# motor set current use

Discussion in 'General Electronics Chat' started by duxbuz, May 22, 2014.

1. ### duxbuz Thread Starter Member

Feb 23, 2014
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I have a small dc motor with a write up like this:
i wanted to try and use ohms law to set it to use a certain current like this chap does with his LED and a resistor

Is this possible? Or is this not possible... using the same method with a resistor and ohms law?

Thanks

2. ### timwhite Member

Apr 10, 2014
50
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Hypothetically, yes. Using a transistor you should be able to feed a controlled current into your motor, if I'm understanding your question correctly.

Jul 18, 2013
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The LED represents a different load than a DC motor, in the case of the LED it is usually either turned ON or OFF, if you want to control the RPM of the motor then you would be operating the transistor in the linear part of the conduction curve for the transistor.
With a LED, the load is known and fairly constant, the load characteristics of a DC motor is very different.
Max.

4. ### duxbuz Thread Starter Member

Feb 23, 2014
133
0
Ok so no. If the circuit had no transistor it would not be possible.

As in the example the calculations are performed before he gets to the transistor part. So in theory it could be a circuit without a transistor for the led calculations to be performed. But not for a motor.

5. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,771
971
There are many ways to "control" a motor.. Why don't you explain what you are trying to do with this motor.

6. ### Austin Clark Member

Dec 28, 2011
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The current drawn will be load-dependent. There is no general I-V characteristic/curve for a DC motor, it varies with load. This is because the faster the motor rotates, the more counter-emf (reverse voltage) will be generated. As you apply a load, the motor slows slightly, less counter-emf is generated so the voltage across the motor effectively increases, and thus current increases as understood by Ohms Law. More current = More Torque by the way.
Hope that helps.

7. ### duxbuz Thread Starter Member

Feb 23, 2014
133
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Thanks for help everyone.

I was hoping to hook up the small dc motor(voltage 1.5 to 4.5v, current 0.083 to 0.41A) with the p2n2222ag as a switch on a microcontroller pin at npn's base, with 3V battery supply across my motor and the npn's collector/emitter.

Being a novice I am looking to just get a quick working example but I seem to have gone off on a tangent now and am reading Kirchhoffs Laws and working through examples of transistor gains, set by a great member on here, that are teaching me absolute valuable information, but I still haven't set up my dc motor circuit.

I don't really know how to proceed.

8. ### BillB3857 Senior Member

Feb 28, 2009
2,400
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If you are wanting to vary the speed of the motor by use of the microcontroller, Pulse Width Modulation (PWM) would be a strong possibility. By turning the transistor ON/OFF rapidly and varying the amount of ON time compared to OFF time, the motor will average the two and speed will be controlled. Most books on microcontroller programming will cover PWM.

Don't forget the diode across the motor to protect the transistor.

9. ### duxbuz Thread Starter Member

Feb 23, 2014
133
0
Would this work with my small dc motor(voltage 1.5 to 4.5v, current 0.083 to 0.41A)

I aim to use Ic at between 80mA - 410mA

If I then chose a value of say 150mA (I don't know why I would choose a value this is probably incorrect am not sure how a range is used )

I calculate using

.150/10(forced beta)=15mA
(4.5V - .7V)/.015 = 253R

is that correct?

Take into consideration max Ic could be 400mA

then
.400/10(forced beta) = 40mA <----- this then is the limit the pin can source, so a problem ?

or does my 253R limit the Ic to always being max 150mA and when the motor wants more current it just stops or slows

Hope this makes some sense

10. ### duxbuz Thread Starter Member

Feb 23, 2014
133
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Just making it spin. To demonstrate switching with a microcontroller.

11. ### #12 Expert

Nov 30, 2010
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Every motor that I know about demands a higher current to start than to run. When you limit the current too much, it won't start. YOU have to find out what this limit is by experimenting with YOUR motor. If it won't start with 22 ohms in series with it, use less resistance. It's that simple when you have the motor in your hand. It's much more difficult from over here.

12. ### duxbuz Thread Starter Member

Feb 23, 2014
133
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I was wondering if these calculations are reasonable? I have edited my quote slightly as I am presuming now that the base resistor limits the Ic's max value?

And I would find a standard resistor value.

13. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,810
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Using a 'forced beta' (i.e assumed) value in calculations is generally fine for obtaining a resistor value which will ensure the transistor is switched fully on (saturated), but cannot give a controlled maximum Ic because that current will depend on the actual beta (which is probably unknown).

14. ### duxbuz Thread Starter Member

Feb 23, 2014
133
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Thanks. So these calculations seem ok, in respect to beta being an approximation?

The maximum Ic is dictated by the base resistor?

The motor would stall if it needed more than the Ic?

15. ### Alec_t AAC Fanatic!

Sep 17, 2013
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No, not for your current-limiting purpose. In reality beta varies widely from one transistor type to another, and even between individual transistors of the same type.
Partly; also by the actual beta and temperature.
As #12 pointed out, if you don't provide enough current it won't even start. Voltage control would be better for you.

16. ### duxbuz Thread Starter Member

Feb 23, 2014
133
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Thanks Alex

What would be a better formula to calculate base resistor for 150mA?

17. ### Alec_t AAC Fanatic!

Sep 17, 2013
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1,105
Check the datasheet for the transistor type you will use. Find the 'typical' beta and use that in your calculations. Bear in mind that the transistor probably won't be saturated, so will dissipate considerable power as heat!

18. ### ronv AAC Fanatic!

Nov 12, 2008
3,292
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I think we got out into the weeds somewhere along the line. If what you want to do is turn the motor on and off the attached circuit will work.
The motor will only take the current it needs to try to supply the required torque. In other words if you keep it from turning it will take the maximum current. If it spins free it will only take the minimum current.

If you want to change the speed you can turn the transistor on and off quickly with the micro (say 5 KHz). If it is on half the time it will turn about 1/2 as fast as it would turned on all the time. This is called PWM.
The diode is there to clamp the voltage when the motor is turned off. - Google clamp diode.
To do a real good job we would need to know all the specs on the motor and the micro.

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Feb 23, 2014
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