What I have: 12V fan, 9V 300mA AC-DC adaptor (or just 9V 300mA DC supply) and soon I will have voltage booster.
My questions:
By boosting 9V to 12V, does the power (in watts) remain the same, and if so, does the current decrease proportionally to the increase of voltage?
Thanks in advance.
Yes, power stays the same.
If secondary voltage goes up, current on the primary side goes up proportionally.
If the 12volt fan is using 200mA, then it will draw 12/9 times the current from the 9volt supply (267mA).
Plus the losses in the DC/DC converter, which can add another 20%.
Leo..
So at 12V (ignoring the losses at the booster) the current will be 267 mA? I dont think so- as we raise the voltage to 12V from 9V then the current will be 9/12 times less than 300mA = 225 mA (if my calculations are correct).
The seller from which I bought the booster says that the device's efficiency is 97%. So 225 * 0.97 = 218.25 = 218 mA. Isn't that way too much for the DC fan to handle?
Dude, what's the current @ 9V?
outsider:
Dude, what's the current @ 9V?
From what I measured using my multimeter the current was 0.29-0.30A (290-300mA).
97% efficiency boost circuit? They do exist but I doubt you have one. Assume 85%.
If the fan draws 200mA at 12V, it draws 200mA from any 12V supply, assuming the supply can
provide the current.
You are imagining that somehow a voltage supply controls both voltage and current into the load.
A voltage source/supply sets the voltage and the load then takes the current according to its
response curve.
12V at 200mA = 2.4W, with an 85% efficient converter thats about 2.8W into the converter, which
is 310mA at 9V
A 300mA supply is probably not going to hold up at that load (unless you really do have an ultra-high
efficiency converter).
A word about ratings for power supplies - choose conservatively rated supplies. If you use 300mA,
get a 0.5A amp supply, not a 0.35A supply. Everything runs better when not at the limit (cooler,
better voltage regulation, less noise, etc etc).
If you buy a cheap unbranded supply from eBay, you can expect it not to meet its ratings anyway,
and possibly not safe either, so caveat emptor.
Haha if you tell you that I found the 9V wall adapter in a box with old computer parts at the ceiling 
But now seriously, the boost converter is indeed from ebay for $1 and it is Buck-Boost converter 3V-35V output, 5V-32V input. I am planning to step-up the voltage from the wall adaptor to 12V and HOPEFULLY get 97% efficiency from it. Still waiting for it to be delivered...
And thanks anyway.
You'll need at least 500 mA at 12V, so your 9V adapter will need to supply at least 700 mA.
Wow I thought DC computer fans run on max 200mA... do you really think that atleast 500mA are necessary?
JMD1:
Haha if you tell you that I found the 9V wall adapter in a box with old computer parts at the ceiling
But now seriously, the boost converter is indeed from ebay for $1 and it is Buck-Boost converter 3V-35V output, 5V-32V input. I am planning to step-up the voltage from the wall adaptor to 12V and HOPEFULLY get 97% efficiency from it. Still waiting for it to be delivered...
And thanks anyway.
You won't get 97% from a $1 cheapo device, trust me! (If I'm wrong let me know, I'll be buying
them...)
Here's one way to get very efficient buck/boost conversion, it uses 4 decent MOSFETs to manage it,
cheap converters use schottky diodes to reduce part count and won't be as efficient.
Okay, anyway, I am not professional electrical engineer so I definitely won't invest $30 in a powerful converter, I just need to step up a few volts above and below the input (talking about powering a bloody DC 12V fan, not a huge lighting on rock festival), and definitely no need of having some ultra-hyper-exact value frequencies and many ports and control pins and so on... even if the efficiency is not 97%, okay. I would be happy even with 85%.
Thanks to all people who commented here.
JMD1:
So at 12V (ignoring the losses at the booster) the current will be 267 mA? I dont think so- as we raise the voltage to 12V from 9V then the current will be 9/12 times less than 300mA = 225 mA (if my calculations are correct).
The seller from which I bought the booster says that the device's efficiency is 97%. So 225 * 0.97 = 218.25 = 218 mA. Isn't that way too much for the DC fan to handle?
Hi!
I've been reading the thread and...
Is it mandatory to run the fan at 12v? If so, you are going to need more than 300mA out of the 9v source.
BUT, if you can live with the fan running at 9v, which most certainly will work (of course it will move less air). But that way you will be running the fan at 9v@150mA.
PD: You could add the boost converter, and run the fan at 10 or 11v, you can measure the current to get the exact voltage, so in that way you will get the maximum air flow with that power supply.
Hope that helps.
Thank you, now I see that it says 12V 160mA. What I did not mention was the reason for this:
I want to make a very simple project with fan that blows all those nasty fumes from soldering away from my face so I dont have to breathe them. I want as much air flow as possible, but at 12V 160mA I am sure that it would be more than enough.