I started out with a program that didn't work, I have gone through a fault finding process and eventually got to a 5v supply a 220 ohm resistor and the motor.
In the circuit the resistor registers no resistance and the motor doesn't run
tested outside the circuit the resistor has 220 ohms resistance
the motor runs fine through a direct 5 v supply.
Why is the resistor not functioning in the circuit please?
Thanks
You really need to post a circuit diagram before anybody can figure out what is wrong... Are you powering the motor using the Arduino 5v supply? A pin on the Arduino?
You would need to post a diagram.
Sam1Bird2:
Why is the resistor not functioning in the circuit please?
Thanks
bad resistor? did you try another one?
Sam1Bird2:
the motor doesn't run
the motor may have an inherent resistance. I would not expect a 5v supply through a 220Ohm resistor to drive a motor. 5/220 =22mA
Sam1Bird2:
In the circuit the resistor registers no resistance and the motor doesn't run
tested outside the circuit the resistor has 220 ohms resistance
Then chances are you are not measuring the resistor in the circuit correctly. Oh wait a minute. You can't measure the value of a resistor while in a circuit. That explains that.
Sam1Bird2:
I started out with a program that didn't work, I have gone through a fault finding process and eventually got to a 5v supply a 220 ohm resistor and the motor.
the motor runs fine through a direct 5 v supply.
Why is the resistor not functioning in the circuit please?
Thanks
A 5 volt motor requires 5 volts, adding a resistor, reduces the voltage across the motor. You must apply a voltage divider equation to determine how much voltage the motor has across it.
V motor = V power supply * R motor / (R resistor + R motor)
Assume motor has 100 ohms of resistance.
V motor = 5 * 100 / (220 + 100) = 1.56V
So your 5 volt motor is now being powered by only 1.56V. It's probably not going to run.
Thanks very much for all the inputs.
The situation just seems illogical!!!!!
is there some sort of impedance in the motor?
I have tried different components and every combination of source supply I can think of !
There are 3 components,
The circuit is an Arduino [is the feed via the Arduino 5 volt supply the source the problem} a 5v supply, through a 220 ohm resistor to a motor that can run on 3.5 volts.
Testing the resistor measures a 5v voltage drop in the resistor.
A LED in place of the motor registers the expected voltage drop across the resistor and the LED
It just doesn't seem to make any sort of sense!
Thanks again for input!
Martyn
Sam1Bird2:
is there some sort of impedance in the motor?
Yes. If there wasn't it would be a short circuit and your power supply would explode.
Why do you think you need a resistor in series with the motor?
If the circuit is that simple then post a drawing of it with all points labelled so we can talk about them in the posts.
Are you trying to connect the motor directly to Arduino 5V pin or to an Arduino input/output pin? Is so, STOP! There is NO way the Arduino can provide the current to run the motor without damaging the microcontroller chip.
It doesn't make sense to you because you are missing pieces of the basics. It doesn't make sense to us because we don't know what the circuit looks like and don't know what you are doing. Let's get a picture up so we can correct where you are going wrong. Please also describe how you are taking these measurements. I suspect you are using the meter incorrectly.
Sam1Bird2:
The circuit is an Arduino [is the feed via the Arduino 5 volt supply the source the problem} a 5v supply, through a 220 ohm resistor to a motor that can run on 3.5 volts.
[/quote]
So based on that you're expecting to start with 5V, drop 1.5V across the resistor and still have 3.5V for the motor.
Have you heard of Ohms Law? V = IR or I = V/R. So for 1.5V across 220 Ohms that would be 1.5 / 220 = 6.8 mA (thousandths of an amp).
Where did you get the idea that your motor will run on such a tiny current? It won't. Both current and voltage are important, you can't just ignore one of them.
Steve
if u've got two NPN-transistors? connect the as a darlington and use the resistor in the base connection.
Assuming you have the generic small dc motor often used with an arduino, the resistance of the motor will be very low. The resistance is not what determines how much current the motor needs to operate, but can give you an estimate of the stall current, which is the maximum current that will flow through the motor when the shaft is held stationary so that it cannot turn. During normal operation, within the normal load ratings of the motor, the motor itself tends to limit the current to what is needed to drive the load connected to it, plus the internal losses caused by friction, heating from wiring resistance, etc.
Thanks to all; I see the problem is insufficient current due to the resistor; thanks again! 
Think of a motor as a piece of wire. That's what a winding is. The resistance will be measured in ohms or fractions of an ohm.
Expect even a small motor to require 0.2A or more to function.
Anyway why put a resistor in series with a motor? That's not how things are done to control power,
power is controlled by switching, as with an Hbridge.
MarkT:
Anyway why put a resistor in series with a motor? That's not how things are done to control power,
power is controlled by switching, as with an Hbridge.
Well if you're old enough you might remember Scalextric racers where the trigger controllers contained a variable resistor in series with the car's motor.
But, nostalgia aside you're right, no-one does it like that any more, not even Scalextric.
Steve
Sam1Bird2:
Thanks to all; I see the problem is insufficient current due to the resistor; thanks again!
Very easily could be insufficient voltage too.