Please help me understand power adapter wattage math...

If a notebook has the same nvidia 45w gpu and the same hardware components, why would the manufacturer ship the 35w i5 with a 90w brick and the 45w i7 with a 130w? that's an extra 30w over the cpu 10w difference .

Is there something wrong with my math? what is the extra 30w needed for?

This question is about ivy bridge notebooks

 

The problem with wattage is it can measure 2 different things: heat and electrical current. Your wattage numbers for the processors seem like they're based on TDP, which means watts of heat, not electric current. So it's entirely possible (even likely) that the electrical current required by a quad-core i7 processor could, at maximum utilization, be proportionately the same as the difference between the 130 watt and 90 watt adapters.

 

Also, power supplies for notebooks seem to come in ratings, like 60W, then 90W, 130W, 150W, 180W, 210W, etc. It seems that they jump at least 20 or 30W as they go higher. So not only is the i7 going to need a bit more power, but perhaps the manufacturer has no choice but to go from the 90W to the 130W model based on the way power supplies are manufactured, even if that much is not needed. Most of these power supplies are not made by the actual laptop maker, but by 3rd parties.

 

For the puposes of notebook design the electrical watts and the heat watts will be the same.

Notebook designers usually allow for a worst case of full CPU load + full GPU load + display (the backlighting uses several watts) + other internal components + some devices attached to USB + charging the battery. And then round up the total to the next standard size.

I suspect that in your case the higher wattage CPU pushed the design total power requirement over the 90W capacity and the next standard CPU size is 130W.

If you have a plug-in mains power meter then you can monitor how much power the computer actually uses and you may discover that you can safely use a smaller PSU.

John

 

I think we also need to keep in mind the efficiency of a power adapter as well. If a power adapter is supplying 130W of power to a device, the actual mains draw will be higher because the difference will be lost as heat. So when using a mains meter we have to keep this in mind. If the power draw at the wall is 150W, that does not mean that 150W of power is being used by your laptop. That would only be the case if a power supply was 100% efficient, which none are. Correct me if I'm wrong here.

 

The point about PSU efficiency is valid. Finding data about the likely efficiency isn't easy and my best guess is that recent PSUs are around 90%. One can get a rough indication of the efficiency of the PSU by feeling how hot it gets and I think the heat dissipation is one of the considerations when designing a PSU. Most tend to be black, which radiates heat better, and a bigger surface area helps. [The new Zolt PSU is probably more efficient because it has a relatively small body for dissipating the heat.]

Therefore it's likely that the power ceiling of a 90W PSU will be 95 to 100W at the mains socket. However, you will probably discover that the actual power drain for the hardware being discussed here is below 90W.

John

 

Some, if not most by now, power adapters list efficiency labels. I have two Dell adapters . One is a Lite-On 90W with a level IV efficiency label, and the other is a Delta 150W with a level V efficiency level. According to this, they are both at least 85% efficient:

http://www.cui.com/efficiency-standards

 

CPU can take far more power wattage than its rated TDP. Read more about Short term and Long term Power limits

James D.

 

Thanks for that link. I hadn't seen it previously. I confirms that my guess of 90% was in the right range.

John

 

New 150W AC must output exactly 150W or above. All those efficiency math is calculated about how much power AC takes from power socket, not how much it delivers.

 

All you need to know is... 1.21 Gigawatts.

 

Thanks for the clarification, I'll test it using a wall monitor. I upgraded my Dell inspiron 7720 from an i5 3230m (90w) adapter to a 3940xm (fastest ivy bridge mobile apparently ) which is 55tdp but the most ever shipped were 45tdp with 130w adapters. It has a nvidia 650m which uses 45 watts max apparently but I hardly use it, most of my apps run on the dedicated.

So far things seem fine. Cpu runs usually been 40 to 60c, never went above 80 yet. And notebook is lying close to the desk so right now unoptimal airflow. Fan comes on hard occasionally but otherwise it's quiet.

When I still had the i5 in, in the computer store we tested the 150w and the notebook wouldn't power on, which is strange because online there are many dell compatible with my model number for sale... with the same watts volts amps as this brick that wouldn't work. The store brick just said dell but didn't list compatible models on the box. The salesman said some notebooks are wired not to accept more than a certain amperage for safety, so that could be why. Still I wonder if maybe it would work with my 3940xm inside now. .or if maybe an online adapter that lists my model number in the description, would work.

