Choosing a power source, how to get voltage and current ratings?

All QuestionsCategory: Electrical EngineeringChoosing a power source, how to get voltage and current ratings?
Choosing a power source, how to get voltage and current ratings?Niels Toft asked 4 months ago
Power supplies are available in a wide range of voltage and current ratings. If I have a device with specific voltage and current ratings, how do these relate to the power ratings I need to specify? What if I don’t know the hardware specifications, but I’m replacing a previous power supply with certain ratings?
Is it okay to lower the voltage or should it always be higher? What about the current? I don’t want a 10 A source to damage my 1 A device.

1 Answers

Best Answer

Choosing a power source, how to get voltage and current ratings?Marcos Oliveira answered 4 months ago

If the device says that it needs a certain voltage, then you have to assume that it needs that voltage. Both lower and higher could be bad.
At best, with low voltage, the device will not work properly in an obvious way. However, some devices may appear to function properly, and then fail in unexpected ways under the just right conditions. When you violate the required specifications, you never know what might happen. Some devices can even be damaged by too low voltage for extended periods of time. If the machine has a motor, for example, the motor may not be able to develop enough torque to spin, so it just sits there while it’s hot. Some devices may draw more current to compensate for the lower voltage, but higher current than intended can damage something. Most of the time, a voltage drop causes the device to not work, but damage cannot be ruled out unless you know something about the device.
Higher than the specified voltage is definitely a bad thing. All electrical components have voltages that they fail to exceed. Components rated for higher voltage generally cost more or have less desirable characteristics, so it is likely that the selection of the appropriate voltage tolerance for the components in the device has received significant design attention. Applying too much effort violates the design assumptions. A certain level of overvoltage may damage something, but you don’t know where that level is. Take what the device says on the nameplate seriously and don’t give it more effort than that.

current rating
The current is a little different. The constant voltage source does not determine the current: the load, which is the device in this case, does. If Johnny wants to eat two apples, he will only eat two apples whether you put 2, 3, 5 or 20 apples on the table. A device that wants 2A of current works in the same way. 2A will plot whether the power supply can only supply 2A, or whether it can provide 3, 5, or 20A. The current rating of the supply is what it can provide, not what it will always force through a load in some way. In this sense, unlike the mains voltage, the current rating of the power supply should be at least what the device wants but there is no harm in being higher. A 9 volt 5 amp source is a superior combination of a 9 volt 2 amp source, for example.

replace existing supply
If you’ve been replacing a power supply previously and don’t know the hardware requirements, then consider classifying that power supply as the hardware requirements. For example, if an unnamed device is powered from a 9 volt, 1 amp source, you can replace it with a 9 volt, 1 amp or more source.

advanced concepts
The above presents the basics of how to choose a power supply for some devices. In most cases, this is all you need to know to go to a store or online and purchase a power supply. If you are still not clear about what exactly the voltage and current are, it is better to leave it for now. This section goes into more detail of the power supply that is generally of no interest at the consumer level, and assumes some basic understanding of electronics.

regulated vs disorganized

Very basic DC power supplies, called unregulated, just compromise the AC input (generally the DC you want is at a much lower voltage than the wall power you’re plugging in), rectify it to produce DC, and add an output cap to reduce Ripple, call it today. Years ago, many energy sources were like this. It was more than just a transformer, four diodes making a full wave bridge (electronically takes the absolute value of the voltage), and a filter housing. In these types of supplies, the output voltage is determined by the ratio of the rotation of the transformer. This is constant, so instead of making a constant output voltage, their output is mostly proportional to the AC voltage. For example, a “12V” DC source may produce 12V at 110VAC, but then it may produce more than 13V at 120VAC.
Another problem with unregulated supplies is that the output voltage is not only a function of the input voltage, but it will also fluctuate with the amount of current being drawn from the supply. A “12V 1A” unregulated supply is likely designed to provide a rated voltage of 12V at full output current and the lowest valid AC input voltage, such as 110V which can be more than 13V at 110V without load (0A) out ) alone, then higher after at higher input voltage. Such a supply can easily output 15 volts, for example, under some conditions. Appliances that need “12v” are designed to handle that, so that was fine.

Modern energy sources no longer work that way. Anything you can buy as consumer electronics will be a regulated power supply. You can still get an unregulated supply from more specialized electronics suppliers targeting manufacturers, professionals, or at least hobbyists who should know the difference. For example, Jameco has a wide range of energy sources. Its wall warts are specifically divided into organized and unorganized types. However, unless you wander where the average consumer should be

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