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Basic Electronics » 5v to 3v using nerdkit parts?

December 20, 2009
by promethean
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Hey all,

I have a small vibrating motor that operates at 3v, max 95ma. Is there any way I can use the parts included with the nerdkit to output this voltage?

December 21, 2009
by promethean
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Hmm, this is odd, but useful-- could someone explain the specifics of why this happens? The output voltage on a Pin out seems to be about 2.8v @ 80ma, which is perfect for this motor. I'm guessing this is due to the internal resistance of the micro, but this brings a few questions for me:

Is this internal resistance a relatively predictable constant? (Outside of environmental factors of temp etc., I mean)

I though resistors caused current drops, but voltage remained somewhat unchanged. I suppose I should have known better, if V = IR. But does this mean I can get to a target voltage (lower) voltage using resistors of a known value?

December 21, 2009
by BobaMosfet
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promethean--

Pay special attention to your DataSheet-- Page 304 (section 28). General I/O pins only pass (MAX) 40mA of current. And voltage presented depends on purpose of pin. Since I don't know what pin you are referring to, I can't tell you why you are seeing the voltage you are seeing.

Another question-- Are you sure you are measuring things correctly? I have to ask this-- I'm curious as to how you measured the current on a single pin, since you can't do that with a voltmeter directly, and that contradicts the datasheet...

:P BM

December 21, 2009
by BobaMosfet
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Oh, and to answer your first question directly you cannot (if it was like mine) limit the voltage to 3V with parts from the NerdKit. You either need a voltage Regulator, or a zener diode of the proper rating, or other parts.

December 21, 2009
by promethean
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I used pin 27, modifying the blinky LED program to simply remove the delay so I had a steady "high" on the pin. Then I removed the LED from the board and used a digital multimeter. I received about 2.8v on the voltage setting and 75-80ma on the ma setting. The reading was obtained by putting the positive (red) lead in row 12 of the board, the row the LED had been in, and the other multimeter lead to the ground rail.

December 22, 2009
by BobaMosfet
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promethean-

I'm creating a thread called "What Current and Voltage Really Is" to helpful provide a better fundamental grasp of current, voltage, how to measure both, and the dangers of potentially misusing your meter/misconnecting things.

December 22, 2009
by BobaMosfet
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Well... I was going to create a thread... I had several hours worth of work into when windows decided to blow up in my face, so that's gone, sorry. :( I'm bummed as it was really nice look at these things.

December 22, 2009
by Hewitt2
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no hijack intended but I always have windows blowing up and so does my son the geek. He does not know he is getting the nerdkit for Xmas. what he asked for was a computer he could play with linux on. I have been playing with his new ( actually a used one but new to us) linux box for two weeks and that thing just works. I am about to break my windows. the wife switched to apple 2 years ago.

December 31, 2009
by carlhako
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BobaMosfet still going to post the thread? I think it would be great as im still getting a grasp on exactly what voltage and current mean and how they relate to each other.

January 05, 2010
by BobaMosfet
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carlhako-

I haven't gotten a chance to write that thread, however I did manage to get some of it out in this thread, on 29th:

http://www.nerdkits.com/forum/thread/392/

This should help you a lot (I hope).

January 23, 2010
by sepeters
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Hello, I'm new to the board and thought I'd post a reply... There is a solution for making 3v with the provided parts but it's not a "good" solution. I would only use it for short term testing since it will suck your battery dry as fast as it can :)

The solution is to use a voltage divider, neglecting the motor (not always a good idea) if you take your 5v output pin and run 3x 10k resistors in series to ground you will get about 3.3v between the first resistor and ground which you could use to power your motor. If it's a really small motor it will probably not affect the voltage divider that much and you won't get a whole lot of power out of the output pins anyway.

The best solution for you would be to either buy a 3v regulator or a 5v-3v regulator.

Steve

February 08, 2010
by treymd
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I was about to ask why a simple voltage divider wouldn't work, but I guess it can, albeit a sloppy solution. Question though, how is it that a regulator would use less power, when the regulators themselves dissipate power through heat? do they somehow use less power in doing so than what a voltage divider would?

February 08, 2010
by mongo
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Here is a simple solution...

A diode generally has a voltage drop of .7Volts. Two in series would be 1.4Volts, etc...

February 08, 2010
by Rick_S
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This thread is a bit old but another easy solution could be a 3v zener diode.

February 10, 2010
by sepeters
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Voltage divider .vs Voltage regulator probably wouldn't make much difference as long as you turned off the pin on the MC when it was not in use. The voltage divider is a less efficient way of producing a required voltage otherwise since if you want to always have the voltage available for multiple uses it's going to burn power that the regulator wouldn't when there was no load.

=}----- Steve

February 21, 2010
by peabo
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The major problem is that the motor wants 80 mA and the microcontroller is not able to provide that current.

The ATmega allows 40 mA per pin with an overall limit of 100mA for sum of all pins (IIRC). High current out of the ATmega is really a hack, but if you aren't using other pins to provide high current, here is a way to do it:

Use three pins to power the motor. No one pin needs 80 mA / 3 ~=27 mA well within spec.

All three pins 0 = no motor current, all three pins = 1 causes the motor to run.

We imagine using series resistors to accommodate the voltage versus current across the load (the motor) which is supposedly 3V.

If the motor drops 3V, we need to drop 2V so at 27 mA that is 75 ohms.

So, between each of the three output pins, wire a 75 ohm resistor to a common point and wire the motor the the common point across ground.

It would be a good idea to wire a diode like 1N4001 (check current rating vs 1N400X) in reverse across the output, to prevent reverse EMF from generating negative voltage across the output.

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