Quickly switchable switch
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Posts: 46,084
Hiya, I'm working on the hardware for my small engine controller,
and I'm trying to find something to drive my 12V lines.
I have NPN switching transistors, and when I use it, in the ON
position it works great, but when it's off, it still leaves a
circuit on, but with more resistance. Am I just hooking this up
wrong? Or do these things not actually open the circuit fully?
Im still waiting for my solid state relays to get here, but I've
been thinking, they have a 1ms turn on time, and I think I need to
do better to work with this project.
Thanks
Bill
and I'm trying to find something to drive my 12V lines.
I have NPN switching transistors, and when I use it, in the ON
position it works great, but when it's off, it still leaves a
circuit on, but with more resistance. Am I just hooking this up
wrong? Or do these things not actually open the circuit fully?
Im still waiting for my solid state relays to get here, but I've
been thinking, they have a 1ms turn on time, and I think I need to
do better to work with this project.
Thanks
Bill
Comments
billak@f... writes:
>
> Hiya, I'm working on the hardware for my small engine controller,
> and I'm trying to find something to drive my 12V lines.
>
> I have NPN switching transistors, and when I use it, in the ON
> position it works great, but when it's off, it still leaves a
> circuit on, but with more resistance. Am I just hooking this up
> wrong? Or do these things not actually open the circuit fully?
>
> Im still waiting for my solid state relays to get here, but I've
> been thinking, they have a 1ms turn on time, and I think I need to
> do better to work with this project.
>
> Thanks
>
> Bill
The NPN should turn off completely!!!!! How low is the voltage to your NPN
base input when it should be off?
Also, it is wise to connect a resistor from the base terminal to ground if
you want the transistor to turn off as quick as possible. There are other
factors to make it turn off quickly but this is quick and easy without much
calculations.......The lower the value the quicker turn off time will be.
There is some capcitance associated with the base, the resistor from base to
ground gives a disharge path for the base capacitance
[noparse][[/noparse]Non-text portions of this message have been removed]
a resistor (10k I think). and the collector to the - of the 12V
thing I'm running.
I have it running as of last night, I don't know what I was doing
wrong before. It's switching my injector on and off quite nicely.
I'll try the small resistor from base to ground.
Do you have any idea what the turn on time is like for these things?
Thanks
Bill
--- In basicstamps@yahoogroups.com, smartdim@a... wrote:
> In a message dated 1/24/2003 6:32:18 PM Pacific Standard Time,
> billak@f... writes:
>
> >
> > Hiya, I'm working on the hardware for my small engine
controller,
> > and I'm trying to find something to drive my 12V lines.
> >
> > I have NPN switching transistors, and when I use it, in the ON
> > position it works great, but when it's off, it still leaves a
> > circuit on, but with more resistance. Am I just hooking this up
> > wrong? Or do these things not actually open the circuit fully?
> >
> > Im still waiting for my solid state relays to get here, but I've
> > been thinking, they have a 1ms turn on time, and I think I need
to
> > do better to work with this project.
> >
> > Thanks
> >
> > Bill
>
> The NPN should turn off completely!!!!! How low is the voltage to
your NPN
> base input when it should be off?
> Also, it is wise to connect a resistor from the base terminal to
ground if
> you want the transistor to turn off as quick as possible. There
are other
> factors to make it turn off quickly but this is quick and easy
without much
> calculations.......The lower the value the quicker turn off time
will be.
>
> There is some capcitance associated with the base, the resistor
from base to
> ground gives a disharge path for the base capacitance
>
>
> [noparse][[/noparse]Non-text portions of this message have been removed]
using an NPN and turning the voltage on and off quickly just isn't
enough to make a spark. What would be a good circuit to maybe keep a
big cap charged, and when the stamp says so, discharge it through
the coil? Or won't this help any?
Thanks
Bill
--- In basicstamps@yahoogroups.com, "billakay <billak@f...>"
<billak@f...> wrote:
> Well I've hooked up the emitter to ground, the base to pin 8
through
> a resistor (10k I think). and the collector to the - of the 12V
> thing I'm running.
>
> I have it running as of last night, I don't know what I was doing
> wrong before. It's switching my injector on and off quite nicely.
> I'll try the small resistor from base to ground.
>
> Do you have any idea what the turn on time is like for these
things?
>
> Thanks
>
> Bill
>
>
> --- In basicstamps@yahoogroups.com, smartdim@a... wrote:
> > In a message dated 1/24/2003 6:32:18 PM Pacific Standard Time,
> > billak@f... writes:
> >
> > >
> > > Hiya, I'm working on the hardware for my small engine
> controller,
> > > and I'm trying to find something to drive my 12V lines.
> > >
> > > I have NPN switching transistors, and when I use it, in the ON
> > > position it works great, but when it's off, it still leaves a
> > > circuit on, but with more resistance. Am I just hooking this
up
> > > wrong? Or do these things not actually open the circuit fully?
> > >
> > > Im still waiting for my solid state relays to get here, but
I've
> > > been thinking, they have a 1ms turn on time, and I think I
need
> to
> > > do better to work with this project.
> > >
> > > Thanks
> > >
> > > Bill
> >
> > The NPN should turn off completely!!!!! How low is the voltage
to
> your NPN
> > base input when it should be off?
> > Also, it is wise to connect a resistor from the base terminal to
> ground if
> > you want the transistor to turn off as quick as possible. There
> are other
> > factors to make it turn off quickly but this is quick and easy
> without much
> > calculations.......The lower the value the quicker turn off time
> will be.
