Sunday, 2 December 2018

Is your laptop running slow ever since you bought a replacement AC power adapter for it?

Did you know that some AC power adapter connectors for laptops have a 'smart' pin?
If this pin does not carry the correct voltage or signal, your notebook will run very slowly.



A tell-tale symptom is that your notebook is fine when running from the battery, but it is really slow when running from the AC adapter. Another indication is that the CPU usage in Task Manager goes to 100% as soon as you connect the AC adapter.

Sometimes the original AC adapter or its wiring or its connector is broken or faulty, sometimes it is because the AC adapter is incompatible (even though it is supplying the correct voltage and current). This may be because someone has accidentally picked up the wrong adapter by mistake or because a non-compatible replacement adapter has been supplied.

Many HP AC adapters have this extra 'smart' pin. Typically this provides a voltage to the notebook which is a kind of 'sense' pin. If the adapter cannot deliver enough power, the voltage will dip and the CPU will go slow (typically it disables level 2 cache on the CPU to reduce power consumption).

On some Dell adapters, they use this extra wire as a serial communication line to read some data from the adapter to ensure it is of a suitable type. See here for details. To troubleshoot Dell power issues - see here.

Dell Power connections - the DS2501 has two pins and a 'one-wire' interface



Some replacement adapters come with a 'dongle' - a different dongle can be used to match different notebooks so that the adapter manufacturer can supply a 'generic' adapter with a specific dongle for a specific model of notebook. So you may need to check that the 'dongle' is the correct one and if the adapter should have one supplied with it for that specific model of notebook.

Another symptom of using the wrong AC adapter (typical for adapters with 2-pin connectors) is that the notebook runs OK on AC power, but it does not charge the battery. This is usually because the output DC voltage of the adapter is too low (or it cannot supply sufficient current).  For instance when you use a 19V AC adapter with a notebook that requires a 21V adapter.

The specified voltages on the two labels should always match exactly, whereas the current (Amps) rating printed on the AC adapter should match or exceed that printed on the notebook label. If the adapter voltage is too high by a Volt or two, it may cause overheating of the notebook - if it is much higher than just a few extra Volts, it may cause damage!

Usually the  required adapter rating is on a label on the underside of the notebook so the first thing you should do whenever you get your hands on a notebook, is to check that the notebook power label matches the label on the AC adapter.