Mosfet problems

[Agent36]

Hi,
I have been trying to control a 12 volt DC car wiper motor, with the load it it draws 2 amps.
I have written a program on a 12f683 using HPWM which works fine. The N channel mosfet (STP36NF06) is causing me a few problems. With this mosfet connected to the pic with a 330 ohm resistor in series with the pic and a 470K ohm resistor connected between the HPWM pic pin and ground the mosfet does as it should. I have also put a diode across the Drain and source (reverse biased) to limit the back emf of the motor. When the terminals connected to the motor are shorted out the thermal fuse (trip current 2.5A) cuts the power but the mosfet shorts out either across gate drain and source or just the gate and source. For test purposes I am running the motor from a 7 Amp hour battery when I short the thermal fuse with my meter reading the current it trips in about 2 seconds to mA's. Yet this still damages the mosfet when the short is tested on the circuit. The mosfet is rated at 30A and 60 volts, has any one had this sort of problem or knows of a way to over come this problem? any help would be much appreciated.
Nick

[Luciano]

Hi,

A thermal fuse is slow. The current peek will be very high before
the thermal fuse triggers. (Lead Battery short-circuit current > 100A).

Poor-man's solution:

In addidion to the thermal fuse, put a 0.47 Ohm/5W wirewound resistor
in series to the thermal fuse. (RadioShack Catalog part #: 271-130).

The resistor will limit the short-circuit current to:
14V / 0.47 Ohm = 29.78 A

Drawback: The resistor will dissipate about 2 watts of power during
normal operation of the motor. (Motor current 2A).

Best regards,

Luciano

[BrianT]

A 12 volt 50 headlight watt globe will allow the motor to run at pretty much full speed and also function as a protection device. The inrush current will be high but it can allow you to fault find easier than replacing a bag of fuses.

How many wires go to the motor?

I blew up lots of switching transistors when trying to make an intermittent control for a windscreen wiper on an old car back in student days. It turned out the wiring of the motor placed a dead short across the armature to provide a dynamic brake and park the wiper at the same spot every cycle.

HTH
Brian

[Agent36]

Hi,
Thanks for the replys, I have removed all the wires from the motor except the positive and negative. The motor runs fine during normal operation via the mosfet, I am planning to build 100 plus of these so not sure a light bulb would be a good idea. I will use that trick for fault finding in the future to save changing fuses/ mosfets all the time.
I was thinking of using a spare ADC channel on the pic to measure the voltage between the thermal fuse and the motor, if the fuse starts to heat up as it does when it trips the resistance will increase causing voltage to drop across it, this would cause the ADC to sense more voltage at that point. I could add some code to compare the ADC value to a preset value in the program, if this value was exceeded the fet would be turned off. Does any one think that I could acheive this in time to stop damage to the Fet? The whole program at the moment takes about 100ms to execute one cycle.
regards Nick

[Luciano]

Hi Nick,

Use a Logic level PowerMOS transistor BUK101-50DL or BUK101-50GL.
The BUK101 has built-in overload protection against short circuit load.

(Logic level = You can drive it directly with an I/O of the PIC).

Datasheets:
http://www.nxp.com/acrobat_download/datasheets/BUK101-50DL_1.pdf
http://www.nxp.com/acrobat_download/datasheets/BUK101-50GL_1.pdf

Best regards,

Luciano

http://img238.imageshack.us/img238/2024/buk101mri6.th.png (http://img238.imageshack.us/img238/2024/buk101mri6.png)
(Click to enlarge the picture)

[Agent36]

Hi,
Thanks for the info on the mosfets, I will try and find a stockist in the uk. My usual stockists RS and RAPID dont seem to hold them.

Kind regards Nick

[Luciano]

Hi Nick,

www.conrad.com (Select UK flag)

BUK 101-50 GL
Part no.: 157996
£4.69

Note that the BUK100-50GL will also work. (13.5 A)
Part no.: 157988
£1.50

BUK100-50GL Datasheet:
http://www.nxp.com/acrobat_download/datasheets/BUK100-50GL_1.pdf

* * *

Make sure you don't buy the "GS" version because the input
it is not logic level 5V.

Best regards,

Luciano

EDIT:

See this link:
http://www.donberg.ie/descript/b/buk_101-50gl.htm

[Agent36]

Hi Luciano,
Thanks for taking the time to post the links to that website, I was just about to get a copy of Every day practical electronics out and check the listings for a stockist of those fets, you have saved me the trouble. I will be ordering a few to test in my application. Out of interest what frequency would be best for DC motor control? At present I have set the HPWM frequency to 7000Hz to remove the noise from the motor windings. I am sure that I have read somewhere that the frequency of the mark to space ratio has a direct effect on the torque and efficiency of the motor.

Thanks for your help. Kind regards Nick