I have a laptop AC Adapter thats rated for 19.5v/3.34A DC output for a total of 65W. The input is rated at 100-240V/1.5A AC. My question is what is limiting the output to 65W? It seems like the input can support up to 100*1.5=150W minimum. I"m assuming you lose some in the AC to DC conversion, but 65W would be more than 50% loss. So, is the adapter able to support more than 65W or is there something here I'm missing?

  • \$\begingroup\$ The input is worst-case scenario. 1.5A may be inrush current or peak draw for the 2 milliseconds it takes for the output side to catch fire and shut down. \$\endgroup\$ – Bryan Boettcher Feb 25 '13 at 19:03

I do power supply design for a living. When I specify that this circuit is rated for 19.5v/3.34A 65W, there could be so many reasons for this. Few arbitrary examples include:

-Transformer saturates at 70 VoltAmps (VA)
-The transistors I am using start breaking down after 21V and/or 4 Amps
-My linear voltage regulators start sinking too much heat due to high voltage differentials
-Capacitors start having huge ripple currents that they cannot handle (ESR losses)
-I am unable to meet compliance in powerfactor/emissions at higher power ratings

any many more possible things...

EDIT: As for the 1.5Amps input current, this is the maximum instantaneous current the adapter will pull from the input. It is NOT an RMS value.

  • \$\begingroup\$ So why is the input power rating so much higher? This doesn't answer that. \$\endgroup\$ – Cybergibbons Feb 25 '13 at 21:50
  • \$\begingroup\$ @Cybergibbons Answered above \$\endgroup\$ – hassan789 Feb 26 '13 at 15:44

The adapter clearly states that it supports only 65W. There could be many reasons for this, but in the end attempting to draw more would either fail immediately or prematurely.

The internal design of the adapter isn't made for more than 65W and shouldn't be used above this. Likely components cannot sustain the extra current required, and the input spec could be generous to be conservative for some losses (not 65W though).

  • \$\begingroup\$ "There could be many reasons for this" - Thats my exact question. Is it just to be conservative? Are there other components that limit this? I'm guessing mostly the first one. I'm not planning on using it for anything, it was more of a theory question. \$\endgroup\$ – 7200rpm Feb 25 '13 at 19:01
  • \$\begingroup\$ @7200rpm The adapter IS made from components. As one chain is strong as it's weakest link, the adapter is powerful as it is weakest component. If the specs are correct, then 65W is maximum power that guarantee correct work of the device without failing. If you want to know what component is in this case, then probably you would have to open it and examine the specs for the internal components, including heat dissipation as it is can be limiting factor too. \$\endgroup\$ – zzz Feb 25 '13 at 19:20
  • \$\begingroup\$ @zzz Very true - So there is probably a component in the adapter that is a weaker link than simply input vs output power. And I probably shouldn't be opeining up my wife's power adapter :) \$\endgroup\$ – 7200rpm Feb 25 '13 at 20:00

The label rating of the power supply will generally reflect the conditions under which it underwent its safety characterizations.

These not only include abnormal tests (shorting a transformer, for instance) but steady-state thermal tests at whatever condition generates the worst-case heating for the part.

It also influences the temperature rating (or "class") of the magnetic components within the power supply. Higher load can require higher temperature rating of the magnetics, which require higher-rated components (tape, magnet wire, bobbin, etc.)

Operating the adapter beyond its label rating is a 'misapplication' in the eyes of the regulatory people - i.e. don't do it.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.