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### Topic: confused on voltages of arduino (Read 1 time)previous topic - next topic

#### trecords

##### Jun 30, 2012, 10:10 am
Hi,

recently i have provided little test on arduino pins. So i have started with simple blink example with pinMode output 9 and increased delay to 5 seconds for tester to catch exact voltage. So when Led was turned off the voltage was 3.22 but when on it was 1.50. Why do this vice versa?
Then i have connected simple motor into same pins where Led is connected and the result was when Led is on motor stops running and when it is off motor starts running.
Please can someone explain me why do this happens?

Thanks,
Turkel.

#### Osgeld

#1
##### Jun 30, 2012, 10:19 am
led's look like infinite resistance to a point, after that they "open up"

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The voltage versus current characteristics of an LED are much like any diode. Current is approximately an exponential function of voltage (see Shockley diode equation), so a small voltage change results in a large change in current. If the voltage is below the threshold or on-voltage no current will flow.

and dont connect a motor direct to the arduino, thats bad mk? (high currents, back emf = damage)

#### trecords

#2
##### Jun 30, 2012, 10:45 am
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led's look like infinite resistance to a point, after that they "open up"

I couldn`t got what you mean, can you tell me where can i read abut this but excatly with same case which i get by motor and led.
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and dont connect a motor direct to the arduino, thats bad mk? (high currents, back emf = damage)

OK thanks will add some 0 ohm resistor, but my motor is very little as it is from MP3 player works with 3v (or low)

#### Grumpy_Mike

#3
##### Jun 30, 2012, 10:49 am
No, please stop damaging your arduino. You need a resistor in seriese with the LED, and you need more than a 0 ohm resistor with the motor.
To drive a motor you need a transistor. Getting more than 40mA from a pin will damage it.

#### trecords

#4
##### Jun 30, 2012, 11:06 am
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No, please stop damaging your arduino. You need a resistor in seriese with the LED, and you need more than a 0 ohm resistor with the motor.
To drive a motor you need a transistor. Getting more than 40mA from a pin will damage it.

OK i am going to see how much wat is my motor then will decide on resistor for motor.
Also how to learn about Led infinite resistance structure ? Why motor runs when Led is off?

#### trecords

#5
##### Jun 30, 2012, 11:08 am
Also very interested on how can i know that is i am loading arduino pins with incorrect voltage? I have tester with 7-8 functioni think it is enough for to know this, is it ?

#### Grumpy_Mike

#6
##### Jun 30, 2012, 11:14 am
You need to measure the current your loads take. Do it with the load just connected to the 5V line. Do you know how to use the meter to measure current?

#### Grumpy_Mike

#7
##### Jun 30, 2012, 11:26 am
I think you need to do some reading, look at these web pages of mine:-
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

http://www.thebox.myzen.co.uk/Workshop/Motors_1.html

#### trecords

#8
##### Jun 30, 2012, 03:26 pm
Thank you very much i have read these tutorials, they were really great
My motor is: 13.10mA and i am not sure about ideal voltage for it and how much RPM is it. Actually i have tested it by myself with use of multimeter with 1.5 AA battery
So if arduino output is 3V then i need to find mA for 3 volt or is it would be same mA for 3V too?

currently the equation is this:
13.10 = 1.5 * X so X is my ohm for resister i think i have got the thinks.

But the problem with transistor, how to find out required transistor ?

#### Grumpy_Mike

#9
##### Jun 30, 2012, 03:40 pm
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My motor is: 13.10mA and i am not sure about ideal voltage for it and how much RPM is it. Actually i have tested it by myself with use of multimeter with 1.5 AA battery

So if it takes 13.1mA at 1.5V then if you use ohms law V = I * R you will see its resistance is 114.5 ohms.
If you then put 3V on it by using ohms law again I = V / R you will see it draws 26.3 mA
Do you have an arduino that puts out 3V?
Most output 5V, at that the current your motor will take 43.6mA - that is above the danger point.
In any case you need a diode across the motor like the link showed.

