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Choosing power supply, how to get the voltage and current ratings?

I have a conceptual question about the rated current shown on a laptop charger. And I assume it is a kind of switch mode power supply. Before I ask my question I would like to mention why I am asking this question. Let’s say I have a 9V battery and I hooked it to the ends of a 1 ohm resistor. Theoretically the current flows would be around 9A. If I increase the resistance (impedance) the current will go to zero linearly. If resistance is 4.5 ohm the current will be 2A and when it is 9 ohm the current will be 1A ect. Theoretically the current decreases linearly and goes to zero as the resistance increases.

In SMPS devices such as laptop chargers the output voltage is regulated and kept at a value around 19V. My question is: “Why is there always rated current such as 4.6A for a laptop charger?” Is that because the total impedance of laptop input is in some interval? Isn’t it possible for a laptop charger (or for any SMPS) to feed a bigger impedance and flow very low currents with the same regulated voltage?

My charger has rated voltage and currents 19V and 4.6A. I really wonder if it is possible to draw 1A with again 19V fixed voltage or draw 10A with again 19V by just adjusting the impedance it feeds. Is regulation related to this rated current in SMSPs?

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user16307
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  • “Why is there always rated current such as 4.6A for a laptop charger?” This is the max current it can supply, AND a laptop needs a lot of power to run and charge the battery at the same time, so 1amp is unlikely to suffice! – Garrett Fogerlie Dec 27 '12 at 15:18
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    Note that the current doesn't decrease linearly with the resistance, because 1/R is not linear. – clabacchio Dec 29 '12 at 13:05

2 Answers2

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The current rating of a constant-voltage power supply is the maximum current it can put out. Devices can always draw less than that if they wish. This answer goes into more detail. In particular, look at the Current Rating section.

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Olin Lathrop
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"Rated" effectively means maximum; it is safe and normal to draw much less current (e.g. when your laptop battery is fully charged) but the SMPS will not allow you to draw more.

If your car's engine always produced its rated power, you'd probably wreck it before you were out of the parking lot!

The exception (currently rare but starting to happen for LED lighting) is a "constant current" supply which will supply, say, 350ma at any voltage up to a certain limit (12 or 20 or 40V), because 350ma is a common current for lighting LEDs.

  • Many switched-mode power supplies have a minimum amount of current they fan supply while remaining in regulation. In some cases, a supply may be able to dump excess current (severely degrading efficiency) if the load doesn't draw enough. In some less common cases, however, a supply's output voltage may exceed the expected level when loaded below the minimum. – supercat Nov 19 '14 at 00:36