pwm 2amp

Tobias

I am having a problem saturating a power transistor, I will be controling it with a pwm command on my bs2 and help will be helpful.
Thank You
Toby

metron9

You must specify the transistor.

I can give a wild guess that the PWM is too fast and another wild guess that the current is not high enough to saturate the transistor.

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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

Tobias

I am using a tip41 or a tip120, I am also using a npn transistor to run the power transistor hooked up in a darlington circuitry. How is the right way to saturate a transistor?
I've got a 2 amp coil and trying to control it with a pwm signal, I've got the diode for the reverse spike. Thanks for your reply Toby

Phil Pilgrim (PhiPi)

The "PWM" command in PBASIC doesn't generate a pulsewidth-modulated output suitable for saturated switching of an inductive load. It is, rather, a duty-cycle output that [noparse][[/noparse]from the manual] "... outputs a rapid sequence of on/off pulses, as short as 1.6 us in duration, ..." — in other words, much too fast for a Darlington, which is notoriously slow to switch anyway. You would be better off with something that will generate a true PWM output, like Parallax's PWMPAL or the MoBoStamp-pe, whose onboard coprocessors provide up to four PWM channels.

-Phil

metron9

"How is the right way to saturate a transistor?"

The way you know the transistor is saturated is when the collector voltage drops down to the same as the base voltage.
Edit Above just something I read a few minutes ago.

I don't know how to calculate all the transistor stuff because I have not yet found the right reading material that will can actually get the information through my thick head. I do know though that you should post more information than you have. Voltages, currents and a schematic so someone who does can answer the question.

In the meantime have a look at this calculator page. I typically use fets so I don't have to try and figure out how to use transistors. Some day i will find the right text to understand them I hope.

http://transistor.20m.com/transistor2.html

What is it you are controlling, what is the 2 amp coil? Why are you using PWM what is the circuit doing? Is it a motor? If you are energizing a coil the PWM frequency has to be slow enough to allow the current to get through, perhaps your frequency is too high.

This link has a bunch of transistor information.

http://www.kpsec.freeuk.com/trancirc.htm

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

Post Edited (metron9) : 12/18/2007 7:19:33 AM GMT

Tobias

I will look at the information you provided, Thank you much.. I have all the circuitry on my bench when I hook up a small 12 volt .25amp motor it works fine I can control my speed and the transistor does not get to hot, but when I replace the motor with a proportional coil that is used to control the flow of hydraulics, which is rated at 12 volt and 2 amps max the transistor turns realy hot after a bit.
Thanks
Toby

metron9

First how hot is too hot. What size heat sink are you using?

If you are operating at too high of frequency the transistor may be operating in its partially on state most of the time, when you use a .25 amp motor it can disapate the heat because it is small. When you go to the 2AMP motor it can't disapate the heat because of various reasons.

Heat sink

Transistor operating in partially on state

Here is a link to a page where the author uses a TIP120 and PWM to control a motor and goes over the math needed to operate the transistor in its saturated state.

http://www.learn-c.com/experiment5.htm

Can you run the motor at full speed without transistor getting too hot and it gets too hot at 1/2 speed for example? That could be too fast of PWM. The pulse turns on the transistor but it never reaches saturation before the pulse drops for example.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

Tobias

I have researched and was lead to use a mosfet IRF530 transistor to control the coil with a pwm command, will I be suprised or suprised, is it going to work?
Thank You Toby

Mike Green

You will run into a similar problem with the IRF530. A MOSFET requires a particular gate voltage to saturate (rather than a base current) and the saturation voltage is greater than 5V with the IRF series ... look at the datasheets ... they have a graph of gate voltage vs. drain current. The IRLxxx series of devices have a lower gate threshold and are designed to be switched by logic voltages. Always check the datasheets.

jmalaysia

With a .25a/12v load you are only asking the transistor to handle a 3 watt load. When you step up to the 2a/12v load you jump to 24 watts, which is 8 times the load and 8 times the heat. The TIP120 can handle 5a, so that's not a problem, but certainly it will get warm, even with a heat sink. Further, the less "on" the transistor is, the higher it's internal resistance is when compared to the load, meaning if it's not fully biased on then it will be dissipating more heat than required.

Tobias

Any solutions? Do you think the IRL series will work better then the IRF?

Tobias

···· I've been playing with this project for a while, I finally got a hold of some mosfets IRF540, I ran a 40ma 12v motor at pwmout 2, 150, 180 to pwmout 2, 150, 600 and it worked good but when I put the coil on the circuit it does not go over pwmout 2, 150, 180, it seems like its reseting it's self. I've also noticed that it makes a difference what value in the command you use, what is the correct value to use for opening and closeing a 12v 2amp coil?

Thanks Toby