Don't you understand the datasheet for the little 2N4401 transistors?
Their absolute max allowed current is only 0.6A but your motor uses 8.6A at 6V.
Your little transistors need a base current that is much too high to be fed from a micro-controller. You should use logic-level Mosfets instead of the little 2N4401 transistors.

Don't use a weak little 12V AC/DC adapter. Instead use a 6V power supply that has enough power.

Hey thankyou for breaking it down for me ... i did a little research and found out about IRLD110(5V) and IRL540N(10V)which one do you recomend for my job... this can be switched on by my PIC but this one will only hold 6A ... i searched but i could not find any higher with a 5V gate.
please guide me if you do know a logic level Mosfets where i can get them @ that rate... My adapter has the option to tune it to 6v so I'll do that or are you recomending to get a normal 220 to 6 V DC

many thanks!!!

 

You could take a look at using a IRL520N or similar. Digikey has this part.

hgmjr

 

You could take a look at using a IRL520N or similar. Digikey has this part.

hgmjr

Correct me of wrong this allows a 5v at the gate so it can be run from my PIC

 

Correct me of wrong this allows a 5v at the gate so it can be run from my PIC

Absolutely.

hgmjr

 

Didn't you look at the datasheets for the Mosfets?
1) The tiny IRLD110 will melt if it has as much as 8.6A in it.
2) The IRL540 is very old and also has a fairly high on-resistance so it will also waste a lot of power.

There are thousands of better Mosfets. For example an IRFZ3711 has a max on-resistance of only 0.0073 ohms when its gate is only 4.5V so with 8.6A in it it will dissipate only 0.54W.

Can your little AC/DC adapter supply at least 8.6A? I doubt it. It will melt.

 

I think what audioguru is trying to say is that you need to be attentive to the Rds(on) specification as well as the gate voltage in order to optimize the performance of the driver.

hgmjr

 

Absolutely.

hgmjr

Thankyou for your support ..so just replace the 4 transistors with 4 IRL520N.. im also assumming the the resistors will not be needed as you said the PIC can directly provide 5V(at each pin?)

 

Thankyou for your support ..so just replace the 4 transistors with 4 IRL520N.. im also assumming the the resistors will not be needed as you said the PIC can directly provide 5V(at each pin?)

That should work.

hgmjr

 

The IRL520 is also pretty old with a fairly high on-resistance. The motor is 0.7 ohms and the IRL520 is 0.27 ohms when it is cold (higher when hot) so it wastes at least 39% of the power that is supposed to go to the motor.

Better and newer Mosfets should be used.
Gate resistors should be used.

 

You need to start with a better power supply.

If you can find a used ATX form factor computer power supply and convert it to a bench power supply, you will have plenty of power at much closer to the correct voltage that you need.

I converted a little 250W Compaq ATX power supply to a bench supply. It can output 14A at 3.3v, 25A at 5v, and +12 at 8A. It also has -12v and -5v available.

If you use the 3.3v output, you can use a power resistor to reduce the current like I calculated before.

Also, in your schematic, you do not show the connection to the motor. I hope you realize that you need diodes from each motor winding to the +V supply. Without that, your transistors will be subjected to a large reverse-EMF pulse when they turn off, and it will probably destroy them.

 

I checked Digikey and they carry the IRL3714 which has an Rds(on) of 0.02 ohms. That should be as better choice than the IRL520.

hgmjr

 

hey all thanks for the responses.. well if i measured the coils properly... each showed me 0.3 ohm in X1k meter.. I will get a few IRL3714 and IRF3711 and test them out...Take account of adding diods to the circuit...the power supply may be a problem for me,No knowledge about them at all...a bit busy but will try to update asap

 

Sgtwookie's suggestion of using an ATX supply is probably the least expensive route. AC to DC power supplies can get a bit expensive in the 100W range.

hgmjr

 

Sgtwookie's suggestion of using an ATX supply is probably the least expensive route. AC to DC power supplies can get a bit expensive in the 100W range.

hgmjr

I could not get any IRL520's so i decided to go with IRF540..This can also be run rom my PIC since the gate voltage is ± 20 and its drain is 22A,
0.055 W

 

The IRF540 is an ordinary old Mosfet that needs 10V on its gate to completely turn on. With only 5V from the micro-controller then it won't or it will barely turn on.

 

The IRF540 is an ordinary old Mosfet that needs 10V on its gate to completely turn on. With only 5V from the micro-controller then it won't or it will barely turn on.

seems that i have to keep searching for other ones then, thank you for noticing

 

In very general terms, International Rectifier and Vishay power MOSFETs start with:
IR
If the 3rd character is an "F", it's a standard-level MOSFET that requires Vgs=10 to turn on.
If the 3rd character is an "L", it's a logic level MOSFET.

This is not 100% true, but it's a good way to narrow down your search.

Also very generally, a power MOSFET with a low Vdss (say, 20v-30v) will have a much lower Rds(on) for a given total gate charge than one with a higher Vdss.

 

In very general terms, International Rectifier and Vishay power MOSFETs start with:
IR
If the 3rd character is an "F", it's a standard-level MOSFET that requires Vgs=10 to turn on.
If the 3rd character is an "L", it's a logic level MOSFET.

This is not 100% true, but it's a good way to narrow down your search.

Also very generally, a power MOSFET with a low Vdss (say, 20v-30v) will have a much lower Rds(on) for a given total gate charge than one with a higher Vdss.

Thanks, this should help me narrow down my search at a great extent