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Re-wiring a 3v hand-fan to run off a USB cable, need help reducing voltage.

Qwidgybo

Aug 10, 2015
3
Joined
Aug 10, 2015
Messages
3
Hi All,

I have bought a 3v (2 X 1.5 v AA batteries) hand fan and also bought a USB phone charger, cut off the phone charger bit, and am trying to get the USB cable to power the hand fan without burning out the motor. Initially I just hooked the red/black cables to the corresponding points on the fan - I call that "Fan No.1" now. Fan No.2, I got a potentiometer from my local Maplins (electronics shop in the UK) and when I wired it up it worked, but had a very, very short "adjustment" phase on turning the knob, that, and it started to smoke and glow red. I did tell the guy at Maplins what it was for so I don't know why he gave me the pot that wasn't the correct voltage or whatever. Got a refund on the pot and bought 3 resistors, a 100ohm, 150ohm, and 300ohm (based on some googling of people attempting similar projects). I think those are the levels, I asked for them but the 100 one looks like it might actually be a different ohm level the colors on it are brown-brown-black-black and what looks like either brown or a darker shade of brown. Which confused me, but anyway. I tried hooking fan no.2 up with the resistors in between the positive and negative wires but the fan wouldn't turn at all, tried jiggling the connections etc no joy. It did work when I briefly touched the pos/neg usb cables directly to the motors points so I know the fans motor is operative, however I only ran it for a second so as not to move onto fan no.3.

This is just a fun project for me, I know I can buy a USB fan for £3 or whatever, I just like tinkering - any ideas on what level of ohms would work if I was to use a resistor or is there some easier way to reduce the voltage from 5 to 3 - keeping in mind I don't own a soldering iron, don't really want to buy a breadboard or make a proper circuit, although if it is really simple/cheap I'd be ok with that... it's just a very simple project and I want to keep it cheap-ish.

I have also done a search of your forums for "reduce 5v to 3v" but couldn't find anything that would help.

Any ideas?


Qwidgybo
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
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Jan 21, 2010
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25,510
A 3V regulator would be the best option.

There are plenty of small buck (voltage reducing) switchmode regulators available on ebay. These can reduce the voltage with good efficiency and cost about a dollar.

Do you know how much current these fans draw?
 

Alec_t

Jul 7, 2015
3,587
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Jul 7, 2015
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3,587
Without knowing how much current the fan draws (a) when running at normal speed and (b) when stalled it is difficult to specify a suitable resistor value.
Using a series resistor (fixed or variable) is not a good way of controlling motor speed. The problem is that (counter-intuitively) on start-up a DC motor of the type used in cheap fans draws much more current than when at full speed. So putting a resistor in series limits the start-up current, perhaps (as you've found) to a point where there's not enough torque to overcome the friction/load and so get the motor rotating.
 

Gryd3

Jun 25, 2014
4,098
Joined
Jun 25, 2014
Messages
4,098
Hi All,

I have bought a 3v (2 X 1.5 v AA batteries) hand fan and also bought a USB phone charger, cut off the phone charger bit, and am trying to get the USB cable to power the hand fan without burning out the motor. Initially I just hooked the red/black cables to the corresponding points on the fan - I call that "Fan No.1" now. Fan No.2, I got a potentiometer from my local Maplins (electronics shop in the UK) and when I wired it up it worked, but had a very, very short "adjustment" phase on turning the knob, that, and it started to smoke and glow red. I did tell the guy at Maplins what it was for so I don't know why he gave me the pot that wasn't the correct voltage or whatever. Got a refund on the pot and bought 3 resistors, a 100ohm, 150ohm, and 300ohm (based on some googling of people attempting similar projects). I think those are the levels, I asked for them but the 100 one looks like it might actually be a different ohm level the colors on it are brown-brown-black-black and what looks like either brown or a darker shade of brown. Which confused me, but anyway. I tried hooking fan no.2 up with the resistors in between the positive and negative wires but the fan wouldn't turn at all, tried jiggling the connections etc no joy. It did work when I briefly touched the pos/neg usb cables directly to the motors points so I know the fans motor is operative, however I only ran it for a second so as not to move onto fan no.3.

