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I am having some problems with understanding power supply. Can a 24VDC 14A rated power supply provide more current than it's rated? I thought the maximum current the power supply can provide is 14A. Imax= V/(Ri+RL), when RL = 0. (Ri=internal resistance, RL=load resistance) Even if I short the power supply with a wire, the maximum current that will flow through the wire is 14A? If not, what's the maximum current? Can you explain why?enter image description here

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It depends

Basically, there are several different possible outcomes, depending on the decisions the designer made when designing the power supply. Some possibilities:

  • The power supply fails. In which case, the current and voltage drop to zero, and possibly the power supply catches fire. This is the cheapest and easiest option for the designer/manufacturer.
  • The power supply has an output fuse, which blows. In which case the the current and voltage drop to zero, and the power supply can be repaired by replacing the fuse. This is only marginally more expensive than the above, so should be the bare minimum in most power supplies you can buy.
  • The power supply has overcurrent protection. In which case the power supply detects the short circuit, and turns the output off, or turns it down to a very low level. It may then automatically reset after some time, or it may need the short disconnected, or a reset button pressed.
  • The power supply has a current limit. In which case the current stays at or near the rating, and the voltage drops to virtually nothing, in keeping with ohms law. This is the most complex to design and most expensive to build option, and a power supply which does this will probably draw attention to it in the manual/datasheet.

In all cases, the power supply will probably go a bit over the rating before the above kicks in. This is done because the manufacturer wants to make sure it never ends up less than rated, so they build in a little bit of margin. But you should't normally use a power supply in the narrow gap between the rating and the point it fails/shuts down, as it will probably be unreliable and/or wear out quickly.

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Although the supply might provided more than its rated current, you can not assume that it will. The manufacturer is guaranteeing that you will get at least 14A.

No one can say, based on the information you give, what will happen if you short the outputs. The current might limit at 14A and the voltage drop to near zero. The current might "fold back" to a smaller value. The supply might catch fire.

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The amount of current that can flow is dependent on how the power supply has been designed. It's a choice of the power supply designer.

The possibilities for a supply rated at 14A are -

a) 14A - if it's been designed to limit at a constant current.

b) much less than 14A - if it's been designed to 'fold back' on overload. This provides a much lower current when overloaded to avoid turning a load fault into a fire.

c) much more than 14A - if it's an old transformer + rectifier type supply. The current will typically stop within one second if a fuse blows, or in a few minutes when the supply catches fire.

d) roughly zero - a more or less full shutdown into an overload. It might pulse occasionally to test whether the short circuit is still present.

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