how simple lab current source and voltage source work?

[PG1995]

Hi

While helping me with the queries please don't forget that I'm a beginner to this technical stuff (or, my knowledge of this area of knowledge is nothing more than beginner's! ).

I'm trying to understand how simple lab current source and voltage source works without getting into too much technical details of their operations.

This lab power supply (variable voltage/current source):
https://img810.imageshack.us/img810/4475/powersupplycurrent.jpg

can deliver maximum 30V and 3A. The funny thing is in the picture it is delivering 4.45A which is more than the maximum rated value for the current. But I think I'm interpreting the meaning of maximum rated value wrongly. I think by "30V/3A" it means when voltage is set at 30V then the maximum current you could get is 3A. In other words, the maximum power the power supply could deliver is: Power = 30V x 3A = 90W. So, if you are using the supply as a constant current source and the current being delivered is 10A, then the maximum voltage it could produce would be 9V.

Please correct me. Thank you.

 

[Nigel Goodwin]

Sorry, but yes you're wrong.

The maximum useable current at any voltage is 3A - if anything it will be LESS at lower voltages (due to the increased heat dissipation).

If you notice on that picture the current light is on, presumably showing it's in current limiting mode, and not working in it's normal operation.

 

[JimB]

To try and expand a bit on what Nigel said...

This lab power supply (variable voltage/current source): ....
can deliver maximum 30V and 3A.

30V and 3A is the RATING of the PSU.
Depending on the detailed specification (read the handbook) it can deliver anything within those limits all day every day without any problem.
Depending on the design of its internal circuitry it could be that it will not deliver 3A at 1V without overheating, (read the handbook).

The funny thing is in the picture it is delivering 4.45A which is more than the maximum rated value for the current.

The current limit control is at maximum, this 4.45A is what the power supply CAN deliver but dont ask it to do it all day or it my overheat and die.
The designer has put in a generous allowance over the published specification. Either that or he was too cheap to design the current limit control to limit the output to 3.06A, 2% over the specification.

But I think I'm interpreting the meaning of maximum rated value wrongly. I think by "30V/3A" it means when voltage is set at 30V then the maximum current you could get is 3A. In other words, the maximum power the power supply could deliver is: Power = 30V x 3A = 90W. So, if you are using the supply as a constant current source and the current being delivered is 10A, then the maximum voltage it could produce would be 9V.

Please correct me. Thank you.

Yes you are reading more into this than is actually there.
The psu does not give 90watts at any setting of voltage and current.
It gives up to 30v, and up to 3A depending on the detailed specification, there may be a de-rating curve when operating at low voltages, it may only be rated for 1.5A when the voltage is below 15V, read the manual.

JimB

 

[crutschow]

If it were a switching type supply, then it might be able to deliver it's rated power at a low output voltage, but a linear regulator, which is the most common type of power supply, can only deliver its rated power at the maximum rated output voltage.

 

[KeepItSimpleStupid]

And, 2.5 * 4.5 is about 11 W and no where near the rating of the power supply. As was said, variable power supplies have built-in limits and sometimes you can go outside of one of the bounds. With transistors, there is a parameter called SOA or safe operating area. Note that on page 2 of a 2N3055 datasheet, http://www.datasheetcatalog.org/datasheet/motorola/2N3055.pdf, there is an SOA graph.