# maximum rated voltage/current value on lab power supply

Discussion in 'General Electronics Chat' started by PG1995, Jun 3, 2011.

1. ### PG1995 Thread Starter Active Member

Apr 15, 2011
753
5
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):
http://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.

2. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,433
1,623
Actually we believe you when you say it delivers 4 amps and a art number or a link to the supply's spec would be better...

Sure, it's giving more then it's rating, and that is as it should be. If I promise you 3 amps I better deliver 3 amps so I'm going to design it to make more then 3 amps.

Noting the light over the current meter we see this is in current limit mode.

With such a low output voltage it is able to provide some extra current.

3. ### tom66 Senior Member

May 9, 2009
2,613
214
That's a lot over.

I have a 35V 3A CC/CV power supply, the maximum current is set to 3.04A on my multimeter.

Perhaps someone has modified it? Often current limits are set by a few resistors which can be changed. The power supply may be perfectly capable of 4.5A but the manufacturer has limited it so they can upsell you to the higher end model...

Most of these power supplies use a single current limit, or sometimes they have a second higher-current range, with limited output voltage. It depends. Either way, I doubt there is an actual power limit, that would require a multiplier circuit (or a microcontroller.... those displays are probably driven by discrete ADC to 7seg ICs.)