# voltage divider ckt

Discussion in 'General Electronics Chat' started by bahubali67, Apr 5, 2013.

1. ### bahubali67 Thread Starter Member

Mar 15, 2012
28
1
hi all. I have 5v power supply and planned to get 2.5v using Voltage divider ckt as show in figure. I got 2.5v on multimeter screen but when I tried to connect to small dc motor it wont turns on. (motor is shown in pic) what is the problem??? I think motor is of 3v I tried to vary voltage divider voltage to 4v but motor is not running???

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2. ### MrChips Moderator

Oct 2, 2009
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The resistance of the motor is much less than 2.5kΩ. Hence the voltage is not going to be 2.5V at the junction.

Another way of looking at it, the 2.5kΩ resistor is going to limit the current to 2mA. Your motor needs a lot more than 2mA in order to run.

Reduce the value of the high side (top) resistor until the motor runs. You don't need the low side (bottom) resistor.

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3. ### bahubali67 Thread Starter Member

Mar 15, 2012
28
1

but my power supply is producing 1a current. ok how to calculate current at voltage divider ckt. i,e, at 2.5v ,what is value of current??

4. ### bahubali67 Thread Starter Member

Mar 15, 2012
28
1
I have place 100ohm(and 220ohm also) and 330 ohm resistor but still couldn't run motor.

5. ### MrChips Moderator

Oct 2, 2009
17,565
5,450
How much current does the motor require in order to run?

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6. ### DerStrom8 Well-Known Member

Feb 20, 2011
2,424
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"ckt" is not a word. It is not that hard to write out "circuit". Also notice, "circuit" does not have a 'k', so "ckt" doesn't even make sense.

Sorry, the grammar nazi part of me just came out....

7. ### #12 Expert

Nov 30, 2010
18,093
9,683
LOL. Yep. I reported a post yesterday for being spam and having bad grammar. Still, I try not to let it annoy me.

About this 1 amp power supply at 5 volts to run a motor at 3 volts and some amount of current. You can go as low as 5 ohms for a test, just to see if the motor will start at all. The problem with adjusting for 3 volts while running with a resistor is that the motor won't start when it only has enough current to run. Better to put 3 diodes in series with the motor so it can have the whole amp to start, then it will reduce its need and use less current.

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8. ### MrChips Moderator

Oct 2, 2009
17,565
5,450
Shucks. You beat me to it. I was waiting for the OP to respond before suggesting the series diode solution. I believe the OP is focusing too much on the voltage and is ignoring the current requirements. That is why the OP fails to see why those resistors will prevent the motor from running.

OP needs to understand Ohm's Law.

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9. ### WBahn Moderator

Mar 31, 2012
23,388
7,098
What makes you think your power supply is producing 1A of current? Because it has a label on it that says 1A? That is just the maximum amouint of current that it CAN produce (or that you can count on it producing). If it were producing 1A and that was passing through a 2.5kΩ resistor, that resistor would have 2500V across it!

Also, if you are going to put a resistor in series with your motor, be sure that it is rated to handle the power.

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10. ### bahubali67 Thread Starter Member

Mar 15, 2012
28
1
sorry I don't know...but I have shown that motor in pic. its a motor from toy car.

11. ### bahubali67 Thread Starter Member

Mar 15, 2012
28
1
we ll discuss this ckt and circuit later, now what abt my question???

12. ### bahubali67 Thread Starter Member

Mar 15, 2012
28
1
ok any other way so that it should divide the voltage but current must remain same.

13. ### WBahn Moderator

Mar 31, 2012
23,388
7,098
It would really help if you would tell us what you are trying to achieve.

You have a motor that you took out of a toy car.

It is normally run from 3V.

You want to do what with it?

Why can't you use a voltage regulator?

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14. ### DerStrom8 Well-Known Member

Feb 20, 2011
2,424
1,356
Do you not know how to type proper english? "abt" is not a word either.

I generally do not answer questions from people who do not know how to type. I also do not answer questions from people who think they're entitled to an answer. We are not required to give you an answer, that is your PRIVILEGE. You can not just expect people to give you answers (in response to the second half of your post).

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15. ### #12 Expert

Nov 30, 2010
18,093
9,683
You refuse to tell us how much current and ask us to spend our time making more designs based on unknown amount of current after you have been given a method that will work?

No.

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16. ### bahubali67 Thread Starter Member

Mar 15, 2012
28
1
I just want control the speed of dc motor.( without using pwm,can we control speed by varying voltage and current). any way to control??

17. ### bahubali67 Thread Starter Member

Mar 15, 2012
28
1
this is not a English grammar forum. its electronics forum, if u know answer then tell or else keep quite. u r not only the person knowing abt electronics, we r here to just discuss abt electronics not English language.

Apr 5, 2008
18,882
3,712
Hello,

Please use correct english over here and not some kind of sms language.

Bertus

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19. ### DerStrom8 Well-Known Member

Feb 20, 2011
2,424
1,356
This is not an English grammar forum but it is an English speaking forum. You are not speaking English. And your attitude is very poor. You think we're required to give you answers? Think again. I am tired of your type coming on here and demanding answers in as rude a way as possible. Trust me, that's a good way to get people to hate you and to not answer your questions. Rudeness will get you absolutely nowhere, except maybe kicked out. If you would change your attitude and be more polite, and type in proper English sentences so people can understand what you're asking, I might consider helping you.

20. ### MrChips Moderator

Oct 2, 2009
17,565
5,450
A typical toy motor will require about 100mA to 1A - 2A depending on the load. The current drawn increases dramatically as the load increases. You really need to state what you are attempting to do.
Do you wish the motor to rotate at constant speed over a range of loads?

If you assume a current of 100mA, determine using Ohm's Law the effect of various series resistances on the current available to the motor. You will easily see that any series resistance will have to be lower than 10Ω and not in hundreds or thousands of ohms.

The two or three diodes in series is worth experimenting with in order to reduce the voltage to 3V from 5V.

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