USB Fan Project - Need Help Bringing Voltage Down from 5volts to 3volts...

[techhappy]

Howdy folks,

I recently put together a fun project to make a usb fan out of a dollar fan from a local store. I followed this tutorial on this site:

**broken link removed**

Basically, everything worked until the 3volt max motor burned out. The author mentioned at the end of the project that he realized the motor could get overloaded since the usb port puts out 5volts and these cheap fans use 3 volts max. He said that he used a resistor to bring the voltage down to 3 volts for the fan motor.

Can anyone here help me to determine what resistor I need and where can I get one? I looked at radioshack.com and did an online search to find a resistor for the job, but it looks like you have to do a math equation to figure out what resistor to use. I am just a hobbyist, not an engineer, so if someone could help me out, that would be wonderful.

I need to go from 5volts to 3 volts, and can you suggest where to buy a resistor?

Thanks,

TechHappy

 

[eblc1388]

The resistor needs to drop two volts at a certain current, the actual value we don't know as current depends on the motor used and there are millions of motors around.

If you have a multimeter that have a current range, try measuring the motor current by placing the meter, set at current measurement range, in series with the 3V batteries, with the fan running.

You can then work out the resistance using ohm's law, value of resistor R=2V/current(Amp). Resistor wattage = 2V x current(Amp).

Edited: For example: if measured current = 0.2 A, then resistor value = 2/0.2 = 10 ohm,
wattage = 2 * 0.2 = 0.4W , choose 0.5W(commonly available type)

So you need to use a 10 ohm 0.5W resistor for your purpose.

 

[techhappy]

Thanks Chung, this is really useful, could I ask just a few more questions:

If this is the motor I'm using, a generic 1.5-3v hobbyist motor:

https://www.radioshack.com/product/index.jsp?parentPage=search&summary=summary&cp=&accessories=accessories&productId=2102822&kw=motor&techSpecs=techSpecs&currentTab=techSpecs&custRatings=custRatings&features=features&origkw=motor&support=support&tab=summary

Would the amperage be around what you said it might be?

Also, what you were recommending for the resistor, I found one that matched your specs, would this be alright:

https://www.radioshack.com/product/index.jsp?productId=2062330&cp=&pg=5&origkw=resistor&kw=resistor&parentPage=search

Also would it matter if I got a higher rated ohm resistor? Would it make it more stable and robust against burning out? Like a 100 ohm 1/2 watt resistor? I am new to all of this, so I apologize if this sounds dumb.

Thanks!

 

[techhappy]

I did my own conversions, trying to figure out the problem, I also did a little research on the 1.5 to 3v hobby motors.

If the usb electrical output to the fan is a consistent 5volts(500ma) and the fan motor I want to power is rated for 3 volts max, then I want to reduce the voltage by at least 2volts.

So if I used a 1/2 watt resistor like you said, 1/2watt(according to this conversion site:http://www.radioing.com/eengineer/rfcalc.html)
says that 1/2 is equal to 5 volts.

Wouldn't using a 1/2 watt resistor effectively cancel all current going to the device? Shouldn't I use a 1/4 watt resistor instead to drop it in half?

I need help, sorry if I'm making things complicated.

 

[eblc1388]

If this is the motor I'm using, a generic 1.5-3v hobbyist motor:

As I said, no one would be sure about the motor current without measuring with the real motor.

Your link for resistor refers to a 10K resistor, is a 10,000 ohm resistor which is way too high for any use here.

Also would it matter if I got a higher rated ohm resistor? Would it make it more stable and robust against burning out? Like a 100 ohm 1/2 watt resistor? I am new to all of this, so I apologize if this sounds dumb.

Thanks!

I reckon you can only use a resistor value upto 30% higher than the value you have calculated from the measured current, otherwise your motor would not start rotating.

Maybe other users can suggest an approximate resistor value for you to try.

Let me tell you the truth, your post is sort of the top class posts around here because it has got all the information required for me and al others to provide you with the sort of help you'll need. You have also put in a lot of effort to indicate some work done on your part. Don't ever apologize for sounding dumb as that is never the spirit of this forum and their users.

We don't like lazy ones who put in a single incomplete sentence and hoping for an answer delivered to him via email. You're most welcome here.

Added: Do you have a meter that can measure current?

 

[techhappy]

Okay, thanks. This helps alot. You were right about that link, it did say 10k, I dug around and found this, 10ohm 0.5 watt resistor:

https://www.radioshack.com/product/index.jsp?productId=2062309&cp=&pg=6&origkw=resistor&kw=resistor&parentPage=search

The 30% rating higher than calculated ohms info is useful to know in the future. Okay, so I'm going to take your advice and measure the rating of the amperage, is that right? What sort of device do I need for this job, a voltage measurement tool?

 

[eblc1388]

Wouldn't using a 1/2 watt resistor effectively cancel all current going to the device? Shouldn't I use a 1/4 watt resistor instead to drop it in half?
I need help, sorry if I'm making things complicated.

Resistor does not work that way. Their function depends on the current flow in them. They drop a lot of voltage if large current is flowing and drop nearly none if no current is flowing. So there is no magic value which says "drop 2V" if the current is not known.

I'll tell you what, why don't you buy two 10 ohm/0.5W resistors and try it on the motor. Initially uses only one resistor only.

