Laptop Adapter current request [closed]

Question

My Lenovo SL500 AC Adapter has broken and left me without a way to charge my PC. It's Output Specs were the following:

• 20 V
• 4.5A (4.5A)
• 90 W

I had another Lenovo AC Adapter for a different PC of mine's, though having the following Output Specs:

• 20 V
• 3.25A (3.25A)
• 65 W

It is clear that the maximum current it can provide is 3.25 A, while the previous one could provide up to 4.5 A.

I would like to know two things:

1. how can I measure current request from my laptop?In order to assess if the working adapter will be able to provide it.

2. are there any counter indications on using this adapter instead of the original one?Can any damage occur to the PC or to the adapter by having a under amperage?

Thanks for any help provided

Answer 1

Part 1. As @retro says in is reply, you can use a multimeter in series with the load. To do this, you need to interrupt the circuit to the laptop. You could do this either by chopping open the existing adapter cable and breaking one of the two wires, or by making an adapter. Then the multimeter is inserted into the break and used to measure the current. The laptop then needs to be connected and in use.

I see two issues with this though. Firstly, a multimeter is only good at measuring a constant current draw. Laptops nowadays can be extremely dynamic with their load, with short pulses of higher current draw that a multimeter may miss.

Secondly, you need to ensure maximum load - screen at full brightness, processor at 100% with no clock scaling, HD in use, optical drive in use, fans at full speed, battery charging, screen at full brightness, graphics card being pushed etc. It would be easy to miss a corner case and find out that the laptop works 95% of the time.

Part 2. I agree with @retro for most laptops - using an underspecced power supply can cause the power supply to overheat. That said, Lenovo seem to be a little bit more intelligent with how they treat this. I have three Thinkpads and they have been supplied with 90W and 65W power supplies. All three work with both power supplies and are shown in instruction manuals to work with both. The price you pay on two of them is that the battery will not charge as quickly when using the smaller power supply and loading the system. The benefit is that the adapter is much, much smaller. All three detect the connected power supply rated power, and two of them warn when the 65W is attached.

The SL500 seems to be able to work with the 65W adapter:

http://forums.lenovo.com/t5/SL-Series-ThinkPad-Laptops/ThinkPad-SL500-AC-Adapter/td-p/315184

http://shop.lenovo.com/ae/en/itemdetails/40Y7696/38/4364BD0F20B94413B5AD6E684D2848E9

A colleague's Thinkpad only requires the 65W adapter but was provided with the 90W - I believe that it may be cheaper to produce as it much larger. It certainly gets much less warm, so may be more reliable.

I do not know how the Lenovo power supply rated power is detected by the laptop - there only appear to be two contacts. Some Dell power supplies use a third contact and Dallas Semi serial ID chip to identify themselves.

Overall, I strongly suspect that it will be ok to use this adapter.

Answer 2

1. You can measure current with a multimeter in series (i.e. you have to break the circuit somewhere and put the multimeter in the circuit). However, how are you going to measure the current draw without the correct rated PSU? If it requires somewhere between 3.25A and 4.5A, you will not get a true representation of its requirement.

2. Yes, you can damage the charger and quite possibly the laptop. I am a laptop engineer and have seen a lot of damage due to incorrect PSU usage. The PSU can overheat, which is a fire risk. This can in turn melt the socket on the laptop, or cause further damage.

Always use a PSU that is rated correctly. If your laptop came with a 90W PSU, don't use a 65W PSU. You'll find replacement PSUs on eBay are very cheap (try searching brand model PSU, e.g. "Acer 5200 PSU", or you can get a universal laptop PSU from many computer retailers - especially the ones that sell spare parts.