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LW1DSE > TUBES 28.02.21 12:41l 332 Lines 16102 Bytes #999 (0) @ WW
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Subj: Blocking Oscillators
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Blocking oscillator
ΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝ
A blocking oscillator is a simple configuration of discrete electro_
nic components which can produce a free-running signal, requiring only a re_
sistor, a capacitor, a transformer, and one amplifying element. The name is
derived from the fact that the tube is cut-off or "blocked" for most of the
duty-cycle, producing periodic pulses. The non-sinusoidal output isn't suita_
ble for use as a radio-frequency local oscillator, but it can serve as a ti_
ming generator. The simple tones are also sufficient for applications such as
alarms or a Morse-code practice device. Some cameras use a blocking oscillator
to strobe the flash prior to a shot to reduce the red-eye effect.
When it comes to the components involved in this circuit, specific
types of each component are needed to have it work to its full potential. The
transformer is a vital component. For example, a pulse transformer creates
rectangular pulses, which are characterized by fast rise and fall times with
a flat top. There are a seemingly endless amount of combinations of voltages,
transformers, capacitors, tubes and resistors that can be used to vary and
model the circuit.
Due to the circuit's simplicity, it forms the basis for many of the
learning projects in commercial electronic kits. The secondary winding of the
transformer can be fed to a speaker, a lamp, or the windings of a relay.
Instead of a resistor, a potentiometer placed in parallel with the timing
capacitor permits the frequency to be adjusted freely, but at low resistances
the active device can be overdriven, and possibly damaged. The output signal
will jump in amplitude and be greatly distorted.
Circuit operation:
ΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔ
o Vb
³
±
± Rp
± Rg
------------³
³ ΪΔΔΔΔΔΔΔΔΔΔΒΔΔΔΔ±±±ΔΔΔo Vc
³ ³Ί³ ³ |
³ ψΫΊΫ ³ ³- |
ΫΊΫ Ns ³ ΔΑΔ |
Vp ΫΊΫ ΔΒΔ Cg ³
Np ΫΊΫ Ls Vs ³+ ³
³ ΫΊΫ ³ ³
³ Lp ΫΊΫ ³ ΔΑΔ ΔΑΔ
³Ί³ψ /// ///
--------ΪΔΔΔΩ ³------
³ ³
³ ³ ³ ³
³ Im³ ³Is ³
³ ³ ³ ³
ίίί ³
Va -----ΔΔΔΩ Vg
ΪΔΔΔΏ
³ ³ ³
³ V1 ³ ³
³
ΔΔΔ ΔΑΔ ΔΔΔ
/// /// ///
The circuit works due to positive feedback through the transformer
and involves two times: the time Tclosed when the switch is closed, and the
time Topen when the switch is open. The following abbreviations are used in
the analysis:
t : time, a variable;
Tclosed: instant at the end of the closed cycle, beginning of open cycle.
Also a measure of the time duration when the switch is closed;
Topen : instant at the end of the open cycle, beginning of closed cycle.
Same as T=0. Also a measure of the time duration when the switch is
open;
Vb : plate source voltage;
Vc : grid bias voltage;
Vg : instantaneous grid volatage;
Vo : Tube cutoff voltage, Va/ζ;
Vp : voltage across the primary winding. An ideal switch will present
supply voltage Vb across the primary, so in the ideal case Vp = Vb;
Vs : voltage across the secondary winding;
Im : magnetizing current in the primary;
Ipkmax : maximum or "peak" magnetizing current in the primary. Occurs imme_
diately before Topen;
Np : number of primary turns;
Ns : number of secondary turns;
N : the turns ratio defined as Ns/Np. For an ideal transformer operating
under ideal conditions, Is = Ip/N, Vs = N * Vp;
Lp : primary (self-)inductance, a value determined by the number of pri_
mary turns Np squared, and an "inductance factor" AL. Self-inductance
is often written as Lp = AL * Npύ * 10-9 [henry];
Rp : primary winding resistance, the winding DC resistace plus HF (core
and eddy and proxy losses);
R : combined switch and primary resistance;
Rg : grid leak resistance;
Up : energy stored in the flux of the magnetic field in the windings, as
represented by the magnetizing current Im;
A more-detailed analysis would require the following:
M : mutual inductance, its value determined by degree to which the mag_
netic field created by the primary couples to (is shared by) the
secondary, and vice versa. Coupling is never perfect; there is always
so-called primary and secondary "leakage flux". Usually calculated
from short-circuit secondary and short-circuited primary measure_
ments;
Lp,leak: self-inductance that represents the magnetic field created by, and
coupled to the primary windings only;
Ls,leak: self-inductance that represents the magnetic field created by, and
coupled to the secondary windings only;
Cwind : interwinding capacitance. Values exist for the primary turns only,
the secondary turns only, and the primary-to-secondary windings.
