Variable vs constant power supply

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Discussion Overview

The discussion revolves around the characteristics of power supplies, specifically comparing variable and constant power supplies, and their implications for devices like CCTV cameras. Participants explore the effects of no-load voltage, voltage sag under load, and the potential risks of using different types of adapters.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions the terminology for power adapters that exhibit higher no-load voltage compared to those that maintain a constant voltage, seeking clarity on the implications for device compatibility.
  • Another participant points out that the datasheet indicates a voltage tolerance of +/-3%, suggesting it applies across the load range, but raises concerns about the absence of no-load voltage data for the Meanwell adapter.
  • There is a discussion about the significance of no-load voltage, with some participants arguing that it can overstress components and affect reliability, while others question its relevance.
  • Participants note that ordinary adapters can experience significant voltage sag (up to 45%) at no load, while branded adapters like Meanwell exhibit much lower sag (around 3%).
  • One participant inquires about the internal components of adapters that contribute to different sag characteristics, suggesting that power control and feedback mechanisms play a role.
  • There is a debate about whether regulated power supplies can be used interchangeably with devices designed for ordinary adapters, with some asserting that devices have specific voltage input specifications that must be adhered to.
  • A later reply suggests that using a regulated power supply should not damage devices, as they typically operate within a specified voltage range.
  • Participants express skepticism about marketing claims related to power supplies, emphasizing the importance of reading datasheets thoroughly.

Areas of Agreement / Disagreement

Participants express differing views on the implications of using variable versus constant power supplies, the significance of no-load voltage, and the compatibility of regulated power supplies with various devices. The discussion remains unresolved regarding the specific characteristics of the Meanwell adapter in question.

Contextual Notes

There are limitations regarding the availability of specific input voltage specifications for devices, and the discussion highlights the complexity of power supply design without reaching a consensus on the implications of using different types of adapters.

  • #31
Ephant said:
In your description, did you mean the voltage is always 12V even when current vary from 0 to 5A?
Yes!

Ephant said:
when the current drawn is less than 5A, the voltage will be greater like 18V (for no load). It is only when the current drawn is 5A that the voltage will be 12V at full load. In both case the power in watt is maintained from voltage times ampere).
Sorry, not quite correct, the delivered power is not constant. Please see the datasheet below for a table of how the output voltage of an typical unregulated supply varies as the current changes.
Beware of the graph on that page, the x-axis is nonlinear. At low currents the voltage drops rapidly with increasing loads.

You will see that at 50mA load the delivered power is 1Watt;
but at 300mA load the delivered power is 4.8W!

https://assets.omega.com/manuals/M1388.pdf

Cheers,
Tom
 
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  • #32
Tom.G said:
Yes!Sorry, not quite correct, the delivered power is not constant. Please see the datasheet below for a table of how the output voltage of an typical unregulated supply varies as the current changes.
Beware of the graph on that page, the x-axis is nonlinear. At low currents the voltage drops rapidly with increasing loads.

You will see that at 50mA load the delivered power is 1Watt;
but at 300mA load the delivered power is 4.8W!

https://assets.omega.com/manuals/M1388.pdf

Cheers,
Tom

How do you know what wattage is the right one, 1Watt for 50mA load or 4.8W for 300mA load? If both is wrong, how do you check the right wattage?
 
  • #33
Ephant said:
how do you check the right wattage?
"...right wattage?"

That is sort of like asking "What is the weight you can lift?". There is no "right" answer because you can lift your limit or anything below your limit.

The Maximum wattage that the power pack can supply is stated, sometimes indirectly, on the pack itself or in the data sheet.

If not explicitly stated, the Wattage is the normal output voltage, V, times the rated output current I; in other words W = V x I -- where V is the output voltage, and I is the current that the load is using (drawing from the power pack).
 

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