voltage divider ckt

Thread Starter

bahubali67

Joined Mar 15, 2012
28
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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MrChips

Joined Oct 2, 2009
30,712
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.
 

Thread Starter

bahubali67

Joined Mar 15, 2012
28
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.

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??
 

DerStrom8

Joined Feb 20, 2011
2,390
"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....
 

#12

Joined Nov 30, 2010
18,224
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.
 

MrChips

Joined Oct 2, 2009
30,712
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.
 

WBahn

Joined Mar 31, 2012
29,978
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??
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.
 

Thread Starter

bahubali67

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

Thread Starter

bahubali67

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

WBahn

Joined Mar 31, 2012
29,978
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?
 

DerStrom8

Joined Feb 20, 2011
2,390
we ll discuss this ckt and circuit later, now what abt my question???
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).
 

#12

Joined Nov 30, 2010
18,224
ok any other way so that it should divide the voltage but current must remain same.
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.
 

Thread Starter

bahubali67

Joined Mar 15, 2012
28
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?
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??
 

Thread Starter

bahubali67

Joined Mar 15, 2012
28
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).
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.
 

DerStrom8

Joined Feb 20, 2011
2,390
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.
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.
 

MrChips

Joined Oct 2, 2009
30,712
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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