> >
> > There is some capcitance associated with the base, the resistor
> from base to
> > ground gives a disharge path for the base capacitance
> >
> >
> > [noparse][[/noparse]Non-text portions of this message have been removed]
without an expensive current probe.....but generally this is what you want...
The voltage you produce from the ignition coil to fire your spark plug is
dependant upon (all other factors being equal) is how QUICKLY you can turn
off the current in the coil.
Vout = L delta i / delta t L = inductance, i = current and t is time. So
with a certain current in the coil, the smaller delta t, the larger the
voltage produced.
You will want to use a mosfet transistor for this. probably a "logic" level
device.
Go to <A HREF="www.glitchbuster.com">www.glitchbuster.com</A> This guy also
replies to posts in this forum and
can probably help you select the mosfet you want.
For your injector circuit- a mosfet is also desireable, but since you already
have a PNP working, tune it a little.....
The turn on time will vary depending on many factors. You will need an
o-scope to know for sure. However, to achieve the fastest possible turn on
and turn off time follow these steps...
First some theory: The more curent you use to "fire" the transistor, the
faster it will turn on. HOWEVER, the more current the transitor base is
using to stay on, the slower it will turn off. The idea is to turn it on
with the mimimum amount of current needed to keep it on, so there is less
current to "get rid of" when the transistor gets turned off.
FOR FAST TURN ON TIME.....there are two things to do
1 Determine the minimum amount of base current (largest possible base
resistor) that will still cause the transistor to turn on fully to open your
injector.
2 Then once you know this value seperate the resistor into two values with an
approximate ratio of 75/25.
example......if you determine the largest value base resistor that will work
is 1000 ohms, get 750 ohms and 250 ohms (although those values are not
standard, just come close to the 75/25 ratio I mentioned, or even 80/20).
3 Connect the two resistors in series going to the base of your transitor.
Connect a tantulum capacitor (~1uf) across the larger base resistor. The plus
end of the cap towards the stamp end.(stamp pin >>>larger reisistor cap combo>
>>>>smaller resistor>>>>>transisor base)
4 AN IMPORTANT CONSIDERATION - verify that the smaller resistor value is not
so small that the initial current will be too much for the stamp to handle.
The initial current is approximately equal to.........(5-0.7) / smaller
resitor value. Thats 5 volts from stamp minus the base emitter voltage drop
divided by the smaller resitor ---Simple ohms law...
Theory.......The faster you can get current into the base, the faster the
transistor will turn on. The capacitor will "temporarily bypass the larger
base resistor. So when you first command the transistor on, the effective
base resistance is very small yielding lots of base current and fast turn on
time. After a very short time, the cap is charged and then the "larger"
resistor is passing current and the base of the transistor is using the
minimun current to stay on.
This will be a balancing act, if the cap is too big, the turn off time will
be slowed down.
For the quicker turn off time, connect a small value resistor from the base
to ground, approximately 1 to 5k ohms. This gives a path to discharge the
the base current once you command the transitor off.
[noparse][[/noparse]Non-text portions of this message have been removed]
"<Transistor type> datasheet". Works just about every time for me. Though
ignore anything with "chipdocs" in the address. Its a pay thing and I guess
they pay the search engines big bucks because they're always on top.
Original Message
From: <billak@f...>
To: <basicstamps@yahoogroups.com>
Sent: Saturday, January 25, 2003 6:08 AM
Subject: [noparse][[/noparse]basicstamps] Re: Quickly switchable switch
> Well I've hooked up the emitter to ground, the base to pin 8 through
> a resistor (10k I think). and the collector to the - of the 12V
> thing I'm running.
>
> I have it running as of last night, I don't know what I was doing
> wrong before. It's switching my injector on and off quite nicely.
> I'll try the small resistor from base to ground.
>
> Do you have any idea what the turn on time is like for these things?
>
> Thanks
>
> Bill
>
>
> --- In basicstamps@yahoogroups.com, smartdim@a... wrote:
> > In a message dated 1/24/2003 6:32:18 PM Pacific Standard Time,
> > billak@f... writes:
> >
> > >
> > > Hiya, I'm working on the hardware for my small engine
> controller,
> > > and I'm trying to find something to drive my 12V lines.
> > >
> > > I have NPN switching transistors, and when I use it, in the ON
> > > position it works great, but when it's off, it still leaves a
> > > circuit on, but with more resistance. Am I just hooking this up
> > > wrong? Or do these things not actually open the circuit fully?
> > >
> > > Im still waiting for my solid state relays to get here, but I've
> > > been thinking, they have a 1ms turn on time, and I think I need
> to
> > > do better to work with this project.
> > >
> > > Thanks
> > >
> > > Bill
> >
> > The NPN should turn off completely!!!!! How low is the voltage to
> your NPN
> > base input when it should be off?
> > Also, it is wise to connect a resistor from the base terminal to
> ground if
> > you want the transistor to turn off as quick as possible. There
> are other
> > factors to make it turn off quickly but this is quick and easy
> without much
> > calculations.......The lower the value the quicker turn off time
> will be.
> >
> > There is some capcitance associated with the base, the resistor
> from base to
> > ground gives a disharge path for the base capacitance
> >
> >
> > [noparse][[/noparse]Non-text portions of this message have been removed]
>
>
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