n-chanel enhancement mode power mosfet

[Goatman]

the tires and motor are just fine. no funny smoke... yet
i found this car in the trash (had a blown cap) lol.

btw, i hate op-amps... the 741 is a menace to get working. sometimes it does sometimes it doesn't. i've tried various ones from differet manufacturers.  damn common mode gain aint helping either lol.

 

[Alun]

By the way while a reverse diode in paralell with a relay is required  but you don't always need on for a DC motor, since when the motor is spinning at full speed the back EMF produced is the same value as the power supply, this is even more true for smaller motors, especially if the MOSFET you're using has a high voltage rating.

The 741 sucks - it's high noise and high power consumption try TL071, TL081 or even LF351.

 

[audioguru2]

Hi Alun,
You are talking about JFET-input opamps and Goatman mentioned common-mode.
Did you know about the cool trick that most JFET-input opamps can do?

1) Make an opamp follower with the output connected to the (-) inverting input and the oscillator (or speech or music) connected to the (+) non-inverting input.
2) Slowly turn-up the input level but don't exceed the p-p of the opamp's power supply voltage.
3) When the negative swings of the input exceeds the opamp's negative common-mode limit (about 1V to 3V above the negative supply voltage) then the opamp suddenly inverts!

The same thing occurs if the non-inverting opamp circuit has two negative feedback resistors for gain. ;D

View attachment 36954

 

[Alun]

This is bad as oscillation could occur given the right (or wrong  ;D) circumstances.

 

[audioguru2]

Well I don't think it would oscillate.
This form of gross overdrive causes the signal to sort of fold over on itself.
Look at it on a 'scope or listen to it happen. It's weird! ;D

 

[Goatman]

I'll just go with my best friend (bd139). never failed me. i'll connect the relay between the collector and the supply. no worries!

At tafe (turtiary and further education) we had a few practicles on setting the offset null on 741s. 9.5/10 times out of ten they don't work. lecturers blame it on our connections. year right. all the bjt amps we build work lol.

one time the +ve supply pin got shorted with the negative input internally inside the op-amp. (-ve and +ve inputs were grounded to set the offset null). with a 15V supply a hole got burned between the +ve supply and the -ve input. i left it on, it was so hot i could melt solder with it easily. took about 2 minuts to finally go open cct. it melted my breadboard, lol. lecturer still blamed it on my connections.  :

when i was a kid my dad bought me an elecrical kit where u have all the components in a piece of cardboard with small springs as tyerminals and all u gotta do is connect up the wires like it says in the manual. It came with a dual op-amp 8 pin IC which i have taken out recenlty. the thing NEVER FAILS ME, everytime it does everything i want it to do. Sad it's only marked with 728 and that's it. i tried googling it. no-go.

 

[Goatman]

Here's some pics of the completed car. It's literally too fast for it's own good  ;D
I used a rellay and a bd139. the pcb is glued with hot glue to the frame of the car.
all the lifted pads are from the original 4 mosfets which controlled forward, and reverse. with 1 relay i can only have forward. I don't want reverse, because if i don't press reverse or forward i can't steer (remote controll problem). So i just use reverse for steering when i'm not trying to break the sound barrier  .

to sum it up, this is a complete success. Thank you all for helping me out

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[Goatman]

more pics

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[audioguru2]

Hi Goatman,
It's great that you got it working well. The sound barrier is more than 700mph!
I like the tie-wrap that holds a wheel on. ;D
I see heavy wear on the tires, they'll need replacement soon.

 

[Goatman]

The tie wrap used to hold it together but i glued the tire on instead. I should remove it, but it adds ghetto-ness lol.
I think the tires wil be fine (hopefully) if they wear out it think i'll just buy a new car  . I've tortured this one enough .  ;D

 

[Goatman]

n000 i fried the motor. It started to smoke, drove at close to full speed for 2 metres or so then smoked heavier, slowed down then stopped alltogether. It was good while itm lasted.
I'm buying a new motor today tho. Also rated at 12V I can't find any motors the same size that are rated at a higher voltage

 

[audioguru2]

Then the car's motor or drivetrain will blow.

I told ya so!! ;D ;D

 

[Goatman]

Ok i've installed this motor
http://www1.jaycar.com.au/productView.asp?ID=YM2716&CATID=&keywords=motor&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=
the problem is that this motor is about half as powerfull as th old one. Car goes as slow as it did before the high voltage mod.
Where can i get a more powerfull motor. The the drivetrain is pretty low, when driving down the hill it is little faster than on flat land, so i need more RPM than i need power.

I could get this 6V motor which does 9k rpm at 6v, so at 12V it would do 18k rpm roughly
http://www1.jaycar.com.au/productView.asp?ID=YM2712&CATID=&keywords=motor&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=

Any suggestions?

 

[audioguru2]

You can't use the motor's high RPM unless your gearing matches it. You need a motor with high torque at the RPM it will be running at in your application.
If you use a 6V motor with a 12V supply, its voltage and current are both doubled so it tries to produce 4 times its rated power. Guaranteed burn-out like your other motor.

 

[Alun]

I agree with you completely audioguru,

But whether the motor burns out depends on the loading as well as the power supply. If you run a 6V motor off load at 12V it woudn't dissipate 4 times the power, because as the motor spinns faster it genterates more back EMF that counteracts the supply, the power consumption will go up but not as much as you think. So you can safely overvoltage a motor if the load is very light. In this application however as the motor speed increases the load will also increase due to increased air resistance so over-voltageing the motor is a very bad idea and it will cause it to overheat.

 

[audioguru2]

I wonder how many overvolts it takes to over-rev an unloaded motor to make it fly apart in pieces. ;D
These little motors draw more than 5A when stalled. At 12V is that enough heat to fry my eggs for breakfast? Nah, the smoke would ruin the taste!

 

[Alun]

I've just done an experiment with an old electric screwdriver, the batteries are dead so it might be srapped anyway.

It's rated for 3.6V

Off load just the motor on it's own not driving any gears:

3.6V 880mA.
7.2V 1050mA

3.6V with just the gears and no additional load, 2020mA.

I haven't tested it with a large load like tightening a screw, but I'd imagine it will draw a lot more than 2A, probably 3 or 4A.

Conclstion, running this motor at a higher voltage would be fine if the load is also reduced.

 

[audioguru2]

I have a no-name-brand electric screwdriver that is over-geared so much that it takes it forever to drive a screw into wood. But when the screw is tight, the thing wrenches my arm off. They should have an automatically variable transmission like a few newer cars have. ;D

 

[Goatman]

I'm gonna try this motor today
http://www.dse.com.au/cgi-bin/dse.storefront/427aa0ae0272ef082740c0a87f9c0719/Product/View/P9004
it has more rpms and more torque @ highest efficiency.