Sign in to follow this  
Goatman

n-chanel enhancement mode power mosfet

Recommended Posts

I attached 2 batteries in series to my RC car, blew the power mosfet after a few days of totally insane speeds.

I've tried to use a bjt transistor as the input voltage is about 1 volt. Didn't work. The output impendance of the car's circuit is so damn high it's not funny!

I'm looking for an n-chanel enhancement mode power mosfet with a really high input impendance and low turn on voltage.

or a nice little non-power mosfet that can turn a rellay on for me.

I have tried a motorolla IRF520 nch emode mosfet but that has both a too-low input impendance and a too high imput voltage.

any suggestions?

I am pretty sure the mosfet i blew up is related to the NEC C4305. I can't read the number off the one i blew up because it no longer looks like a transiostor at all  :o

Share this post


Link to post
Share on other sites

Hi Goatman,
Welcome to our forum. ;D


I'm looking for an n-chanel enhancement mode power mosfet with a really high input impendance and low turn on voltage.

All Mosfets have an extremely high input resistance but a fairly high capacitance. If the input must switch at a high frequency for pulse-width-modulation control of the motor's speed then adequate current must be provided to charge and discharge it. Most Mosfets need a gate voltage of 10V to turn on fully. A few require only 5V.

or a nice little non-power mosfet that can turn a relay on for me.

A small Mosfet like a 2N7000 also needs a high gate voltage. A relay can't switch quick enough for PWM control of the speed. Why not use a power Mosfet to drive the motor and a transistor or two to amplify the control voltage to feed it?

Did you use the double normal voltage (and therefore double the current) with an ordinary RC speed controller? Then the current was much too high for it. Replace the ordinary Mosfet with a more modern one that has a very high current rating. Then the car's motor or drivetrain will blow. :o

I've been there, done that! My real car's turbo got stuck for a few days. It produced double the normal power for totally insane speeds. May that car rest in peace. ;D 


Share this post


Link to post
Share on other sites
Guest Alun

audioguru,
The 2N2000 only very low gate voltage, 3V is enough to allow an Id > 500mA the maximum pulse current rating.

http://www.supertex.com/pdf/datasheets/2N7000.pdf

post-0-14279142205186_thumb.gif

Share this post


Link to post
Share on other sites

The motor is rated at 12V, I used 2x7.2V batteries. 14.4V won't hurt it.

If i short the drain and source pins the motor will go. so no high speed switching found here.

I need a mosfet that will turn on with 1 or so volts.

I tried a unity gain op-amp cct + BJT. didn't work.

I don't have a circuit to show u guys because i didn't design it.

AFAIK, there is a 6-7V (off-on) output from an IC which goes to a transistor (totally unknown). That transistor outputs 0.3-1.2 (off-on) to where the gate of the original mosfet used to be.

Share this post


Link to post
Share on other sites

Hi Alun,
You're relying on typical values for design spec's.
Ron Mancini, a scientist at Texas Instruments agrees with me in his recent article titled, "How to read a semiconductor data sheet" in EDN here: http://www.edn.com/article/CA514964.html?industryid=2281&nid=2433

He says, "Typical values can get novice or naive designers in trouble because they show desirable parameter values that the IC cannot dependably deliver. There is a tendency to lust over typical values even when the data sheet gives minimum/maximum values, and this path can lead straight to an unreliable design."

The 2N7000 Mosfet actually has a very high maximum gate voltage at a very low output current. The scientist above and I design circuits using the minimum and maximum spec's to be certain that all circuits with them work.

post-1706-14279142205457_thumb.gif

Share this post


Link to post
Share on other sites

The motor is rated at 12V, I used 2x7.2V batteries. 14.4V won't hurt it.

44% more than its power rating doesn't sound like it is good for the motor.

If i short the drain and source pins the motor will go. so no high speed switching found here.

Doesn't it have a variable speed control?

I need a mosfet that will turn on with 1 or so volts.

Impossible. Most Mosfets are spec'd with a 10V input.

I tried a unity gain op-amp cct + BJT. didn't work.

I think that the transistor connected to your blown Mosfet is another blown Mosfet that was in parallel with it.

I don't have a circuit to show u guys because i didn't design it.

I have seen schematics on cheap RC car manufacturer's websites and yours might also be able to be found.

AFAIK, there is a 6-7V (off-on) output from an IC which goes to a transistor (totally unknown). That transistor outputs 0.3-1.2 (off-on) to where the gate of the original mosfet used to be.

I have never seen a transistor that is off with a 6V input and on with 7V. I think that the voltage goes near 0V for off. Then the unknown transistor is a Mosfet that is also destroyed by your over-current. A Mosfet at the output needs a lot more than only 1.2V on its gate to turn on.

post-1706-14279142205579_thumb.gif

Share this post


Link to post
Share on other sites

ok. turns out that unknown transisor had a terminal going to the reverse controlling mosfet which isn't there anymore (I don't intend to replace it).
If i get rid of that transistor, the IC is outputing 2.2V (on, 0V off) when Vbatt is 8V, 2.0V (on, 0V off) when Vbatt is 16V The Ic's VCC runs off a sepparate 9V battery.
I'm gonna measure the output resistance of that IC to see if it can run a BJT. if it can, i'll hook up a 3055 and see what happens.

