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Simple Motor Problem! Help! (Newbie)

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This one would be good if The current weren't so high..

**broken link removed**

When VGS = 5v and VDS = 12v, ID = 25A...

I need ID to be 1.25A right?
 
You are confused because the device from Rapid listed in your post #20 is a TRIAC which is completely different to a Mosfet.

I don't think NTE make anything. I think they buy ordinary transistors and Mosfets, mark them with their own part numbers and increase the price to make a huge profit. You should buy the original part that they buy at a much lower price.

The Mosfet listed in your post #21 is barely turned on with a current of only 0.25mA when it has a gate-source voltage of 2.0V. With a gate-source voltage of 10V then it conducts up to 30A.
 
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AHHH I had no Idea that a TRIAC is a different type of transistor... right things make more sense now...

Also the transistor in my post #21 conducts up to 35A at 5v VGS(figure 4)... but does that mean that I could actually use it even if I only need 1.25A?? I dont want to burn out my motor by supplying it with too much current, but if its "up to" 35A then its fine isnt it?
 
The battery in my car supplies 400A to the starter motor when it is cold.
Does the clock in the car draw 400A all the time? No, it draws as much current as it needs which is about 1mA (0.001A).

Your motor also draws as much current as it needs. When it is stalled and the moment when it starts running it draws its max amount of current which is only 1.25A (when its supply is 3.0V). When it is running then its current is less and depends on its mechanical load.

You need a Mosfet that turns on with a max on-resistance of 0.24 ohms so that the voltage loss is 0.3V (10% of the 3V supply to the motor) and when its gate-source voltage from the microcontoller is only 5V.
 
Ah, ok I think Iv'e got it.

I can use the transistor above as a switch which turns when 5v is on the gate (from the microcontroller output). I have a potential divider (the resistance of the transistor and the motor) Wil will makes sure there is about 3v across the motor when the transistor is ON. The motor will draw current from the supply (max 1.25A) and will run.

(sorry for being so noobish, but I am learning!).
 

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You do not know the resistance of the Mosfet. Its maximum resistance is listed on its datasheet. It could have a resistance that is much less.

Your motor will probably get almost 9V and blow up and the resulting high current will also kill the battery!
Why do you use a little 9V battery that lasts for a short duration?
Use two AA cells in series that can supply the 3V at 1.25A to start the motor running and can supply a few hundred mA to run it for hours at the correct voltage.
 
Yeah I did mean using the maximum resistance as a starting point for my potential divider (but now I realize that it makes no sense). Oh and do you mean just use three 3v AA bat's for the whole circuit (I need 9v for my op-amp) and only connect one of them up to the motor? (I didn't know it makes a difference whether you use batteries in series or just use one battery by itself..)
 
If you limit the max voltage to a DC motor with a series resistor then it probably will not start running if it has a mechanical load because its current increases when it has a mechanical load. When it has a resistor in series then its voltage drops when it tries to draw more current.

It is better if you use a weak little 9V battery to power the opamps plus two powerful AA cells for the motor. Then when the motor is loaded the voltage to the opamps will not be affected.
 
Ah, Ok so I'll just use two AA batteries for the motor. So would it work if I just replace the 9v supply with 3v (produced by the two AA batteries) and then replace the transistor with the MOSFET from post #21?
 
Ah, Ok so I'll just use two AA batteries for the motor. So would it work if I just replace the 9v supply with 3v (produced by the two AA batteries) and then replace the transistor with the MOSFET from post #21?
Where will you find an opamp that works with a supply of only 3V that drops to 2V when it has run down?
The Mosfet also might not work because its lowest Vgs is spec'd at 5V.
Will the motor run OK when the 3V battery voltage has dropped to only 2V?
 
Gosh..this is more complicated then I thought (SO MANY VARIABLES!)

Anywho, I meant that I will run the Op amp with the 9v supply and just use 2 AA batteries to run the motor (should work). The MOSFET will have 5v across it since the chips output produces 5v (roughly) so that shouldn't be a problem.

Then as your saying, the motor needs to run even when the battery has gone down to 2v... well, if it's spec is 3v then I guess it shouldn't draw a lot of power...so wouldn't that take a while?
Maybe I can use rechargeable batteries?. At this point im not going to worry about when the batteries run down as I'm just building a prototype of my circuit. (I might change the motor to conserve power etc)

For now, let's not worry to much about the power problem.
 
Anywho, I meant that I will run the Op amp with the 9v supply and just use 2 AA batteries to run the motor (should work). The MOSFET will have 5v across it since the chips output produces 5v (roughly) so that shouldn't be a problem.
Good.

Then as your saying, the motor needs to run even when the battery has gone down to 2v... well, if it's spec is 3v then I guess it shouldn't draw a lot of power...so wouldn't that take a while?
The battery's datasheet shows how long. 10minutes? 20minutes?

Maybe I can use rechargeable batteries?
Ni-Cad and Ni-MH cells are about 1.2V each so two are only 2.4V which might be too low. Three are 3.6V which might be too high.
My electric RC airplanes and helicopter use 3.7V Li-Po cells like modern hybrid cars.
 
Right, hmm... Looking at the motor's data sheet it says that the rated voltage is 3v, though it does operate between 1.5~4.5v so really, I could use 3 AA batteries? Also, the motor will actually act as a switch and turn a nob, so it shouldn't be on very often and when it is on it will be a few seconds.

I think using 3AA batteries should do the trick, and since the transistor above should be able to handle 5v I think we may have a good chance of it working?
 
Three AA alkaline battery cells are 4.5V only when they are brand new. When their voltage drops to 3.0V then the Mosfet might not work (some Mosfets will not work).
 
Well, that really dosent matter because the mosfet is connected to the Chip's output which is ~5v, the mosfet is connected up to a ~5v source (i'll just use the 4.5 AA Battaries).

The thing is, as long as I use batteries I guess I have to just accept the fact that they will go down to about 3v later on and need replacing....
 
Well, that really dosent matter because the mosfet is connected to the Chip's output which is ~5v.
Isn't the chip powered from the 4.5V battery that drops to 3.0V? Then the gate of the Mosfet will get only 3.0V to 4.5V and some Mosfets will not work.
 
Eh? Why would the battery drop to 3v when I connect the IC? (you mean as it uses more and more power or just straight away?)
 
Eh? Why would the battery drop to 3v when I connect the IC? (you mean as it uses more and more power or just straight away?)
The battery is 4.5V only when it is brand new. It drops to 3.0V when it is nearly dead and needs to be replaced.
Is the chip powered from that same battery?
 
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Well yeah I was thinking to power it all up from the 4.5V AA batteries. I guess I could just use two separate batteries (one for the motor and another for the rest of the circuit).
 
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