If motor runs too fast, put another resistor in series.

If it runs too slow, put another resistor in parallel with the existing resistor. This way, you have three choices to test your motor and the cost of an extra resistor is cheap too.

 

[eblc1388]

Please try the two resistor method mentioned in my last post first. Obviously you have not got a multimeter at the moment so I won't go into the details of current measurement.

 

[techhappy]

Okay, I feel like I'm the right track. I'm going to pick up a multimeter tomorrow and try the two resistor approach you mentioned.

I'm starting to grasp, from reading other sites and the discussion here that knowing the current from the device is crucial to calculating a resistor that works properly and in a stable fashion.

Its a good thing I'm trying to do this the right way, so hopefully this device will actually last awhile and not burnout like the last one I put together, doh! Too much voltage and the motor fried! Once I sort out the current and get a resistor that works, I'll have a post a pic of what it looks like. A truly nerd project, but fun if you have spare usb cables sitting aroud, hehee.

 

[eblc1388]

Happy experimenting. It is really the fun part with electronics.

By the way, I have assumed you want to run your motor at 3V maximium voltage and gave a formula for dropping 2V. In real life you might want to run it at a lower voltage still to prolong its life and have less noise.

In that case, just use a higher value of voltage drop in the equation like 2.5V or 3V.

 

[Dr.EM]

Alternatively, 3 of these:

https://www.radioshack.com/product/...a+rectifier&kw=1a+rectifier&parentPage=search

in series would always drop around 2.1v.

 

[audioguru]

I have a little 3V fan like that from the dollar store. With two Ni-MH battery cells giving it 2.4V, it draws over 1A. When something stops its blades it draws over 2A. With 3V it might draw 3A! That would fry the USB power supply parts of my computer and the cheap little motor wouldn't last very long anyway.

 

[Dr.EM]

It looks like one of these:

**broken link removed**

they are very common. The datasheet says 0.34A no load and 1.28A "fan load" (presumably these are designed to power small fan blades). Either way, that is too much and I wouldn't risk it. I'd also be worried about the inductance creating noise or worse on the USB system?

 

[audioguru]

Hi Dr. EM,
Thanks for attaching the excellent datasheet for the cheap little motor. Its details puts RadioShack's to shame!

Did you see where it draws 3.45A when stalled? That should smoke a computer's USB power supply pretty good, and probably the little motor too.

 

[hantto]

Putting that discussed 10ohm resistor would effectively limit the current to 5v/10ohm=0.5A in the worst case scennario (better make that a >2.5W resistor)

 

[Dr.EM]

It may work with that resistor, though will probably need to be started with a finger and will only run very slowly. It shouldn't damage the computer that way though...

 

[techhappy]

Okay, so I went to Radio Shack and got the current numbers for the 1.5-3v motor that were talking about:

https://www.radioshack.com/product/index.jsp?productId=2102822&cp=&origkw=motor&kw=motor&parentPage=search

According to the specs on the back, the current at maximum efficiency is .70a (+ or - 15%) and the output .31 watts.

So based on those numbers, what would the Ohm rating be and what resistor should I get? Is a Micro 1-Amp Rectifier Diode a better solution than a resistor? So far, it seems if I add or decrease diodes, I can decrease the volts .8 volts per diode, making it customizable to a degree if I wanted to keep the motor not running at max voltage.

Although, I like the idea of dealing with just 1 transistor. Let's say, I wanted to run the motor at 2.5 volts, so as not to overload the little motor. What ohm rating and wattage rating of resistor should I get based on the current flow indicated in the above specs?

Some of the discussion about burning out the usb ports has me concerned. I thought with a proper resistor in place, the device can't draw more power that what it's given?

So many questions, so much to learn, I had no idea how complicated this could get to get the voltage down to safe levels for this motor. Given, the cost of resistors(10-25cents each) and the $1 fan, this makes it a fun budget project, although it seems like there are several different ways to approach it.

If I was going to take in all the info, from this post, all I want is a way to decrease the 5v usb voltage to 2.5 volts for the motor, in such a way as to make it safe so it won't burn out the fan or usb port.

 

[eblc1388]

So based on those numbers, what would the Ohm rating be and what resistor should I get? Is a Micro 1-Amp Rectifier Diode a better solution than a resistor? So far, it seems if I add or decrease diodes, I can decrease the volts .8 volts per diode, making it customizable to a degree if I wanted to keep the motor not running at max voltage.

Don't fall into the trap of using 0.7A to calculate the resistor value.

If a survey said a person works with best efficiency at 25 hours per week, does it means the person is actually working at 25 hours per week? Of course not, it depends on his boss or the workload.

A motor has a load(fan) and the load determines at what current and efficiency the motor is working. With a stalled motor and four diodes, the current will be very high and you have a real risk of damaging the USB port.

With a resistor setup to drop 2.5V, the maximium current is limited to double the running current and one can be sure that it is still within the allowable limit of the USB port. So that would be the preferred method for your setup. If you have planned to buy a multimeter, you can then fit a resistor best suited to your case by actually measuring the motor current with fan together with 2.5V across them.

 

[audioguru]

The little fan might not start running with 10 ohms in series with it from a 5V supply.

 

[eblc1388]

Ever seen an propeller plane starting off in a war documentary film?