Need to use mosfet as a switch instead of transistor

[Tipu606]

i have a Chinese AC-DC fan where the fan motor is provided
with two 2SD882 transistors(40V, 3A,10W) connected in parallel and this is then connected in series with the DC motor
The speed of the fan is rather slow due to the resistance drop in the transistors
Can anyone please advise what equivalent rating mosfet i can use to get more speed as then the ohms is less in mosfet?
The fan operates on 6V 7AH battery and has a 30W DC motor
thanks

 

[jiripolivka]

Are the transistors used as a power switch, or as phase switches?
If you need a lower-loss switch a MOSFET like IRF520 or similar can be used. Its channel resistance is low, but make sure the gate-channel voltage cannot exceed TTL logic limits. I use always a 5.1 Zener across the gate/drain input, and a 10 kOhm resistor from the input to gate to make sure the MOSFET is safe.
Make sure you use a good heat sink as the MOSFET could be hot due to transitions from OFF to ON state.

 

[Tipu606]

thanks!
the transistors are used as a power switch
IRF520 will be just fine rated at 10 amps whereas mine is set for 6 amps for parallel connection of the transistors
Just downloaded the data sheet for it.
regards

 

[kassimdandago]

You have got a reasonable advise to use irf 250 with a heat sink, it is ok.

 

[Tipu606]

You have got a reasonable advise to use irf 250 with a heat sink, it is ok.

sorry but a TO-3 package mosfet will not fit on the pcb as only a TO-220 type package mosfet is required

 

[kassimdandago]

Then use irf 3205 which has an rds on of 0.008 ohms at atleast 3v and at most 5v on the gate wrespect to the source since it is an n mos. It has a tj temp of upto 170 oc and can tolerate 250watts. I use a pair on an inverter to drive 500 watts load with a centre tap transfomer. I hope iam helped.