Mosfet switch issues
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Posted by Jon Slaughter on October 30, 2007, 4:18 pm
Please log in for more thread options I setup a mosfet to switch power to a pin on a PIC. The circuit works only for low R. Gate voltage is about 3.5-4.5V. Everything works fine with R about 2k but with 4.7k or higher it does not. Vcc is 5V. Why would this be the case? Surely 4.7k or even 10k isn't all that high in this case? The pic(link to pic attached to the drain) is a schmitt triggered input. So it shouldn't be drawing all that much current? I'm not sure how to interpret the datasheet but it has a section for timing conditions and a schematic for "load conditions" with a pullup of 464 ohms and bypass cap of 50pf. I imagine that if the capacitance of a pin is about 50pf, and lets say I double it to 100pf, then a resistance of R = 4k gives me a 400ns rise time and 1k is obviously 100ns. Surely both are reasonable? Here I'm neglecting the speed of the parallel port but I imagine its independent of what R is? So what I'm really interested in is what is going on? What am I not taking into account that is causing timing issues? I imagine it might be the capacitance but I'm not sure. I just can't see why I would need to supply so much current to something let a mosfet gate which is what I imagine the input pins are. (I realize that for high speed switching it requires more current but its a schmitt trigger so I should be able to run it as slow as I want?) Thanks, Jon | ||||||||||||||||||||||
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Posted by ian field on October 30, 2007, 5:49 pm
Please log in for more thread options >
> I setup a mosfet to switch power to a pin on a PIC. The circuit works only > for low R. > > Vcc ---> R ----> Drain + link to pic > Source ---> GND > Parallel port --> Gate > > Gate voltage is about 3.5-4.5V. > > Everything works fine with R about 2k but with 4.7k or higher it does not. > Vcc is 5V. > > Why would this be the case? Surely 4.7k or even 10k isn't all that high > in this case? The pic(link to pic attached to the drain) is a schmitt > triggered input. So it shouldn't be drawing all that much current? > > I'm not sure how to interpret the datasheet but it has a section for > timing conditions and a schematic for "load conditions" with a pullup of > 464 ohms and bypass cap of 50pf. > > I imagine that if the capacitance of a pin is about 50pf, and lets say I > double it to 100pf, then a resistance of R = 4k gives me a 400ns rise time > and 1k is obviously 100ns. Surely both are reasonable? > > Here I'm neglecting the speed of the parallel port but I imagine its > independent of what R is? > > So what I'm really interested in is what is going on? What am I not taking > into account that is causing timing issues? I imagine it might be the > capacitance but I'm not sure. I just can't see why I would need to supply > so much current to something let a mosfet gate which is what I imagine the > input pins are. (I realize that for high speed switching it requires more > current but its a schmitt trigger so I should be able to run it as slow as > I want?) > > Thanks, > Jon Did you select a specifically "logic level" type MOSFET? | ||||||||||||||||||||||
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Posted by Jon Slaughter on October 30, 2007, 6:40 pm
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>
>>
>> I setup a mosfet to switch power to a pin on a PIC. The circuit works >> only for low R. >> >> Vcc ---> R ----> Drain + link to pic >> Source ---> GND >> Parallel port --> Gate >> >> Gate voltage is about 3.5-4.5V. >> >> Everything works fine with R about 2k but with 4.7k or higher it does >> not. Vcc is 5V. >> >> Why would this be the case? Surely 4.7k or even 10k isn't all that high >> in this case? The pic(link to pic attached to the drain) is a schmitt >> triggered input. So it shouldn't be drawing all that much current? >> >> I'm not sure how to interpret the datasheet but it has a section for >> timing conditions and a schematic for "load conditions" with a pullup of >> 464 ohms and bypass cap of 50pf. >> >> I imagine that if the capacitance of a pin is about 50pf, and lets say I >> double it to 100pf, then a resistance of R = 4k gives me a 400ns rise >> time and 1k is obviously 100ns. Surely both are reasonable? >> >> Here I'm neglecting the speed of the parallel port but I imagine its >> independent of what R is? >> >> So what I'm really interested in is what is going on? What am I not >> taking into account that is causing timing issues? I imagine it might be >> the capacitance but I'm not sure. I just can't see why I would need to >> supply so much current to something let a mosfet gate which is what I >> imagine the input pins are. (I realize that for high speed switching it >> requires more current but its a schmitt trigger so I should be able to >> run it as slow as I want?) >> >> Thanks, >> Jon >
