-voltage power supply. You might try connecting the positive terminal to ground, but since the negative terminal is already connected internally to ground in this inflexibly designed power supply, you will in fact be causing both terminals to be connected to the same point, shorting out the power supply.
The reference for voltage is ground, absolutely 0 Volts. But if the output of the isolated power supply [either terminal] is not connected to ground [or a voltage that has been measured with respect to ground], then the "absolute" voltage of the power supply - relative to ground - cannot be assumed.
For instance, you can have a 9 Volt battery on which a joker has just deposited a tremendous static charge without telling you. If you measure the voltage from the battery to ground, you will see that it has 10,000 Volts on it. (+/- 9 volts on the other terminal.) This demonstrates that you can't assume that the negative terminal of a 9 Volt battery is at 0 Volts (relative to ground), nor that the voltage of the positive terminal is at 9 Volts (relative to ground). Only when one side of the battery is connected to ground can you say what the absolute, real voltages are of the two terminals. But while the battery is floating, you only know that the voltage difference between the two terminals is 9 volts, but you don’t know what their absolute voltages are, relative to ground.
Best luck,
-Neil-
The choice of having the + source tied to ground is not arbitrary but chemistry. Metal is a positive ion and that is why metal corrodes. Make the metal negative and it does not rust away. To preserve the metal wires and connectors in communication systems, they are typically operated negative relative to ground. Another problem that occures is a surface oxidation in connectors when no current is flowing. That is why some circuits have a "sealing" current even though the application does not require any current. The problems encountered by poor connections in many electronics are a direct result of these often lost or forgotten lessons of the past.
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