Electronic Boost Control

[Tom Cloud]

>In a message dated 97-04-02 18:26:29 EST, you write:
>
>> My first query is to a power supply to produce a clean 5V from the 
>>       dirty 12V supply of a running engine. Does anyone have a schematic of
>
>>       a typical power supply which may be used for such an application, be 
>>       it switch mode, linear, or whatever? I will need to drive a 12V pulse
>
>>       solenoid valve, so will need sufficent curent to run driving
>circuitry 
>>       for this valve (I expect that this will be biggest current demand on 
>>       the system)
>>       My second query is in regards to the driving of the valve. It uses a 
>>       12V coil which requires around 800mA holding current. Does anyone
>have 
>>       any recommended methods to drive coils like this from a logic output?

Get a copy of the National Semiconductor Linear Applications book.
(check their web page -- if they won't send you one, find the
name of their rep in your area -- should be able to get that
from their web page, else e-mail me and I'll get it for you)

BEWARE of *anything* you get from RatShack!!!  I recommend some
of their literature to my students -- I teach power supply design
and their book on power supplies is good for the beginner to
look through, but it's certainly not meant to be used as a
design aid -- only for concepts (there are some useful examples
in it, though).

For 5 volts: you can use the 7805/340-5 regulator.  They can
be had up to five amps.  Assume 5 amps, and 15 volts (max) out
of your alternator, you'll have to dissipate (15-5V)(5A) = 50W
of power.  That's do-able, but it's a lot of heat.  The switching
route will produce less heat, but will generate noise that'll
get into your electronics (radio, ecu, etc), so you'll have
to be real careful -- look in the National Linear App book.

As far as driving 800 mA.  That current pretty much exceeds
most TO-92 case transistors (2N3904, 2N4401, 2N2222, etc).
Note that transistors have an Ic(max) parameter, but they
also have a PD(max) parameter.  In a switching app, you
should have very little heat -- unless you're switching
often and the t-on and t-off rise and fall times are
lengthy WRT the PRR (pulse rep rate).

The xstors I mention are not necessarily my favorites -- they
are what our students use in their lab kits.  Ic(max)
ratings: 2N3904(200 mA), 2N4401(600 mA), 2N2222(800 mA) **note
that this is the metal case version, the plastic is lower**.
Don't run devices at or near their max (assuming you want
them to live a long time) -- especially if they're going
to be living at elevated temps.


As far as MOSFETS.  (1) they are NOT faster than bipolars.
Theoretically, they ought to be, but they're not.
(2) MOSFET switches are supremely easy to use.  Get
the type that you can drive directly from 5 volt
logic.

My preference?  I still like some vacuum tubes  ;-)

I'll be glad to offer some help with the hookup if you need
it.  CMOS logic typically doesn't have enough drive
current to properly drive a bipolar transistor (depends).
It will drive a MOSFET.  You DON'T need a resistor
between any driving circuit and the gate of a MOSFET
(I usually put one there for old time's sake).  You
MUST put a current limiting resistor between the driver
and the base of a bipolar.  The value of that resistor
is R = (driving voltage minus .7) / (desired base current).
In fact, if you are building a saturated switch (and you
are), the desired base current should be approximately
three times the amount of current you get by dividing
the collector current by beta.

Lost?  If so, ask and I or one of the other lurkers can
clear it up.


Tom Cloud <cloud at peaches.ph.utexas.edu>