PWMPAL and MOSFET

tehmoordar

hi,
I am using pwmpal and MOSFET 2N700 but I feel since my motors are consuming more than 5 amps these MOSFET are getting very very hot, what all options do I have. Can I use some other MOSFET having higher current capacity,
can I use power MOSFET ( IRF511).
Or suggest me some other good MOSFETs which can take more than 5 amps
thanks

Jon Williams

You may want to give the IRL520 a try.· We carry it for your convenience:

http://www.parallax.com/detail.asp?product_id=604-00039

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Jon Williams
Applications Engineer, Parallax

Jonathan

One thing to remember is that a heavy heat sink will help a lot when pushing components. I have a project that draws 1.2 amps from a voltage reg designed for a max of 1A. I have a large heatsink, and it has run for years without problems. I'm not reccomending pushing things like that, just trying to say that a good heat sink goes a long way.

Jonathan

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
www.madlabs.info - Home of the Hydrogen Fuel Cell Robot

KenM

I would like to add one point to the heat issue. If I read the correct spec sheet, the 2N700 is good for a drain current of 500mA pulsed. Far less than·the 5 amps you may get to when near 100% duty cycle.

Even with a low on resistance, such as the IRL520 the device·might still become quite warm. It depends on how·"clean" the signal is driving the transistor.

When the transitor (mosfet·or BJT) is·ON·very little heat is developed in the device. The same goes for when the device is OFF.

In the·transition period from OFF to ON and visa versa, heat will be developed. Do you have an o-scope to see the driving waveform?

Even without an oscope you can perform the following simple test to determine if the majority of heat generated is due to the switching transient or not.

Make the motor run at full speed by turning on your mosfet and let it run at full speed for about 5 minutes. Does the mosfet still become very hot? If not, that would confirm the majority of heat is generated in the transistion period.

How do I know this? I had this issue occur using a low on resistance Mosfet. With a crappy drive circuit the device was quite hot. Improved the drive circuit, less heat. On continuous....almost no heat.

tehmoordar

Hi,
for the time being I am using MOSFET IRF511 with heat sink on.
Now another problem , I am usin the attached code to run two motors. When I run the code and download into BS2 it starts runing the motor. Now it is saved in the BS2 but once I switch the BS2 off and turn it on after few minutes then it does not execute the code which is stored in the BS2.
I have to run again from the editor.
If I turn BS2 off and switch on againThe debug terminal screen is there I can see the motor numberes increasing but motors does not run until or unless I run the program again.

I have increased the voltage supply from 9 to even 10+ Volts but still facing the problem

Whats going on.
Thanks

Jon Williams

According the the IRF511 spec it is good for 5.6 amps ... are you pushing it? In your original post you stated that your motor is drawing more than 5 amps.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Jon Williams
Applications Engineer, Parallax

KenM

Have you confirmed the mosfet gate voltage during the time the motor should be running?

tehmoordar

Hi,
I am exactly using the same circuit which is given in the help documents of PWMPAL on the parallax.com website (NUTS & VOLTS).· So I dont thinkso I am pushing it I have just connected motor between "drain of MOSFET and Vdd on BS2 (BOE). Vdd is 5Volts.

I dont know how to confirm gate voltage during motor runing.How to confirm that and what effect would it have on code.

·I· measured voltage across motor terminals which were showing me like 2.8Volts. Iam using motor with these specs (see attachment) I am using RM2 motor. I dont thinkso this is correct voltage I beleieve it shoul be near 5V.

But why BS2 is not runing once I try to run it again.

Thanks for the continous help

KenM

tehmoordar,

The gate voltage does not affect the code. When something does not work certain steps are taken to diagnose the problem, and that is why I asked about the gate voltage.

One of those steps would be to see if the mosfet is receiving a signal or not. It appears that there is a signal to the motor based on your 2.8 volt reading.

tehmoordar said...
I dont know how to confirm gate voltage during motor runing.How to confirm that and what effect would it have on code.

·I· measured voltage across motor terminals which were showing me like 2.8Volts. Iam using motor with these specs (see attachment) I am using RM2 motor. I dont thinkso this is correct voltage I beleieve it shoul be near 5V.

But why BS2 is not runing once I try to run it again.

Thanks for the continous help

tehmoordar

hi,
2.8 Volts I measured once motors were runing. Now I switch off the BS2 but my debug terminal screen was still there which was definitely froze now I again I turned on my BS2 after few mins ( not immediately)·the debug terminal started runing as usual but motor was not responding at all. Therefore I have to run again from th editor.
If I immediately turn on my BS2 then motors respond normally but not if I let BS2 stay off for while.
Thanks

Tom Walker

At first glance, that sounds like (OK, mixed metaphor) the gate capacitance is keeping the MOSFET in an ON state which is why it works if your Stamp is only off for a little while. After a time with your Stamp turned off, this gate capacitance discharges and there is insufficient voltage to turn it back on. Without seeing your code and circuit, everything is just educated guesses....

FWIW, once you have fixed the problem, your circuit will probably end up with a resistor between the MOSFET gate and ground to drain off this capacitance and speed up the ON/OFF transition of the MOSFET.

Good luck!

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Truly Understand the Fundamentals and the Path will be so much easier...

tehmoordar

hi

my code is attached and circuit is very simple it is the one which is mentioned in PWMPAL (nut and volts) example on the parallax.com website.

thanks

tehmoordar

hello,
I am still waiting as to why my volts are only 2.8 V and why not it is 5v and how can increase the speed of my wheels and why BS2 is not working by it self
thankyou