If running the motor at 13.1mA is fine for you and you want to run it off 5V then the resistance that pulls this current is found again by ohms law R = V/ I is 382 ohms. You know that 115.5 ohms is your motor so to make up the difference you need 382 - 115.5 = 266.5 Ohms. This is not a standard value so use the next highest standard value of 270 Ohms to put in line with your motor.

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how to find out required transistor

This is down to knowing what transistors are around. For this sort of load use the popular 2N2222 transistor with a 10K base resistor.

#### trecords

#10
##### Jun 30, 2012, 07:18 pm
My arduino is UNO (and have another mini pro) it has 3.22v output i have done short test with it  ]
So my motor is 3.22v and 16.1mA
I need 200ohm resistor exactly (0.01% tolerance

Now about transistor, i have read about it as it is stable output regulator regarding to middle pin input. Actually i don`t have any transistors now, but i will get some nearly, or i will grab into some old home electronics  \$)
Do we need transistor to prevent interchangeness of amperage by motor? because if i block motor rotation the amperage increases up to 140mA. I have read about transistors but there are were not any equation or any rule for it, just explanation. can ou forward me to right direction to understand transistors.
One more question, you said 2n2222 transistor which resistance is included, and it is 10k so do i need to subtract this resistance and get ohm for separate resistor regarding to transistor`s internal resistor? do all transistors has resistor inside or it is just resistance of transistor itself?

Thank you very much Sir, you helped me to understand this think very much

#### smeezekitty

#11
##### Jun 30, 2012, 07:42 pm

My arduino is UNO (and have another mini pro) it has 3.22v output i have done short test with it  ]
So my motor is 3.22v and 16.1mA
I need 200ohm resistor exactly (0.01% tolerance

With 3.3v, the motor should be fine without a resistor without exceeding the current limits.
But if you do use a resistor, the value does not need to be that precise e.g. you can use 150 ohms or 220 ohms.
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Now about transistor, i have read about it as it is stable output regulator regarding to middle pin input.

Sorry, not following you here.
Basically the current on the base (center pin) controls the (usually higher) current between the emitter in the collector.
Quote

Actually i don`t have any transistors now, but i will get some nearly, or i will grab into some old home electronics  \$)

Definitely something worth having and 2n2222 are really cheap.
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Do we need transistor to prevent interchangeness of amperage by motor? because if i block motor rotation the amperage increases up to 140mA. I have read about transistors but there are were not any equation or any rule for it, just explanation. can ou forward me to right direction to understand transistors.

That means the motor stall current is 140ma. A 2N2222 should be able to handle that.
Quote

One more question, you said 2n2222 transistor which resistance is included, and it is 10k so do i need to subtract this resistance and get ohm for separate resistor regarding to transistor`s internal resistor? do all transistors has resistor inside or it is just resistance of transistor itself?

Very few transistors have resistors inside. You need a base resistor somewhere between 220 ohms and 100K depending on the current you need to switch. Without a proper base resistor, the transistor will draw too much current and risk damaging your Arduino pin.

Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
Resistors for LEDS!

#### Grumpy_Mike

#12
##### Jun 30, 2012, 08:54 pm
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can ou forward me to right direction to understand transistors.

Try this:-
http://www.kpsec.freeuk.com/trancirc.htm

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i have read about it as it is stable output regulator regarding to middle pin input.

What ever it is you think you have read you have misunderstood it.

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you said 2n2222 transistor which resistance is included,

No resistor is included, this is an extra resistor that goes between the base and your arduino output pin.

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I need 200ohm resistor exactly (0.01% tolerance)

No, it does not need to be this accurate, 20% will be fine.

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Do we need transistor to prevent interchangeness of amperage by motor?

No idea what interchangeness means. You need a transistor to switch the required current. Quite honestly I think you are mistaken with your measurements.

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