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Help with MOSFET/Transistor to power motor

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I need a some help trying to get a motor working by switching it on through a more powerful battery. I havent actually done any electronics in a good 3 years and I wasn't very good at the higher power stuff anyway!

Basically I have a circuit that looks something like the one attached and I have a couple of MOSFETS and various high power transistors lying around, can some one help me with the calculations required for R1 and R2 and what sort of parameters would be good for the MOSFET/transistor?

I know the motor usually runs on 8.7v and draws around 2amps. If possible i would also like to reduce the draw of the motor and reduce its RPM, can that be done?
 

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The most power-efficient switching device would be an N-channel MOSFET. It should be rated for at least 20V, 20A, have a low Rds(on) (<50mΩ) and be a logic-level type (since you are using only 6V to drive it). R1 could then be 100Ω (for damping any spurious oscillation) and R2 could be, say, 10k. The motor would need the freewheeling diode connected across it (as shown) and perhaps a snubber.
 
ok so I have an IRF540A:

(https://www.electro-tech-online.com/custompdfs/2013/04/irf540a.pdf)

which has a Vgs threshold of 2.0v - 4.0v, is this the voltage needed at the gate to activate the mosfet?

In the arrangement ive shown with 100ohm R1 and 10kohm the motor has a function like this:

pressing the switch sometimes activates the mosfet and sometimes doesnt then the motor will run continuously even if the switch is released. what could be the problem?

Also im not entirely sure that the mosfet isnt blown!
 
infact it appears that the motor runs even in the mosfet is not activated at all...

Either you have source and drain reversed, or the FET is blown, or when the input switch is open, there is no rpull-down resistor from gate to source.

Make sure that with the input 6V removed, there is a resistor from gate to source. It can be a few hundred to a few tens of thousands of Ω
 
a Vgs threshold of 2.0v - 4.0v, is this the voltage needed at the gate to activate the mosfet?
The threshold voltage is just that: a threshold above which the FET will begin to conduct noticeably. To turn the FET fully on would usually need about 10V. That FET is not a logic-level one. A logic-level one has a much lower threshold (e.g. ~1.5V) and can be turned fully on with Vgs = 5V.
 
So I've followed your advice and should I be building something like what I've attached? It seems a little unusual for me to have the two ground rails the same but otherwise how can I link the Gate resistor to the Source while still having a complete circuit for the Gate to go high (in this example)?
 

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It seems a little unusual for me to have the two ground rails the same
Your circuit has only one ground rail (to which the -ve terminals of both batteries are attached), but is missing the essential freewheeling diode. If you have built this on a breadboard then it's not surprising you get erratic results; stray inductance/capacitance can cause FET oscillation and poor contacts/connections probably won't allow peak currents of ~10A at motor start-up to flow without problems.
 
Ok I will take it out of the breadboard then and also I've realised the diode I have is actually a zener diode, is this any good for these purposes?

Not if the Vz voltage is less than the motor supply voltage. Even so, the Zener is not likely to like the 2A or so that flows through the snubber diode just as the FET turns off. See if you can get a 1A to 3A rectifier diode with a PIV rating of 100V or more.
 
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