Usually combined into a single value;
Operation during Tclosed (time when the switch is closed):
ΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔ
When the switch (tube) closes, it presents the source voltage Vb a_
cross the transformer primary. The magnetizing current Im of the transformer
is
Vprimary * t
Im = ΔΔΔΔΔΔΔΔΔΔΔΔΔΔ [Amper]
Lp
here t (time) is a variable that starts at 0. This magnetizing current Im
will "ride upon" any reflected secondary current Is that flows into a secon_
dary load (e.g. into the control grid of tube V1; reflected secondary current
in primary = Is/N). The changing primary current causes a changing magnetic
field (flux) through the transformer's windings; this changing field induces
a (relatively) steady secondary voltage Vs = N * Vp. In some designs (as shown
in the schematic) the secondary voltage Vs adds to the source voltage Vc; in
this case because the voltage across the primary (during the time the switch
is closed) is approximately Vb, then:
Vs = (N + 1) * Vb + Vc
Alternately the switch may get some of its control voltage or cur_
rent directly from Vb and the rest from the induced Vs. Thus the switch-con_
trol voltage or current is "in phase" meaning that it keeps the switch closed,
and it (via the switch) maintains the source voltage across the primary. This
is a positive feedback from plate to grid via the transformer.
In the case when there is little or no primary resistance and little
or no switch resistance, the increase of the magnetizing current Im is a
"linear ramp" defined by the formula in the first paragraph. In the case when
there is significant primary resistance or switch resistance or both (total
resistance R, e.g. primary-coil resistance (Rp) plus a resistor in the tube's
cathode, the Lp/R time constant causes the magnetizing current to be a rising
curve with continually decreasing slope. In either case the magnetizing cur_
rent Im will come to dominate the total primary (and switch) current Ip.
Without a limiter it would increase forever. However, in the first case (low
resistance), the switch will eventually be unable to "support" more current
meaning that its effective resistance increases so much that the voltage drop
across the switch equals the supply voltage; in this condition the switch is
said to be "saturated". In the second case (e.g. primary and/or cathode resis_
tance dominant), the (decreasing) slope of the current decreases to a point
such that the induced voltage into the secondary is no longer adequate to keep
the switch closed. In a third case, the magnetic "core" material saturates,
meaning it can't support further increases in its magnetic field; in this con_
dition induction from primary to secondary fails. (If there is no current li_
mit in this situation, and the core saturates, the switch will be destroyed).
In all cases, the rate of rise of the primary magnetizing current
(and hence the flux), or the rate-of-rise of the flux directly in the case of
saturated core material, drops to zero (or close to zero). In the first two
cases, although primary current continues to flow, it approaches a steady
value equal to the supply voltage Vb divided by the total resistance(s) Rp in
the primary circuit. In this current-limited condition the transformer's flux
will be steady. Only changing flux causes induction of voltage into the secon_
dary, so a steady flux represents a failure of induction. The secondary vol_
tage drops to zero. Then, the switch opens. Cg then becomes charged because
of grid current with the polarity in the schematic, and quickly the triode is
carried to cutoff.
Operation during Topen (time when the switch is open):
ΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔ
Now that the switch has opened at Topen, the magnetizing current in
the primary is:
Vp * Tclosed
Ipkmax = ΔΔΔΔΔΔΔΔΔΔΔΔΔΔ
Lp
and the energy Up is stored in this "magnetizing" field as created by Ipkmax
(energy Um = « * Lp * Ipkmaxύ). But now there is no primary voltage (Vb) to
sustain further increases in the magnetic field, or even a steady-state field,
the switch being opened and thereby removing the primary voltage. The magnetic
field (flux) begins to collapse, and the collapse forces energy back into the
circuit by inducing current and voltage into the primary turns, the secondary
turns, or both. Induction into the primary will be via the primary turns
through which all the flux passes (represented by primary inductance Lp); the
collapsing flux creates primary voltage that forces current to continue to
flow either out of the primary toward the (now-open) switch or into a primary
load (Clamp). Induction into the secondary will be via the secondary turns
through which the mutual (linked) flux passes; this induction causes voltage
to appear at the secondary, and if this voltage is not blocked (e.g. by a
diode or by the very high impedance of a triode grid), secondary current will
flow into the secondary circuit (but in the opposite direction). In any case,
if there aren't components to absorb the current, the voltage at the switch
rises very fast. Without a primary load or in the case of very limited secon_
dary current the voltage will be limited only by the distributed capacitances
of the windings (the so-called "interwinding capacitance"), and it can destroy
the switch. When only interwinding capacitance and a tiny secondary load is
present to absorb the energy, very high-frequency oscillations occur, and
these "parasitic oscillations" represent a possible source of electromagnetic
interference. This oscillation is a sine wave whose amplitude will be decaying
in an negarive exponential way, the time constant will be dependent on the
leackages (damping) in the circuit.
