Re: Simple mosfet question

On Aug 3, 7:57 am, default <defa...@xxxxxxxxxxxxx> wrote:
On Fri, 03 Aug 2007 10:42:56 +0100, Nobody <nob...@xxxxxxxxxxx> wrote:
The RF modulation is PPM, but the signal between the receiver and
servo is PWM, with a 1500us pulse corresponding to neutral. The pulse
frequency isn't significant.

You are right. It has been awhile since I messed with them. Pulse
frequency is around 80 HZ

To control the servo, you command it to a certain angle, measured from
0 degrees. You send it a series of pulses. The ON time of the pulse
indicates the angle to turn to; 1ms = 0 degrees, 2ms=max degrees(about
120) and anything in between gives a proportional output angle. 1.5ms
is generally considered to be the "center". The 1~2ms limit is
manufacturers' recommendations; you can usually use a wider range
around 1.5ms for grater throw. I did not do this, but you can use
pulses of less than 1ms, and more than 2ms for an output angle of
180degrees or more. The limiting factor is the feedback pot and the
mechanical limits built in the servo.

Years ago I built a little joystick pan and tilt from a pair of servos
to aim a tiny camera. I used a quad op amp to generate the pulses for
both servos.

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Yeah, you are right about servos and they way they are controlled.
The entire circuit I have is basically taking the servo output and
creating a digital signal from it that throws a relay, so commanding
one direction throws the relay one way, and commanding the other
direction throws the relay the other way. This all works great -
until the transistor blows. I've posted the entire portion related to
the transistor, it is basically just inverting the input and changing
it to 0-12V instead of 0-4.5V.

I'm basically just looking for reasons why the transistor is blowing
and I think you guys are right that there is some sort of transient
noise. I'm going to add a resistor between battery and the 470pF cap,
but I'm not sure if I should add a small resistor between input and
gate, or if I should hook up a zener, or both? I'm also not sure how
the zener would connect, I'm new enough at this sort of thing
(transients, noise protection, etc) that I don't know how the zener
helps in a case like this. I'm assuming it would be cathode at the
gate and anode at the ground, and would limit the input voltage to a
"max" of whatever the zener is rated to? So I would use a zener of
like 4V? (The input signal is 0-4.5V). Is that the idea?

.

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