Share this post


Link to post
Share on other sites

Hi Goatman,
If the voltage went to half the supply when the transistor was powering the motor then it might be getting very hot.

Just look at the datasheet for the 2N3055 and use arithmatic to see if it will blow up:
1) You don't say how much max current the motor uses, but assume it is 10A. The base current into a 2N3055 could be as high as 2A. Therefore the 800 ohm output resistance of the IC driver would need a 1600V power supply! A 2N3055 that operates at a high current is usually driven by another power transistor.
2) With a 10A load and a whopping 3.3A base current, a 2N3055 could cause a voltage loss in the circuit of 3V. Mosfets create a much lower voltage loss.
3) With only a 4A load, the base voltage of a 2N3055 could be as high as 1.5V when driven by a very low-resistance source. With a 10A load the base voltage will be much higher.

With an NPN power transistor driving the base of a PNP output transistor, the input voltage will be much lower. Better yet is an NPN small transistor for its low input voltage and current requirement, driving a P-channel output Mosfet.  ;D

post-1706-14279142206255_thumb.gif

Share this post


Link to post
Share on other sites
With an NPN power transistor driving the base of a PNP output transistor, the input voltage will be much lower. Better yet is an NPN small transistor for its low input voltage and current requirement, driving a P-channel output Mosfet.

like one of those darlington pair things minus the extra 0.7V drop? HFE=HFE1*HFE2 :D

I was thinking thesame thing. if the HFE was say 400 i don't think i would have a problem.

I've decided to get a relay. All i gotta do is hook up an opamp with say a gain of 20 and the output will hit supply rails no problems. I've just learned that when the 9V battery is a bit flat that 2V can drop down to just below 1V. with a gain of 20 i can't loose!. surely the 50 or so ohms outa an opamp will drive a relay. With a relay i get no losses as i would with BJTs or MOSFETs.

AFAIK relays are quite tollerant.  The relay i'm using to turn the pumps on for my computer's water cooling is rated at 12V, it turns on at 5V  :o

Share this post


Link to post
Share on other sites

A modern Mosfet works the same or better than a relay. It conducts better than the connection wires and battery's internal resistance.
Make sure you connect a reverse-bias diode across the relay's coil to arrest flyback voltage spikes.
Relays are guaranteed to activate at 0.75 of their coil's voltage rating.

Are you an over-clocker? No wonder you use double the battery voltage for your RC car.
My 2.93GHz Pentium 4 has its fan running very slow most of the time. When it is doing some heavy number-crunching I hear the fan speed-up a bit. If I ever hear it running faster I'll just clean-out the dust. ;D

Share this post


Link to post
Share on other sites
Guest Alun

This transistor buffer has a very high inpuit impedance, low output impedance and a very high gain.

post-0-14279142206369_thumb.gif

Share this post


Link to post
Share on other sites

BTW, somewhere you posted a coloured statement that didn't move. I think the moving command should be the final one.
This image is "marquee'd" then coloured:

[move]Move!                Move!                Move!                Move!                [/move]

This image is marquee'd last:
[move]Move!                Move!                Move!                Move!                [/move]

Share this post


Link to post
Share on other sites

yea i could do that buffer idea. i could use a bd139, bd140 and a 3055. I think i'll just go with a relay tho. cheap and easy. I doubt silicone could conduct better than copper/steel connections inside the relay. Before the mosfet blew up it lost 3 or so volts across it anyways.

Make sure you connect a reverse-bias diode across the relay's coil to arrest flyback voltage spikes.

Is that because of the inductive nature of the motor?

Are you an over-clocker? No wonder you use double the battery voltage for your RC car.
My 2.93GHz Pentium 4 has its fan running very slow most of the time. When it is doing some heavy number-crunching I hear the fan speed-up a bit. If I ever hear it running faster I'll just clean-out the dust. Grin

Hell yea, i have an athlon 3000+ (1.8GHz) at 2.6GHz which is about as fast as a 4000+. a 4000+ actually runs at 2.4Ghz but it has double the L2 cache (1024kb).

Share this post


Link to post
Share on other sites

Is that because of the inductive nature of the motor?

I don't think the relay contacts will care about inductive spikes from the motor.
Whatever drives the relay coil could easily be destroyed by the hundreds of volts produced by the high inductance of the relay's coil when it is switched off. Hold the bare wires connecting a relay's coil to a 9V battery. Keep holding the relay coil's wires while you disconnect the battery.
He, he. ;D The transistor that would be driving that coil will yell, OUCH! louder than you.
The transistor or IC coil-driver will be destroyed without a diode connected in reverse across the coil.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites
Guest 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.

Share this post


Link to post
Share on other sites

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

post-1706-1427914220697_thumb.gif

Share this post


Link to post
Share on other sites
Guest Alun

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

Share this post


Link to post
Share on other sites

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. :(

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this