n-chanel enhancement mode power mosfet

[Goatman]

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 

 

[audioguru2]

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.

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 

 

[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

 

[Goatman]

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.

 

[audioguru2]

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.

View attachment 36941

 

[audioguru2]

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. View attachment 36942

 

[Goatman]

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.

 

[Goatman]

ok, I hooked up a potty and adjusted untill it went to 1/2 vout. the output resistance of that IC is approx 800ohms

is that enough to drive a 3055 without blowing it up?

 

[audioguru2]

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

View attachment 36949

 

[Goatman]

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

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 

 

[audioguru2]

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

 

[Alun]

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

 

[audioguru2]

Hi Alun,
You circuit has lotsa gain!
You could mount a propeller above the hot 47 ohm resistor and use it to propel the car. ;D

 

[audioguru2]

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]

 

[audioguru2]

He, he. I'm wrong!
Leave them running for a while. Notice how they lose their sync? ;D
[move]1

 

[Goatman]

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).

 

[audioguru2]

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.

 

[Goatman]

yea, that's the way switch mode powersupplies work AFAIK.

Thanks, you've saved me destroying the main IC of that RC car. 

 

[audioguru2]

Hi Goatman,
You're welcome. ;D

..... after a few days of totally insane speeds.

Haven't you burned-off the tread on the tires yet? ;D ;D