> Did you select a specifically "logic level" type MOSFET? ? Its 2N7000. Shouldn't any general purpose small signal mosfet work? It seems that the resistors are the key to getting it to work(in the sense that it doesn't work if R is to large). I can't understand why the resistance has to be so low in this case ;/ I can get the circuit to do its job but its taking about 30mA max(on average its probably only about 5mA I suppose) and that seems like way to much(To program a pic using mosfets) ;/ | ||||||||||||||||||||||
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Posted by w_tom on October 30, 2007, 7:53 pm
Please log in for more thread options > I can't understand why the resistance has to be so low in this case ;/ I can
> get the circuit to do its job but its taking about 30mA max(on average its > probably only about 5mA I suppose) ... Assuming the circuit is as described, then a 2N7000 gate is blown so that gate current flows into the Drain-Source channel. | ||||||||||||||||||||||
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Posted by Jamie on October 30, 2007, 8:39 pm
Please log in for more thread options Jon Slaughter wrote:
>
>>
>>>I setup a mosfet to switch power to a pin on a PIC. The circuit works
>>>only for low R. >>> >>>Vcc ---> R ----> Drain + link to pic >>>Source ---> GND >>>Parallel port --> Gate >>> >>>Gate voltage is about 3.5-4.5V. >>> >>>Everything works fine with R about 2k but with 4.7k or higher it does >>>not. Vcc is 5V. >>> >>>Why would this be the case? Surely 4.7k or even 10k isn't all that high >>>in this case? The pic(link to pic attached to the drain) is a schmitt >>>triggered input. So it shouldn't be drawing all that much current? >>> >>>I'm not sure how to interpret the datasheet but it has a section for >>>timing conditions and a schematic for "load conditions" with a pullup of >>>464 ohms and bypass cap of 50pf. >>> >>>I imagine that if the capacitance of a pin is about 50pf, and lets say I >>>double it to 100pf, then a resistance of R = 4k gives me a 400ns rise >>>time and 1k is obviously 100ns. Surely both are reasonable? >>> >>>Here I'm neglecting the speed of the parallel port but I imagine its >>>independent of what R is? >>> >>>So what I'm really interested in is what is going on? What am I not >>>taking into account that is causing timing issues? I imagine it might be >>>the capacitance but I'm not sure. I just can't see why I would need to >>>supply so much current to something let a mosfet gate which is what I >>>imagine the input pins are. (I realize that for high speed switching it >>>requires more current but its a schmitt trigger so I should be able to >>>run it as slow as I want?) >>> >>>Thanks, >>>Jon >>
>>Did you select a specifically "logic level" type MOSFET? >
if memory serves. that number turns on fully at 2.5 or around there.
> > ? > > Its 2N7000. Shouldn't any general purpose small signal mosfet work? > > It seems that the resistors are the key to getting it to work(in the sense > that it doesn't work if R is to large). > > I can't understand why the resistance has to be so low in this case ;/ I can > get the circuit to do its job but its taking about 30mA max(on average its > probably only about 5mA I suppose) and that seems like way to much(To > program a pic using mosfets) ;/ > we use those at work in switching circuits and 2.5 volts is the bias voltage that gets the gate to turn it on which then drives a bipolar that operates a incandescent lamp. -- "I'm never wrong, once i thought i was, but was mistaken" Real Programmers Do things like this. http://webpages.charter.net/jamie_5 | ||||||||||||||||||||||
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Source ---> GND
Parallel port --> Gate