About power supplies

[thememo]

Hello , i am studying electronics engineering and this is my 1st year, as i beginner i want to ask one question about power supplies...This question may be funny to you Also this is my homework but don't worry i don't ask what is that ... how is work... etc.    My lab Prof wants a report about ' what is the difference between power supplies  with details' . Actually i am about to finish this report i explain at great length: number of channels, maximum power& voltage&current limits, noise effect, resolution , programmable capabilities etc.  But i am confused about one subject - Why we need different level outputs? If we compare Rigol DP832A and DP821A power supplies , one of them gives 30V/3A x2 and second of them gives 60V/1A & 8V/10A . I think it can not be the only reason to choose to have high current or high voltage, am i right? What is the reason of this difference ? Also if we use op-amp , we may need two voltage level like +12 and -12, how can give this voltages to op-amp with Rigol DP821A because it's max voltage level is 8. Obviously i confused about this issue and i don't find sufficient information. Thanks for all reply... 

[helius]

But i am confused about one subject - Why we need different level outputs? If we compare Rigol DP832A and DP821A power supplies , one of them gives 30V/3A x2 and second of them gives 60V/1A & 8V/10A . I think it can not be the only reason to choose to have high current or high voltage, am i right?

It pretty much is the only reason. It appeals to users who want the option of higher voltage, and higher current, without buying two separate power supply units. You could test a 60V VFD at the same time as a 10A battery charger, for example.

Also if we use op-amp , we may need two voltage level like +12 and -12, how can give this voltages to op-amp with Rigol DP821A because it's max voltage level is 8.

You can use a single output of the supply with a "virtual ground" circuit to power an opamp.

[jeroen79]

There are different power supplies for the same reason that there are different cars.

Opamps often don't perform well near their supply voltages. If it has to work near circuit ground this can be a problem.
By connecting the negative supply to a lower voltage you get around this.

[bitseeker]

Two reasons: requirements and resolution.

1. Requirements: If you have a circuit that requires 50V, then an 8V power supply isn't going to work. Similarly, if you have a circuit that requires 10A, but your power supply can only provide 2A, then it isn't going to work.

2. Resolution: Digitally controlled power supplies use a DAC to set the output voltage and current. A DAC has a limited resolution (e.g., 1024 steps for a 10-bit DAC). The wider the range that must be supported by the power supply, the larger change each step of the DAC has on the output and, hence, less resolution (e.g., 1024V power supply controlled by a 10-bit DAC yields a very coarse resolution of 1V per step).

These reasons (along with cost) are also why every power supply isn't 1000V and 1000A. You choose the range to fit the requirements and resolution for the task.