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Simple Voltage questions

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RazorM

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Standard electrical computer tower fan. 12V, 0.23 Amps.

I plugged this to a USB plug that has an output of 5V & supposedly 0.5 Amps (the USB reported standard).

The fan worked, but what are the reprocussions after long use due to the mis-matched Voltage & current?

Plz excuse my lack of electronics knowledge...but is there a simple - DIY - way to hook up few of these fans to a USB port safely & properly (by electronics standards) for long term use?? eg. solder in a resistor in the circuit or something??

Much appreciated.....
 
Russlk said:
There should be no problem connecting a 12 volt fan to a 5 volt source. The fan will run slower than it would on 12 volts, but it will not draw more current.
True , but how much current can the usb supply..?
 
Re: Current

RazorM said:
My original concern was that the over supplied current would blow the fan eventually.

The fan will only take what current it needs, the supply it's normally fed from in the PC will be able to supply many amps!. As you're running the fan slow due to the low voltage, the current will be lower as well.
 
Question

Now, on a similar note, I have a USB wire that was given to me w/ some device. I was told that i can use it to charge me cell phone.

I checked my cell phone charger. It's got an output of 3.7V ... 355mA.

I've always wondered if the USB being 5V, 500mA output would damage my Nokia cell in anyway if I plugged it directly to the USB port. ??

I always thought not coz I was always under the impression that w/ electrical devices, esp. stuff w/ chips & sensitive components - eg. alot of the computer devices these days, mp3 players, flash media devices, etc - that over voltage supplied to the device will damage it.

Is it always true that devices will only 'take' the current it needs regardless of over supply? What happens when not enough current is supplied? ...or does it depend on the type of device?

I'm thinking now w/ the fan, coz it's mostly mechanical & magnetic, that the difference in voltage doesn't matter so much whereas more delecate elec. equipment will be more sensitive to these differences....or have i over simplified this all??

Electronically curious....
 
there is probably some kind of fail-safe device on the phone so that if too much or too little current/voltage is supplied, the phone simply won't charge, but dont take my word for it, let someone else confirm that b4 u do it :lol:
 
Re: Question

RazorM said:
Now, on a similar note, I have a USB wire that was given to me w/ some device. I was told that i can use it to charge me cell phone.

I checked my cell phone charger. It's got an output of 3.7V ... 355mA.

I've always wondered if the USB being 5V, 500mA output would damage my Nokia cell in anyway if I plugged it directly to the USB port. ??

I always thought not coz I was always under the impression that w/ electrical devices, esp. stuff w/ chips & sensitive components - eg. alot of the computer devices these days, mp3 players, flash media devices, etc - that over voltage supplied to the device will damage it.

Is it always true that devices will only 'take' the current it needs regardless of over supply? What happens when not enough current is supplied? ...or does it depend on the type of device?

I'm thinking now w/ the fan, coz it's mostly mechanical & magnetic, that the difference in voltage doesn't matter so much whereas more delecate elec. equipment will be more sensitive to these differences....or have i over simplified this all??

Electronically curious....


At 3.7 volts, the phone will only take as much current as it needs... via Ohms law. V=IR, or I=V/R. Becuase of this, we can solve for the resistance the fan normally has at 12volts and .23amps by manipulating Ohms law, so R=V/I = 12volts/.23Amps = 52ohms.

Now, the volts applied to a circuit will ALWAYS be how much voltage "drops" over the circuit. So if 10 volts is applied, there's going to be 10 volts over that circuit... however, a circuit will only TAKE as much current as it needs.

What this means is, at 5 volts, the fan is going to have a 5 volt "drop" over it, but it won't take .23amps... it will draw only what it can. Since we discovered earlier that R=52ohms, and now V=5Volts, using ohms law again we can solve for the current the fan is eating up. I=V/R=5volts/52Ohms=.096amps. So at a little then half the volts originally supplied to the fan, it's taking a little then half of the amps it originally consumed... makes since becuase it's all a ratio.

---------------

On to the cell phone question, your USB port has the power needed to charge your cell phone and very well can, but i'd be weary of doing it... becuase we've looked at Ohms law a few times now, you can see that at a higher voltage, the phone is going to draw more current then what is specified for charging... i doubt it would damage the phone in anyway becuase i'm sure it's built to handle a little bit extra, but it could reduce the life of your battery by charging it rapidly.

On the other hand, the extra current going into your phone could very well damage it, although this probably isn't a very likely situation.

Do you have an extra USB cable and extra cell phone charger or something that you are going to try to splice togethor or what? I'm confused on where this USB cell phone charger is going to come from... if you splice the two togethor you could easily calculate and stick in the right value of resistor to drop the voltage from 5volts down to 3.7 volts, or even better use a voltage regulator that will drop the 5 volts down to 3.7.
 
USB cable

Plot,

The wire actually has a uSB port on one end & on the other, the same plug for my Nokia cell phone. I don't think there's any extra circuit components in it as there's no extra bulges in it & the 2 plugs look very standard.

It came w/ my USB 2.5" external hard case for a laptop harddrive, but the sales guy said that i could charge my cell phone with it too....This was in Singapore.

I have thought about adding resistors & the likes of DIY...I'm just not well versed enough to try it yet...thus all the questions....
 
Just to add to this I am sure USB initally limits current to 100mA, and only supplies more current when requested or initiated by a specific signal? I may be wrong though....
My phone came with a USB cable, in fact the connector on the phone is a mini USB A itself :D Also I know if the sync manager is inoperative on my computer my phone won't charge.
If you are unsure google for your phone and usb charger and see what it comes up with?
 
Cell charger

I took a look at my faster cell charger. It's rated at 5.3V, 500mA.....which is alot closer to what the USB port is supplying.

From the formulaes above, then I guess it should work, it just won't charge as quickly using my USB port as using this fast charger. But it would charge faster than using the slow charger above. Either way though, it seems to me that with the range of the 2 phone chargers, that since the USB port is somewhere inbetween, charging by USB port should not be a problem.
 
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