Maximum rated voltage/current value on lab power supply

In summary, the conversation discusses the different modes of operation of power supplies, such as constant voltage (CV) and constant current (CC) modes. The maximum rated values for voltage and current are also explained, and how they can be interpreted differently. The conversation also mentions the relationship between voltage, current, and power in a series pass transistor supply.
  • #1
PainterGuy
940
69
Hi :smile:

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. BTW, you may wonder why I don't confirm this by experimenting with the supply itself, well, I don't have access to that supply.

Cheers
 
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  • #2
Good question. I might not answer all parts in your post, but I'll get into the main modes of operation of power supplies:

1. CV - Constant Voltage mode - In this power supply you adjust the output voltage, and the current that will flow out is dependent on the load which also follows the ohm's law.

If you set the power supply to 10V output voltage, and now connect a 5 ohm resistor, then the power supply will output a current of I = V/R = 2A. This works as long as you don't exceed the max rated current. And also, these are very common power supplies.

2. CC - Constant Current mode - This power supply is often called a 'current source' because you get to adjust a constant output current.

If you set it to 1.0A output current, and connect a 5 ohm resistor, then the power supply will generate a voltage across the resistor: V = IR = 5V. This works as long as you don't exceed the max rated voltage.

3. CV and CC - These are the more expensive types that operate in these two modes. You can see in your picture, there is a 'CV' above the voltage knob, and 'CC' above the current knob which could be obscured by the bright light indicating the current has exceeded max rating. This combination in a power supply can allow you to adjust max allowable voltage and current that can go into a circuit.

PainterGuy said:
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.

That depends on the load resistance and ohm's law as described above:
V = IR = 3A * 1 ohm = 3V

hope that helps
 
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  • #3
I think somebody connected a low resistance load across the outputs or something's wrong with the supply.
As soon as the current exceeds the rated current(3A), the power supply will drop the output voltage to avoid damaging itself. (short ckt protection).If I was looking to buy a power supply, then I would interpret 30V and 3A as 3A max current at any voltage and max voltage as 30V.

The lab supplies I have used usually go to 10% over their rated current before the current limit light comes on and voltage drops.
 
  • #4
Should also point out that power supplies have constant wattage so a curve of spec'ed I vs. spec'ed V is hyperbolic (P=IV=constant): you can't get spec'ed maximum current at the same time you are at spec'ed maximum voltage.
 
  • #5
jsgruszynski said:
Should also point out that power supplies have constant wattage so a curve of spec'ed I vs. spec'ed V is hyperbolic (P=IV=constant): you can't get spec'ed maximum current at the same time you are at spec'ed maximum voltage.

Hmm . . power supplies do not regulate a constant wattage. If I connect a 10 ohm on its outputs, its wattage is very different than if I connect a 1M resistor.

Anyway, you should be able to get maximum current rating at the same time as maximum voltage rating unless stated explicitly. Or else they would give you a maximum power output specification in addition to the max current and max voltage. If the voltage is below the maximum voltage, the current may or may not be able to increase depending on how they have specified the maximum current since just the power output is not the only possible limitation on the maximum current.
 
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  • #6
jsgruszynski said:
Should also point out that power supplies have constant wattage so a curve of spec'ed I vs. spec'ed V is hyperbolic (P=IV=constant): you can't get spec'ed maximum current at the same time you are at spec'ed maximum voltage.

In a series pass transistor supply drawing max current, the time when the supply is LEAST labored is max voltage.
 
  • #7
DragonPetter said:
Hmm . . power supplies do not regulate a constant wattage. If I connect a 10 ohm on its outputs, its wattage is very different than if I connect a 1M resistor.

Anyway, you should be able to get maximum current rating at the same time as maximum voltage rating unless stated explicitly. Or else they would give you a maximum power output specification in addition to the max current and max voltage. If the voltage is below the maximum voltage, the current may or may not be able to increase depending on how they have specified the maximum current since just the power output is not the only possible limitation on the maximum current.

Physics regulates at constant wattage. Engineers design around that.

If you have a supply that only spec's V & A, the supply is leaving capability on the table. That's fine and all but essentially they are squeezing a single square range under the power hyperbola. The rest could be used but isn't.

Most higher power http://www.home.agilent.com/agilent/product.jspx?nid=-536902290.384194.00&cc=GB&lc=eng" into the power hyperbola.
 
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  • #8
jsgruszynski said:
Physics regulates at constant wattage. Engineers design around that.

If you have a supply that only spec's V & A, the supply is leaving capability on the table. That's fine and all but essentially they are squeezing a single square range under the power hyperbola. The rest could be used but isn't.

