[SOLVED] What is it that defines how much voltage rating a device needs?

[matrixofdynamism]

We have circuits of so many kinds but I am talking about whole systems here like our PC or Washing Machine or the heavy duty machines in factories that use 3 phase power supplies. We come across voltage ratings as 9v, 5v, 12v, 3.3v, 120v, 240v, and also ratings in kV.

What no one has ever told me is where these numbers come from. Why 9v, why not 8v for example. Why 12 and not 15v? Certainly there is a range in between which the components/systems shall work but what describes this range? Why don't the industrial machinery work on 240v or 10kV instead of 2kV or 5kV? You will say that it shall force too much current through it, well we can limit the current flow using special circuits right ?

Than there is this confusion related to how we model the real world components during the circuit analysis. I understand that batteries have a voltage and with a load shall also give current. But certainly for any power source whether it be the 1.5v battery or the car battery or the Power plants, there has to be a limit to how much current they can supply right? So what decides the max current limit?

 

[FvM]

Generally speaking, it sounds quite reasonable that machines are designed to work with standard power grid voltages.

 

[nandal]

depends on the kind of use of the device and inside circuitry ... ac is steped up or down ,rectified accordingly

 

[jeffrey samuel]

the chief thing in mind when the no is printed is the average maximum voltage and current for which the response of the system is maximum

 

[tpetar]

We have circuits of so many kinds but I am talking about whole systems here like our PC or Washing Machine or the heavy duty machines in factories that use 3 phase power supplies. We come across voltage ratings as 9v, 5v, 12v, 3.3v, 120v, 240v, and also ratings in kV.

What no one has ever told me is where these numbers come from. Why 9v, why not 8v for example. Why 12 and not 15v? Certainly there is a range in between which the components/systems shall work but what describes this range? Why don't the industrial machinery work on 240v or 10kV instead of 2kV or 5kV? You will say that it shall force too much current through it, well we can limit the current flow using special circuits right ?

Than there is this confusion related to how we model the real world components during the circuit analysis. I understand that batteries have a voltage and with a load shall also give current. But certainly for any power source whether it be the 1.5v battery or the car battery or the Power plants, there has to be a limit to how much current they can supply right? So what decides the max current limit?

Device designers decide which voltage, and how much current will use that particular device. They must plan usage of their devices on standard mains power, depending from country standards 220V/110V, or if they make smaller devices such as laptops, imaging devices,.... they looks power needed for thise devices and decide appropriate voltage to supply internal parts, such as batteries. Often is avoided usage of high currents, intead of that is used higher voltage, to achieve higher power of device. In some cases when needed higher power, we use three phase. Device designers pay attentions on safety and country standards, also they trying to standardize power supply needs of their devices (you can buy universal charger for laptop,....).

Why transmission line of some electricity distributer works on high voltage lets say 110KV ? Why they use higher voltage over some cable to supply whole city ? Why they dont use 220V voltage and very higher amperage (what will be diameter of that transmission cable) ?

 

[jeffrey samuel]

also note that the higher voltage results in lesser loss and so the distribution lines have lower losses
else higher the current the more losses in the form of I^2R

where I= current
R= resistance of the cable

 

[matrixofdynamism]

So the bottom line is that the designers choose what voltage their circuit shall work optimally.
I understand the point of AC-DC conversion and that all devices shall connect to the main grid. By question had to do with this varying DC input required for different devices. Like Laptops vs Electronic guitars vs Painos vs Printer vs Mobile phone e.t.c They all certainly use a lot of semiconductor devices but do not need same exact DC to work.

 

[FvM]

Some power supply ranges are suggested by etablished battery standards, e.g. 3.6V Li-Ion for mobile phones, 10.8 and 14.4 V battery blocks of notebook computers. For others tpetar's comment applies "Device designers decide which voltage, and how much current will use that particular device."

There are often specific technical reasons, why e.g. "12 not 15 V" has been selected, but they are rarely forcing.

In my view, you should rather start a detail analysis of existing designs than adding up more questions.

 

[matrixofdynamism]

Hmm, ok so this thread is solved than. The solution really is to check present existing designs and see why they use the voltage they do.

 

[tpetar]

Hmm, ok so this thread is solved than. The solution really is to check present existing designs and see why they use the voltage they do.

Good existing power supply solutions are compromis between input voltage/amperage and needed power for device. Often is avoided to use higher currents, and input voltage is always several volts higher then needed because voltage regulation. There is no point to give 25V on input if device need 12V, some part must take job for lowering that difference and this can be heat and additional losses and useless dissipation of power.

:wink: