# maximum rated voltage/current value on lab power supply

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#### PG1995

##### Full Member level 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.

Please correct me. Thank you.

#### yadavvlsi

##### Advanced Member level 3
If you apply a load that took current more than specified limit, your voltage will drop down. But if you remove the load current should drop to zero and voltage will increase again. By the way what you have connected to your supply.?

#### BradtheRad

##### Super Moderator
Staff member
Maybe the designers gave it some headroom to handle brief startup surges, etc.

Or maybe it's heating up as it puts out 4.5A, and it will last for thirty seconds before thermal protection shuts it down.

Or the 3A rating may be conservative.

#### FvM

##### Super Moderator
Staff member
Seriously spoken, your question can't be answered without consulting the specification and users manual of the power supply. For the time being, I can offer this explanations:
- The marking 30V/3A is incorrect and the instrument is able to safely supply more current at low output voltage.
- The instrument isn't designed correctly and can supply a current beyond it's safe ratings.
- The instrument is defective

It's quite normal to have some current reserve above specified limits for an instrument with analog current control. But nearly 50% doesn't sound regular.

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