I’ve built a circuit using a classic 2 transistor oscillator running at around 1khz , driving a FET . It is intended to drive a 6v motor off 12v .
Works fine driving a resistor ( wave form A) , but if I connect a motor ( ~5A running ) the waveform becomes very distorted, the FET ( P type) getting very hot .
Running off 12v lead acid battery , plenty umph .
Waveform A with resistor
B with motor ( connect o/p to 0v)
See attached diagram for values etc .
Any thoughts ?? It’s looks like the FET is not switching off fast enough .
One thing I would check first: if this is a P-channel MOSFET used as a high-side switch, then as far as I understand it should normally be connected like this:
Source → +12 V
Drain → motor
motor → GND
I may be reading the hand-drawn schematic wrong, but in the drawing it looks as if source and drain might be swapped.
Not sure if that is the whole problem, but I would verify the MOSFET orientation first ...
Keeping in mind that a) it's early here and I'm insufficiently caffeinated and b) I've been battling a chest cold for two weeks now and my mind is mushy even with caffeine, so I may be completely, totally and in every other way utterly wrong, but isn't that P channel MOSFET the wrong way around? Isn't the source supposed to go to 12V and the drain to ground through the load? Isn't the body diode always conducting in this configuration?
Thanks, I did find information on the F9540N but it was in Tywaninese and I did not understand it or the translation. So I then made the assumption it was a P-Channel device with source and drain swapped in the schematic.
All datasheets draw a P-fet the same way as an N-fet (drain at the top).
That is confusing, because you usually use and draw a P-fet in a schematic diagram upside down (source on top).
Leo..
Nice notation, I never noticed that, I just checked the pin out designation before using. Now every time I look at a MOSFET data sheet I will think of you.
That's wrong.
Source (right pin) must be connected to 12V.
Drain (the middle pin) drives the load.
Gate (left pin) with 1k to 12volt.
Forget about the darlington. Just use a single transistor. Or no drive transistor at all.
The flip-flop can drive the gate directly if you replace the 5k6 on the right with 1k (and re-calculate C).
Note that switching 5A with this fet will make it warm (at least 40C above ambient)
Leo..
Easy thing to remember...
Switching low-side with an N-channel fet or high-side with a P-channel fet, The load always connects to the middle pin.
Leo..
