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MOSFET help?

S

Stefan Heinzmann

Jan 1, 1970
0
Ed said:
Winfield said:
Ed wrote...
[...]
The application is simply this. I'm building a microcontroller based
pyrotechnic firing system. The fet is needed to switch on and off the
supply to isolate the modules from the pyro. Since typically these
systems will be running from battery, and it involves reasonably long
wire lengths I want to introduce as little voltage drop as possible.

This is somewhat at odds with the current limiter. The series resistor
drops up to 0.65V, as you can see from Win's formula. It makes no sense
to use a fat MOSFET to minimize the voltage drop through it when most of
the loss occurs elsewhere.

If you are very serious about minimizing voltage drop and you still want
current limiting then you may want to use the drain-source resistance of
the MOSFET itself to measure the current. The resulting circuit is more
complicated. You may be better off to look for integrated solutions.
I would imagine I could create a zener gate voltage limiter with a
suitable resistor? This should solve the problem I would imagine, and
surely allow me to operate the fet at a suitable gate voltage level
across a wide range of supply rails?

Yes, that's a possibility. For turning off the MOSFET quickly you may
still want to have a resistor in parallel to the Zener.
 
J

Joerg

Jan 1, 1970
0
Hi Ed,
Hmm I have access to a 100mhz digital scope. Ive no idea if this would
be suitable. I shall just do my best with stopping such things
happening in the first place. Thanks for the advice though, I totally
forgot that the time scale of these oscillations if they happen would
be very short indeed.

That might not be fast enough. What matters is the realtime sampling
rate and some scopes state a certain bandwidth but then the sampler only
runs at a fraction of that so they are really only good for very
repetitive signals. For example, the Tektronix TDS series and others
have really fast samplers in the gigasample per second range so they can
be used to capture single or irregular events. Other scopes sample at
100 Ms/sec yet might have the same analog bandwidth.

Anyway, as Winfield said the transition time is just microseconds so if
there was oscillation it would only become serious if you'd switch the
FET very frequently.
(Boy this newsgroup makes me feel dumb!)

Please don't. We all learn every day and like many of us, I also learned
a lot in newsgroups. It's all give and take, as it should be in our
daily life :)

Regards, Joerg
 
E

Ed

Jan 1, 1970
0
Joerg said:
Hi Ed,


That might not be fast enough. What matters is the realtime sampling
rate and some scopes state a certain bandwidth but then the sampler only
runs at a fraction of that so they are really only good for very
repetitive signals. For example, the Tektronix TDS series and others
have really fast samplers in the gigasample per second range so they can
be used to capture single or irregular events. Other scopes sample at
100 Ms/sec yet might have the same analog bandwidth.

Anyway, as Winfield said the transition time is just microseconds so if
there was oscillation it would only become serious if you'd switch the
FET very frequently.



Please don't. We all learn every day and like many of us, I also learned
a lot in newsgroups. It's all give and take, as it should be in our
daily life :)

Regards, Joerg

Hi Joerg,
Thanks for this information, I now have a working fet prototype that
seems to be doing all it should do very nicely :) I also tried with a
zener diode to clamp the GS voltage and again this seems to work fine. I
need to test it at more extreme loads and voltages now.

As for osciliscopes, I just had to fix my old one - A Hameg 20mhz one,
all the caps are drying out and its becoming increasingly unreliable.
But its working again now and will do for the meantime. Just been
looking on ebay, there are some really nice ones - if only I had the
spare money at the moment!

Thanks again for the advice, this newsgroup and people in it have so far
been a great help!

Ed
 
J

Joerg

Jan 1, 1970
0
Hi Ed,
As for osciliscopes, I just had to fix my old one - A Hameg 20mhz one,
all the caps are drying out and its becoming increasingly unreliable.
But its working again now and will do for the meantime. Just been
looking on ebay, there are some really nice ones - if only I had the
spare money at the moment!

That must be a classic scope. I still have my old Hameg 207 from 30
years ago. Can't part with it. 8MHz bandwidth and it doesn't even have a
trigger. Instead it uses a "synchronizer" but you have to use the
coupling of that into the time base sparingly or it will distort the
waveform. When I place that next to the big HP it looks like a Moped
next to a truck. Hameg created some nice stuff and one day I might get
their spectrum analyzer for off site EMI work.

Regards, Joerg
 
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