This is just a fun project for me, I know I can buy a USB fan for £3 or whatever, I just like tinkering - any ideas on what level of ohms would work if I was to use a resistor or is there some easier way to reduce the voltage from 5 to 3 - keeping in mind I don't own a soldering iron, don't really want to buy a breadboard or make a proper circuit, although if it is really simple/cheap I'd be ok with that... it's just a very simple project and I want to keep it cheap-ish.

I have also done a search of your forums for "reduce 5v to 3v" but couldn't find anything that would help.

Any ideas?


Qwidgybo
Perhaps you didn't explain in enough detail what you were doing, or the maplins guy had a brain-fart.
As mentioned above, without knowing the motor's current draw, any resistor values will be a shot in the dark... you see, the amount of voltage a resistor drops is directly proportional to the current going through it. So we need to know how much current the fan takes.
You can experiment and find a resistor value that will work, but be warned that most resistors are pretty tiny and can only dissipate an 1/8 Watt.

Voltage = Resistance * Current
Power = Voltage * Current
Therefore : Power = Resistance * Current^2

Using the above formulas, you end up with:
Current = SquareRoot( 0.125 / Resistance )
So a 100Ω resistor can only handle 35mA and will drop 5V down to 1.5V. Will only be a problem if the fan's resistance is too low.
A 150Ω resistor can only handle 29mA *Requires almost all 5V across to reach limit. Mostly safe.
A 300Ω resistor can only handle 20mA *Cannot get 20mA from a 5V source. Safe!

This is also why you burnt out the potentiometer. They are not meant to handle any current. At all. So, as you adjusted the potentiometer, the power it was dissipating grew rapidly and it burnt up.

So. If I were you, I'd try the 150Ω resistor first... then perhaps I'd try the 100Ω, but would keep my eye on it.
Additionally, Diodes have a unique property in addition to only letting voltage go one way... They drop 0.7V almost regardless of current.
If you put 2 diodes in-line with your fan, ignore the resistors and you should be able to get 3.6V to the fan. Now... that a 20% increase in voltage to the fan, so it's not perfect, but it may be good enough. Which route you take it up to you.
 

Gryd3

Jun 25, 2014
4,098
Joined
Jun 25, 2014
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4,098
.... or use 3 diodes to drop about 2.1V.
I don't know why I only said 2.. for some reason I had it stuck in my head that 3 would be too much ;)
Perfectly valid! (and cheap)
 

KJ6EAD

Aug 13, 2011
1,114
Joined
Aug 13, 2011
Messages
1,114
Put a switch across the third diode labeled "TURBO MODE". :cool: When the switch is closed, you'll get ≈3.8V which the motor can probably handle, at least for short periods. You'll be able to tell if it can't by it heating up or emitting a burning insulation odor.
 

Qwidgybo

Aug 10, 2015
3
Joined
Aug 10, 2015
Messages
3
I did a bit of research and decided on the silicon diode solution, I put 3 in series and it seems to work although the diodes are getting quite warm/hot when it's running... I assume they will eventually burn out? Is it dangerous?

Regarding the resistors I tried all 3 but the fan would not turn at all, even slowly, it was just dead.

I'm considering putting a pot in with the diodes so I can dial it down, however ideally I'd like to remove one of the diodes as the fan seems a bit slower and provides less air than when it was running on batteries. However I have achieved my goal, I do not need to buy batteries for my fan anymore! Hooray! However it is going into winter anyway so it was more just to have a bit of a fiddle around.

Thank you for all the responses, this has been a great re-learning experience (I say re-learning because I used to do electronics as a kid).
 
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