Most higher power http://www.home.agilent.com/agilent/product.jspx?nid=-536902290.384194.00&cc=GB&lc=eng" into the power hyperbola.

I wasn't arguing with your point that you can get same power with more current if you lower voltage.

But physics does not regulate power, and watts are not being regulated by a power supply :P Physics might be putting a maximum capability on power, but it does not make the watts a constant. I read your original comment as saying that the power supply is maintaining a constant power output at all times.
 
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  • #9
Max power transfer from a power supply to a load can occur at a certain load resistance. If 30V and 3A are max ratings of either a CV or CC power supply, then RL = V/I = 10 ohms for which Pmax = VI is the max rated power, in this case it's 90W.
 
  • #10
DragonPetter said:
I wasn't arguing with your point that you can get same power with more current if you lower voltage.

But physics does not regulate power, and watts are not being regulated by a power supply :P Physics might be putting a maximum capability on power, but it does not make the watts a constant. I read your original comment as saying that the power supply is maintaining a constant power output at all times.

Power consumption is generally fixed for any given implementation of a power supply. That establishes a constraint on what ever downstream circuitry you can design, be it power supply or network analyzers or anything else. It is an implicit upper bound to what you can ever get with that spec'ed instrument system wattage. This effectively regulates how much voltage or current you can ever get. Sometimes this is only implicitand hidden (a single range PS) while other times it's explicit (a multiple range PS).

Ultimately it's simply what ever is spec'ed but you have to look carefully at what the specs actually are. Sometimes it's a single-range supply - then it's like you describe.

This is not what most instrumentation power supplies have - most are multiple range. They do this to squeeze out every last amp or volt out of what the system power supply can deliver. And at any given range, you can't get the maximum spec'ed current while at the maximum spec'ed voltage range, and vice versa. The result is you get a http://i.imgur.com/bSWUE.png".

This is why you have to look very carefully at the specific power supply's specs. It is a fact that sometimes this reality is hidden deep in the spec details but not explicitly described (for marketing/sales reasons) in the headlined specs of the power supply.
 
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  • #11
jsgruszynski said:
Power consumption is generally fixed for any given implementation of a power supply. That establishes a constraint on what ever downstream circuitry you can design, be it power supply or network analyzers or anything else. It is an implicit upper bound to what you can ever get with that spec'ed instrument system wattage. This effectively regulates how much voltage or current you can ever get. Sometimes this is only implicitand hidden (a single range PS) while other times it's explicit (a multiple range PS).

Ultimately it's simply what ever is spec'ed but you have to look carefully at what the specs actually are. Sometimes it's a single-range supply - then it's like you describe.

This is not what most instrumentation power supplies have - most are multiple range. They do this to squeeze out every last amp or volt out of what the system power supply can deliver. And at any given range, you can't get the maximum spec'ed current while at the maximum spec'ed voltage range, and vice versa. The result is you get a http://i.imgur.com/bSWUE.png".

This is why you have to look very carefully at the specific power supply's specs. It is a fact that sometimes this reality is hidden deep in the spec details but not explicitly described (for marketing/sales reasons) in the headlined specs of the power supply.

Don't want to argue because I agree with most of what you say, but an upper bound is not regulation of the power, it simply limits the maximum before the device breaks down. And power consumption is not fixed, it depends on the load you have attached to it. That's all I was getting at.
 
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1. What does the maximum rated voltage/current value on a lab power supply mean?

The maximum rated voltage/current value on a lab power supply refers to the highest amount of voltage or current that the power supply can safely deliver without causing damage to the equipment or posing a safety risk.

2. How is the maximum rated voltage/current value determined for a lab power supply?

The maximum rated voltage/current value is determined by the manufacturer through rigorous testing and quality control processes. It is based on the capabilities of the components used in the power supply and is typically stated on the product label or in the user manual.

3. Can a lab power supply be used above its maximum rated voltage/current value?

No, it is not recommended to use a lab power supply above its maximum rated voltage/current value. Doing so can cause damage to the power supply and connected equipment, and may also pose a safety hazard. It is important to always use a power supply within its specified limits.

4. What are the consequences of using a lab power supply beyond its maximum rated voltage/current value?

Using a lab power supply beyond its maximum rated voltage/current value can result in overheating, component failure, and damage to the equipment being powered. It can also pose a fire or electrical shock hazard.

5. Is it possible to increase the maximum rated voltage/current value of a lab power supply?

No, the maximum rated voltage/current value of a lab power supply is determined by its design and cannot be changed. Attempting to modify a power supply to increase its maximum value can be very dangerous and is not recommended.

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