So recently, a family member of my brought me an old macbook from 2007. It's input is 16.5 volts and maximum current 3.65A.(Though, the current changes all the time, it can be 1A then 2 or 0.5 etc.) I can't afford to get an original charger, since I dont even know id this laptop works! So I would just waste my money. I do have though a 19.5V and 4.7A old sony Vaio laptop Charger/PSU. So, what can I do to -cheaply- get it down to 16.5? Could I use like a resistor in series or something? Thank you.

 

Most of the notebook chargers use ID for identification.
Magsafe should use a 64bit identification code via 1-wire protocol, so connecting just right voltage to power pins is probably not enough. Try study site below.

Example on my old Dell laptop, i can use any of chargers for just running the notebook, but not for charging.

See site:

http://www.righto.com/2013/06/teardown-and-exploration-of-magsafe.html?m=1

 

Most of the notebook chargers use ID for identification.
Magsafe should use a 64bit identification code via 1-wire protocol, so connecting just right voltage to power pins is probably not enough. Try study site below.

Example on my old Dell laptop, i can use any of chargers for just running the notebook, but not for charging.

See site:

http://www.righto.com/2013/06/teardown-and-exploration-of-magsafe.html?m=1

The laptop works perfectly fine with the correct voltage input to it. It is a pretty old laptop and I have tried it with a 16Volt 500mA adapter. It did turn on and was usable but the adapter got REALLY hot and then the laptop shut off bc of the not enough current

 

OK, the very least efficient but simple way to provide a dropped voltage is with series diodes able to handle the power.
BUT now I see in post #3 that you have verified that the computer works. One option is to get a switch-mode supply with an adjustable output and an adequate current rating. A 15 volt supply should be adjustable up to 16 volts and that should work.

 

Welcome to AAC!

So, what can I do to -cheaply- get it down to 16.5? Could I use like a resistor in series or something?

Use an LM338 to drop the voltage.

If you want something cheaper, you could put 3-4 rectifier diodes in series, a power zener diode is also an option.

 

Note that any series resistive method (resistor, diodes, linear regulator) to drop the voltage 3V will dissipate up to 11W, so that must be considered.

To avoid that high dissipation, you could use an inexpensive, step-down switching buck-converter module (as suggested by MB2) such as this, which will likely dissipate no more than a watt (mostly from the small heat-sink on top of the IC).
This will also reduce the current out of the supply by at least 10% as compared to the computer current so it's a win-win.

 

You can also purchase a supply rated with at least that current that can be adjusted to 16.5 volts. I did that for a desktop computer three years ago It cost about $25 (USD) and it had an aluminum housing with a terminal strip on one end.

 

oltage regulator have something called voltage dropout and it is a minimum voltage difference between input and output. for most regulators this is at least 2.5V. and you need to watch for max current.

if regulator is linear that means high heat loses. same goes for resistor and diodes. 3V * 4.5A = 13.5W. one of my soldering irons is 8W. think about that.

if regulator is switching, it may not handle the current (only go to 3A for example) or voltage drop requirements. many of the modules are built around LM2596 which is only good for up to 3A, but the Amazon sellers will happily glue dime sized heatsink on it and try marketing it as a 5A version just to make a sale. buyers beware....

opening PSU to adjust feedback can be done but it need to be done carefully and will most likely not look nice.

also voltage drop using resistor is not the great because drop depends on current and in this case current fluctuates.

the laptop has internal regulator but powering it from higher voltage PSU is causes stress and limits immunity to transients.

just get cheap psu with correct voltage. anything else is going to be a pain and bulky- who likes having large laptop PSU or one with extra "tumor"? not me. there is an abundance of PSUs on the market and many are really low cost.

 

oltage regulator have something called voltage dropout and it is a minimum voltage difference between input and output. for most regulators this is at least 2.5V. and you need to watch for max current.

if regulator is linear that means high heat loses. same goes for resistor and diodes. 3V * 4.5A = 13.5W. one of my soldering irons is 8W. think about that.

if regulator is switching, it may not handle the current (only go to 3A for example) or voltage drop requirements. many of the modules are built around LM2596 which is only good for up to 3A, but the Amazon sellers will happily glue dime sized heatsink on it and try marketing it as a 5A version just to make a sale. buyers beware....

opening PSU to adjust feedback can be done but it need to be done carefully and will most likely not look nice.

also voltage drop using resistor is not the great because drop depends on current and in this case current fluctuates.

the laptop has internal regulator but powering it from higher voltage PSU is causes stress and limits immunity to transients.

just get cheap psu with correct voltage. anything else is going to be a pain and bulky- who likes having large laptop PSU or one with extra "tumor"? not me. there is an abundance of PSUs on the market and many are really low cost.

The last suggestion is a lot like I have already suggested twice. Of course, it might work to just change the reference voltage in the existing power supply. Maybe. I find commercially produced switcher supplies difficult to follow, and never with available circuit schematics.