running multiple relay''s from BS-1
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--- In basicstamps@yahoogroups.com, "Albert Catano"
<acatano2002@y...> wrote:
> --- In basicstamps@yahoogroups.com, "Allan Lane" <allan.lane@h...>
> wrote:
> > In 'normal' use (while the relay is activated)
> > the diode across the relay coil
> > should be reverse-biased (Cathode to +5, Anode to
> > the Stamp).
> >
> > The purpose of the diode is that when you release
> > the coil, there is a current flowing, maintaining
> > a magnetic field, that is holding the contacts
> > closed. When you switch off the coil,
> > the magnetic field collapses. This generates
> > additional current in the coil which wants to go
> > somewhere. The diode gives it a place to go.
> >
> > Without the diode, the current can only go into your
> > relay-driver circuitry -- which tends to burn it out.
> > Some Darlington transistor IC's have built-in
> > diodes for this purpose.
>
> This statement needs a clarification.
> The purpose of the diode is not to prevent "additional current".
> is to clamp the kickback voltage generated when the semiconductor
> switch is turn off.
> This effect in math terms is defined as V=L*di/dt
> where V is the generated voltage, L is the inductance of the coil
in
> Hy, di is the instaneous current and dt is the time it takes the
> swith to open.
> The faster the switch, the larger the generated voltage.
> Imagen as dt approaches zero, what is the voltage equal to?
In reading the post you mentioned, I thought I wrote it for a
second. The only part that I would have changed is the statement
"This generates additional current in the coil which wants to go
somewhere."
The coil (inductor) only stores the energy, it does not create it or
any additional energy, voltage or current.
It does, however, release the stored energy quickly as per the
formula above.
As per the response, the coil is not isolated and will contain
components of resistance and capacitance so dI (current) is limited
and will drop sharply as the effects of L change during the discharge.
The spike of discharge will increase, but resistance and capacitance
will alter the spike over time.
The arc across the relay coils (when using a relay) will burn the
contact material of the relay. Over it's lifetime the coil material
will burn, carbon will start to cover the relay and prevent contact
making the resistance begin to increase, sometimes to a point of
preventing current from passing.
But, the more important aspect for our area is that the pn junction
of the mosfets and transistors cannot withstand any high voltage
spike without being destroyed.
The diode offers a path that will drain the induced voltage and limit
the voltage by shunting it to ground. The diode will pass the
voltage *as* it is being released. I have a desire to use the term
generated, but since it is only stored, I don't think generated fits.
Therefore the opening of the coil circuit and subsequent release of
energy does not follow V=L*dI/dt IN THE CIRCUIT with the diode as
this does to allow for any other effects of any other components in
the circuit.
so, your 60uH coil that is storing 1 amp will only theoretically
release 60 volts back into the circuit. Like a capacitor, the time
constant of the inductor limits how fast the stored charge is
released.
Inversely, the time constant of the inductor also limits how fast the
magnetic field will be generated. This has two ramifications on us.
If the resistance of the coil was zero, it would act as a dead short
and the current entering the coil would approach infinity and that
would cause ripples on the power supply that would not be removed
easily.
The opposite end is that if the resistance was high, the magnetic
field would take a long time to form and we would not have the pull
of the motor or solenoid.
One additional note is that in a motor, a rotating magnetic field CAN
generate (exchange, convert) power as the energy is stored both in
the coil and the mass of the rotating shaft of the motor. This adds
a whole new set of calculations on how much and how fast this energy
is transferred from rotation to voltage
And that, we'll save for another day. : )
Personally, I wish Horrowitz and Hill went into more details about
inductors. Anybody know of a primer for inductors and coils
Dave
<acatano2002@y...> wrote:
> --- In basicstamps@yahoogroups.com, "Allan Lane" <allan.lane@h...>
> wrote:
> > In 'normal' use (while the relay is activated)
> > the diode across the relay coil
> > should be reverse-biased (Cathode to +5, Anode to
> > the Stamp).
> >
> > The purpose of the diode is that when you release
> > the coil, there is a current flowing, maintaining
> > a magnetic field, that is holding the contacts
> > closed. When you switch off the coil,
> > the magnetic field collapses. This generates
> > additional current in the coil which wants to go
> > somewhere. The diode gives it a place to go.
> >
> > Without the diode, the current can only go into your
> > relay-driver circuitry -- which tends to burn it out.
> > Some Darlington transistor IC's have built-in
> > diodes for this purpose.
>
> This statement needs a clarification.
> The purpose of the diode is not to prevent "additional current".
> is to clamp the kickback voltage generated when the semiconductor
> switch is turn off.
> This effect in math terms is defined as V=L*di/dt
> where V is the generated voltage, L is the inductance of the coil
in
> Hy, di is the instaneous current and dt is the time it takes the
> swith to open.
> The faster the switch, the larger the generated voltage.
> Imagen as dt approaches zero, what is the voltage equal to?
In reading the post you mentioned, I thought I wrote it for a
second. The only part that I would have changed is the statement
"This generates additional current in the coil which wants to go
somewhere."
The coil (inductor) only stores the energy, it does not create it or
any additional energy, voltage or current.
It does, however, release the stored energy quickly as per the
formula above.
As per the response, the coil is not isolated and will contain
components of resistance and capacitance so dI (current) is limited
and will drop sharply as the effects of L change during the discharge.
The spike of discharge will increase, but resistance and capacitance
will alter the spike over time.
The arc across the relay coils (when using a relay) will burn the
contact material of the relay. Over it's lifetime the coil material
will burn, carbon will start to cover the relay and prevent contact
making the resistance begin to increase, sometimes to a point of
preventing current from passing.
But, the more important aspect for our area is that the pn junction
of the mosfets and transistors cannot withstand any high voltage
spike without being destroyed.
The diode offers a path that will drain the induced voltage and limit
the voltage by shunting it to ground. The diode will pass the
voltage *as* it is being released. I have a desire to use the term
generated, but since it is only stored, I don't think generated fits.
Therefore the opening of the coil circuit and subsequent release of
energy does not follow V=L*dI/dt IN THE CIRCUIT with the diode as
this does to allow for any other effects of any other components in
the circuit.
so, your 60uH coil that is storing 1 amp will only theoretically
release 60 volts back into the circuit. Like a capacitor, the time
constant of the inductor limits how fast the stored charge is
released.
Inversely, the time constant of the inductor also limits how fast the
magnetic field will be generated. This has two ramifications on us.
If the resistance of the coil was zero, it would act as a dead short
and the current entering the coil would approach infinity and that
would cause ripples on the power supply that would not be removed
easily.
The opposite end is that if the resistance was high, the magnetic
field would take a long time to form and we would not have the pull
of the motor or solenoid.
One additional note is that in a motor, a rotating magnetic field CAN
generate (exchange, convert) power as the energy is stored both in
the coil and the mass of the rotating shaft of the motor. This adds
a whole new set of calculations on how much and how fast this energy
is transferred from rotation to voltage
And that, we'll save for another day. : )
Personally, I wish Horrowitz and Hill went into more details about
inductors. Anybody know of a primer for inductors and coils
Dave