Re: Mosfet Homebrew Question


"GPE" <GPE_NoSpam@xxxxxxx> wrote in message
news:agZyl.44218$5t4.28465@xxxxxxxxxxxxxxx
I think this is being made harder than it is.

The 20N10L is fine for driving that coil - this MOSFET has been used in
thousands of machines with no problem being driven by standard logic.

The problem lies with your biasing circuit.
For a suggested biasing circuit for driving a coil such as yours, see most
any Stern schematic.

Most any -NEW- Stern schematic.


Also, be aware that MOSFET's do NOT like fast current surges (high di/dt)
that happen everytime the gate is turned on. Stern fixes this by slew
rate limiting the gate voltage with a small ceramic cap. One board
designer (not Tony) has decided to not include these caps... this will
result in a reduced lifespan.

-- Ed



"martin" <martin.reynolds@xxxxxxxxx> wrote in message
news:d2be760e-1fda-48a4-9e4d-79a9f6f5f473@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Mar 26, 2:10 pm, "Tony @ GLM" <cl...@xxxxxxxxxxxxxxxxxxxxx> wrote:
On Mar 26, 4:24 pm, "kendallvanp...@xxxxxxxxx"

<kendallvanp...@xxxxxxxxx> wrote:
On Page 26 is the spects on it. It's output is 30ma max as far as my
controlled current and I'm at 3.5v so maybe I'm short the current
needed to push the gate on this thing.

It isn't current that drives MOSFETs, it is almost purely voltage
levels, that's why
you need to do whatever you can to get the input gate voltage as high
as possible.
To do so, I'd get rid of the LED and switch it out for a schottkey
diode. That
should get you up to 4.1V or so at the gate pin.

Also, the HUI datasheet seemed to indicate that when the outputs turn
off, it
is an open-circuit, not a sink to ground. That means that you need to
ensure
that the MOSFETs actually turn off when the HUI turns off. In this
case, I'd lower
the 10K gate resistor down to 1K. That way any energy at the gate pin
is quickly
dissipated.

Got a suggestion on the right fet to do this? Would the one you
suggested still work?

The one I recommended should work quite fine (100V 40A) as an
upgrade to the STP20N10L (which doesn't seem to be available
anymore). Overkill? Yes, but again you won't have to worry too
much about the power dissipation at those lower input drive levels.

There are plenty of MOSFETs out there which will do the job, it
just depends on what your budget is.

Tonywww.greatlakesmodular.com- Re-Engineered Pinball Parts!

Check that 10k resistor - kmake sure that it is connected at both
ends. What you describe could happen if the resistor is missing.

And for test, I'd suggest using a lamp and a 5V supply. Then, you can
find the bugs without cooking coils and transistors.



.

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