The potential of the secondary voltage now flips to negative in the
following manner. The collapsing flux induces primary current to flow out of
the primary toward the now-open switch i.e. to flow in the same direction it
was flowing when the switch was closed. For current to flow out of the switch
end of the primary, the primary voltage at the switch end must be positive
relative to its other end that is at the supply voltage Vb. But this repre_
sents a primary voltage opposite in polarity to what it was during the time
when the switch was closed: during Tclosed, the switch-end of the primary was
approximately zero and therefore negative relative to the supply end; now du_
ring Topen it has become positive relative to Vb. Because of the transformer's
"winding sense" (direction of its windings), the voltage that appears at the
secondary must now be negative. A negative control voltage will maintain the
tube open, and this situation will persist until the energy of the collapsing
flux has been absorbed (by something). When the absorber is in the primary
circuit, the current waveshape is a triangle with the time topen determined
by the formula:
Vz * Topen
Ip = Ipkmax - ΔΔΔΔΔΔΔΔΔΔΔΔ
Lp
here Ipkmax being the primary current at the time the switch opens. When the
load is a capacitor the voltage and current waveshapes are a « cycle sinewave,
and if the load is a capacitor plus resistor the waveshapes are a « cycle
damped sinewave. The voltage at the plate of the tube becomes:
Va = Vb + Vp
and the grid voltage is:
Vg = V(Cg) + Vs = C(Cg) + (Vp * N)
ussualy a large negative value that maintanis the tube far into the cutoff
voltage Vo.
When at last the energy discharge is complete, the volatge at the
grid starts increasing toward conduction, as negative voltages is lost though
the grid leak resistor, Rg, and when the cutoff Vo value is crossed, the tube
starts conducting, and the control circuit becomes "unblocked". Control vol_
tage (or current) to the switch is now free to "flow" into the control input
and close the switch. This is easier to see when a capacitor "commutates" the
control voltage or current; the ringing oscillation carries the control vol_
tage or current from negative (switch open) through 0 to positive (switch
closed).
Repetition rate:
ΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔ
1
f = ΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔΔ
Tclosed + Topen
In the simplest case, the duration of the total cycle (Tclosed +
Topen), and hence its repetition rate (the reciprocal of the cycle duration),
is almost wholly dependent on the transformer's magnetizing inductance Lp,
the supply voltage, and the timing constant in the grid circuit. When a capa_
citor and resistor are used to absorb the energy, the repetition rate is de_
pendent on the R-C time-constant, or the L-C time constant when R is small or
non-existent (L can be Lp, Ls or Lp,s).
Notes
AL represents the geometry of the coils (their length and area and
separation, etc), the geometry of the magnetic path through the magnetic
material (if present) -- its area and length -- the magnetic material (if
present), and fundamental physical constants. Ungapped "cores" in continuous
magnetic materials have AL ranging from 1000 to 10,000; gapped cores have AL
ranging from 100 to 1000. Rods, "plugs", half-cores etc have AL in the 10 to
100 range. A similar formula exists for the secondary inductance Ls. For re_
ference see the Ferroxcube "big catalog" pages 7-13 dated 2008 Sep 01. How to
determine inductance of coils without magnetic material can be found in Chap_
ter 10 Calculation of Inductance in Langford-Smith 1953:429-449.
This is accurate when the primary and switch resistances are small
with respect to the voltage-drop across the inductance:
dIprimary
L * ΔΔΔΔΔΔΔΔΔΔΔ
dt
di/dt is the change in current with repect to time.
ΙΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝ»
Ί Compilled from Wikipedia.com and some comments added. ASCII Drawings and Ί
Ί translation by LW1DSE Osvaldo Zappacosta. B§ Garay, Almirante Brown, Bs As,Ί
Ί Argentina. Made with MSDOS 7.10's Text Editor (edit.com) in my AMD's 80486.Ί
Ί November 01, 2013. Ί
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Ί Osvaldo F. Zappacosta. Barrio Garay (GF05tg) Alte. Brown, Bs As, Argentina.Ί
Ί Mother UMC ζPC:AMD486@120MHz 32MbRAM HD SCSI 8.4Gb MSDOS 7.10 TSTHOST1.43C Ί
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