Re: Simple mosfet question

On Aug 2, 12:50 pm, default <defa...@xxxxxxxxxxxxx> wrote:
On Thu, 02 Aug 2007 15:18:21 -0000, Andrew <xxrag...@xxxxxxxxx> wrote:
I have a simple circuit where I want to convert a digital input from
0-5V to an inverted signal from 0-12V. ie, 0V into the circuit = 12V
out, and 5V into the circuit = 0V out.

I don't have the means to get a schematic up, but I've tried to draw
it with text below (but it probably won't show up right), so I will
describe the simple circuit and I think it will be clear:

There is a 2N7000 N-ch mosfet transistor, with the source tied to
ground. The gate is tied directly to the input. The drain is tied to
a resistor, 10k, and the other side of the 10k resistor is tied to a
battery, +12V. The output is connected to the drain, and is used to
feed the clock inputs of 2 CMOS ICs. There is a 470pF filtering
capacitor tied from the output to ground.

My question is, can anyone see how this circuit would blow the
transistor? Is this an OK way to hook it up? Should I maybe add
another 10k resistor between drain and output (before the cap) to
better protect the transistor? I realize that doing this would bring
the output voltage down very slighlty, but that's fine if it is needed
to protect the transistor.

I'm asking because I've hooked up the circuit more than a few times,
and it always works at first / for a while, but sometimes the
transistor blows unexpectedly and I don't know why yet. I think it is
because I was damaging the transistor during install (with ESD, I was
not using protection at the time, but I learned my lesson!), but I
want to make sure it isn't actually a circuit problem too.

Sorry that I can't get a schematic up, I don't have a place to upload
files while I'm at work. I hope I have made it clear enough :)

+12V
|
|
/
\ 10k
/
|_______OUTPUT
__|
| |
INPUT _____| |__
|
|
|
GND

mosfets are very sensitive to gate drive and spurious signals / static
electricity etc.. A small transient in the gate lead can eat a mosfet
where a BJT will shrug it off. You are doing something that kills the
transistor - bread boarding? Keep the gate lead short to minimize
inductance. Protect the gate and source/drain if there is an
inductance in the load. Use a scope and watch the transients.

Look at the data *** for the 7000 - particularly look at the safe
operating area and you see how fragile they can be.

A good source for the use of mosfets:http://www.irf.com/technical-info/appnotes/an-937.pdf
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I hope I don't get a double post here, but it has been a bit and I'm
not seeing my message pop up, so I'm thinking I forgot to hit reply!
If it double posts, please forgive me.

Anyway...

I was thinking the same thing, if it isn't ESD, it must be some kind
of noise or transients on the transistor leads, which is why I was
looking for a way to protect the transistor.

I should mention that the input is being driven by a servo motor
driver which I did not design or have the schematic for, or really
know much about the design. I am just intercepting the signal and
using it to drive my circuit.

Is it common practice to use a zener as you mentioned, Fred? I'm not
sure exactly how you meant to hook it up? Did you mean with the
cathode at the gate and anode at ground? Or cathode at the input and
anode at the gate? Is doing it this way superior to using a small
resistor in between the input and gate as Tim suggested?

After protecting the gate with a zener or resistor, I should still
probably look at protecting the drain further with either a small
resistor in line with the cap or a small resistor between the cap and
10k resistor, right?

I think you are right Tim, I could use a small resistor such as 200
ohms. The Ids current capacity of the transistor is 200mA, and even a
200 ohm resistor with the levels of voltage I'm expecting wouldn't be
close to this limit.

Default - the design is actually on a PCB right now. There is a bit
of wire coming from the servo driver to the input of the board (maybe
6 inches of 24awg wire (standard servo wire)) and due to space
constraints, there is about 3 inches of 12mil trace on the PCB from
the input to the gate of the transistor. I hadn't considered the
inductance problem that longer traces could pose.

.