It was absolute hell to take the think apart and put it back together. The cpu is so inaccessible.

And now I really wonder if I made a mistake.

I left the white rectangular sticker that came on the cpu on it. Should I have removed it? If I leave it on, is it OK? I don't relish the idea of taking the notebook apart again. It was on the green area of the cpu.

 

Sticker on green pcb? Forget it.
Theory about too high A is false. Just AC wasn't compatible for some reason.

 

Higher amperage ratings on a power supply will not harm your laptop. Higher voltage may, but not higher amperage. Your laptop will just draw what it needs and the power supply will supply it. If the laptop will draw a maximum of 90W and the power supply is rated at 150W, the power supply will still only provide up to the 90W that the laptop needs, which means it has plenty more to spare. It's generally better to have a power supply rated higher than what components need, but you don't want to go lower.

I'll give you an example. The original Xbox 360 came with a power supply rated at 16.5 Amps @ 12V DC (203 W). As Microsoft started making revisions to the Xbox 360 hardware, the power demands went lower and lower. The last and final model of the Xbox 360 has a power supply rated at 9.8 Amps @ 12V DC ( 120W). This means that if you have the latest model that draws 120W you can safely use an original 203W power adapter, but at the same time you can't use a 120W power supply from the latest model on the original Xbox 360 that requires 203W because it is not enough power. Hope this made sense

As far as the 150W adapter not working. It probably has to do with the way Dell makes their power adapters. You see that little center pin on the power adapter plug? Well that pin actually sends info to the laptop as to what kind of power adapter it is. If the laptop's BIOS doesn't like the info it's getting, then it will either reject it, give you a warning, or not charge your battery. Perhaps all 3. Sometimes you can turn off adapter warnings in the BIOS.

So the salesman was partly right in his assessment. Take a look at this if you are interested:

http://www.laptop-junction.com/toast/content/inside-dell-ac-power-adapter-mystery-revealed

 

So, at least the notebook on the 150w should be turning on so that I would see some error or warning?

He was pointing out that there was no connection light on the notebook when we first tried the 150w on the stock i5 configuration but then with the 130w therr was. So I had a real disheartening moment when I had completed the i7 XM upgrade and the 130w I purchased was plugged in but no lights were on on the front panel of the notebook. But! It still booted... hmm, now I wonder if that is indicating that charging is not working correctly? Just checked, battery is still at 100% after in sleep overnight. So I guess charging works. Maybe the salesman just confused me about these lights... generally the notebook is working very well. And blazing fast.. I had a chrome session saved with 40 windows and several tabs in each window and I restored it all in 12 seconds, I mean all pages loaded in 12 seconds. On my i5 that would take many minutes, maybe even temporarily hang chrome.but my understanding is that this is probably the last laptop I'll ever buy that I can upgrade everything... cpu upgrades are getting to be a thing of the past.

 

This could indicate some other issue, most likely a damaged ribbon cable from the power board to the motherboard. Once those ribbon cables get damaged, weird things like this can occur. What model laptop is this anyway? Can you inspect that ribbon cable? I had the same situation with my M15X. The ribbon cable had a fine cut on it from bending and it sometimes would work, and other times it would power on the laptop with no lights on the power board, and other times it would not work at all until I replaced the cable.

And are you sure the 150W adapter the salesman tried was indeed a genuine Dell adapter. I know some HP adapters have similar plugs to what Dell uses, but they are wired differently.

 

Oh no definitely not genuine but a big dell on box, definitly meant for dell. . Generic brand they had in various wattages for each computer manufacturer ... Unfortunately this computer store was quite successful in recent years at the expense of others meaning basically there is it, best buy and online. So I can't really find another store with a different 150w brand adapter to go in and check.

Inspiron 17R 7720 SE, ribbon looked fine when I was doing the upgrade.

I'm starting to think 130w will be fine, I hardly ever use gpu and cpu spikes to 100%, it doesn't stay there long. Wattage as reported by core temp software also spikes at that time. So I'm rarely at 40w on the cpu.

 

Invest in one of these and you will be more confident about how much power your computer actually takes plus the PSU losses so the 130W PSU might take 150W at the outlet when supplying 130W to the computer.

I also doubt that you will get above 130W even with CPU and GPU fully loaded and recharging the battery but the meter will give more confidence.

John