Driving MOSFET from CMOS gate - powering 1A fan

[konradIC13]

Hello,

Im making an electronic circuit. Its purpose is to switch on 5V DC fan when both of my switches are on and turn off the led when both of my swithes are off.

The circuit is powered from Li-po 7.4v battery pack (voltage range 6.3v nearly discharged and max 8.4v fully charged). Then the voltage is stabilised at 5V by 7805IC.

Two SPDT switches and 4011IC are preventing connector bouncing and they distribute low/hi signal to logic 4081 and 4071 devices.

Next the 4081IC is responsible for turning on fan. I have read somewhere that the 4081 output signal is too weak to turn on MOSFET transistor so i put a BC548 transistor there and the nthe BC548 transistor is driving an n-MOSFET IRF710 transistor which turns on the 5V fan (fan max current is around 1A)

The other 4071IC is driving 2n2222 transistor which is responsible to turn on and off Blue LED (3V voltage drop, 22mA current).

So far i have only simulated the circuit in my PC but before ill try to do anything 'in real' i would be gratefully if someone with bigger experience could verify my schematic and check if i have correctly attached transistors if i didnt missed anything. Im not sure if they transistors are right ones and if they will function properly (if the voltages/currents will be enough to turn them on and not big enough to melt everything). Also are the capacitors values good enough for 7085 stabiliser (i took them from example uses from is datasheet)?

Thank you very much for any help in advance.

ps. i also heard that IRF3711 transistors could be useful here and would be able to turn on the fan when attached directly to the DC4xxx output an they can power a fan with 1A current. Is it true?

 

[Misterbenn]

No need for Q3 you can drive Q2 directly.

Also put a freewheeling diode in parallel with your motor.

 

[alec_t]

If your motor draws >800mA then a 7805 will be working near its maximum rated current (1A). Although it has internal protection against overload it would be better to use a regulator with a higher current rating.
An IRF710 is not suitable. You need a FET with a low turn-on voltage and a low Rds(on). The IRF 3711 would be ok but is maybe over-specified for the task. It would be drivable directly from CMOS logic providing you did not want to switch the motor on and off repeatedly at a high rate.
You must include freewheel diodes to protect the FET from back-emfs when the FET switches off.
The cap at the output of the regulator appears not to have its negative leg grounded.

 

[konradIC13]

Hey thanks for answers, in meantime i did some google research. I would like to avoid using IRF 3711 (they are 'quite' expensive in where i buy my components).

I have found out a an intresting nMOSFET BUZ11 ( http://www.datasheetcatalog.org/datasheet/fairchild/BUZ11.pdf ) it has Rds(on)=0.04 OHM

As voltage regulator ill use 78S05, its 2A (http://pdf1.alldatasheet.com/datasheet-pdf/view/22621/STMICROELECTRONICS/L78S05C.html)

I put the diode parrarel to the fan, and corrected the capacitor leg. How does it look now?
ps. And the diode switching part is good? 2N2222 will be enough for it?

 

[konradIC13]

Oh, silly me, here is updated curcuit:
View attachment 64926
Ps. My previous post with changes i made is avaiting moderator approval since i posted links to datasheets there ._.

 

[Misterbenn]

I've only looked quickly but seems good to me. I'd recomend sperating your motor drive and logic power supplys, the high current draw of the motor is likely to pull your voltage rail down a bit and put noise on the power lines. If you can't do this then a few extra caps close to each of the logic chips should help.

 

[konradIC13]

I think that making separate power line for IC chips is not an option since i think that it would require another voltage regulator or even another power supply?
How high value the capacitors should have?

 

[Misterbenn]

It would need another regulator, or you could just run the motor from the non-regulated supply.

I'd add 1uF and 10uF at the regulator and 1uF at each of the logic chips that should remove quite a lot of voltage distortion due to the motor.

 

[audioguru]

A BUZ11 is not a low voltage Mosfet. It is spec'd when it has 10V from gate to source. You need a "logic-level" Mosfet that is spec'd with an input of 4.5V to 5.0V.

You also need a low-dropout voltage regulator because an ordinary 5V regulator stops regulating when its input is less than 7.0V.