View Full Version : Low output voltage

Robert B
09-22-2007, 06:49 AM
I am using an output to control an N channel mosfet but have been getting a low output voltage from the propeller chip 1.62V instead of the advertised 3.3V . I am using an OUTA command and a short loop to control the time the output stays high. I had been using a 10K resistor·pulling the gate of the mosfet down so it wasnt floating but even removing the·resistor did not bring the output up to its claimed 3.3V. The voltage regulator feeding the propeller has been sitting rock steady at 3.3V. Am I missing anything or does anybody know how to ensure I get the 3.3V output.·

Phil Pilgrim (PhiPi)
09-22-2007, 06:56 AM
Is the output voltage low without the MOSFET? If not, you might have a bad transistor. Also, since you seem to be PWMing the gate in a loop, anything you measure with a voltmeter will be the average voltage over some finite time span. If you're modulating with a 50% duty cycle, 1.62V is just about what you'd expect.


Robert B
09-22-2007, 07:05 AM
I actually have 6 outputs and 6 mosfets and every one of them is doing the same thing. I am using the max reading on a voltmeter to record the voltage being applied to the gate. I saw a few posts about duty cycle but my manual does not go into any kind of detail. I assumed that the output would stay high until my count completed and then I tell it to go low.

Robert B
09-22-2007, 07:30 AM
The code I am using to controll two outputs which both drive a mosfet each is: It should hod the output high for about a third of a second then go low.

waitcnt (clkfreq / 3 + cnt)
waitcnt(clkfreq / 2 + cnt)

Phil Pilgrim (PhiPi)
09-22-2007, 07:41 AM
It will depend on the sample time of your voltmeter. If it's one second, like mine is, you'll read less than 3.3V — even as a maximum, since maximum readings are sampled, too. ("Maximum" isn't the same as "peak", in other words.) You could try extending the "on" time to several seconds and measure it that way, or you could look at the output on a scope.


Robert B
09-22-2007, 07:57 AM
The problem is the mosfet has a .8 to 2.0 Gate threshold voltage and the output of the mosfet is controlling 12v however I am only getting about 6V out of my mosfet when it is being turned on, when I started tracing the reason back I was finding that I was only getting 1.62 V on the gate which is connected directly to the output of the propeller chip. I have used a more sensitive mosfet and have gotten a little higher reading but it is also just receiving 1.62V on its gate as well. There is no possibility of the mosfet bringing the output of the propeller chip down due to the inherent high impedence of the mosfet gate. The regulator output is rock solid at 3.3V and has several filter caps afterwards to ensure voltage stability. I will turn up the on time to see if my meter will show a 3.3 volt signal since I do not have a scope handy but I do have another output being held on for 1 second and am having very similar results. Looking at my piece of code does the output cycle on and off or will it stay high for the specified time and then turn off?

09-22-2007, 08:35 AM
You did use the DIRA command to make all those pins outputs right?

Robert B
09-22-2007, 09:04 AM

Phil Pilgrim (PhiPi)
09-22-2007, 09:05 AM
Again, your 6V reading is consistent with a short 12V pulse and a long sampling interval on your voltmeter. The MOSFET is probably switching just fine. If you want to measure the peak voltage on the gate, connect a diode between the gate and a 1uF cap to ground (anode end on the gate). Measure the voltage on the cap, and add 0.6V. That will give you the approximate peak voltage at the gate.


Robert B
09-24-2007, 01:44 AM
Ok, I think I have narrowed it down, I changed the signal time to a second and I am getting 3.3V on the gate of the MOSFET, I have substituted a more sensitive MOSFET in its place, unfortunately I did not follow my schematic well enough and changed out the wrong MOSFET so I dont know if it fixed the problem. I ended up just working around the problem externally using my weaker MOSFET output to activate a relay instead of powering the circuit directly. I really appreciate your input and I have learned a bit more about the Propeller. My engineering skills need some more work as well as my programming skils. Thanks again for the help.