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March 1967 Radio-Electronics
 [Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
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sink-me
The Tunnel Diode Really Works
These two practical circuits can be duplicated by the average experimenter
By I. Queen
Editorial Associate
The tunnel diode is an amazing semiconductor. Articles on it and its applications
have appeared in periodicals, but have often been too theoretical for readers who
like to experiment. Exact voltage and component values are often missing, making
it hard to duplicate the stated results. (The operating bias for a tunnel diode
is generally very critical.)
The July issue of this magazine, page 26,carried an excellent discussion of basic
principles of the diode. It showed that this device has a negative resistance sandwiched
between regions of positive resistance. Fig. 1 shows a highly simplified form of
the familiar tunnel diode curve. Current increases as voltage is increased up to
about 0.1 volt, then decreases as voltage increases from 0.1 to a little less than
0.5 volt, when it again starts to rise with increasing voltage. Voltage and current
values are typical for a 1N653 (Texas Instruments). Circuit theory indicates that
such a device must be an excellent oscillator and switch. This article will give
full details on a crystal oscillator for 27-30 mc and a switch that is triggered
by light.
Simple tunnel-diode oscillator.
The oscillator
The negative resistance region means that the tunnel diode is a ready oscillator,
As explained in the July issue, the load line must intersect the region of negative
resistance (Fig. 2). This load resistance is smaller than the negative resistance
of the diode, specified as -40 ohms for a 1N653. E1, the circuit voltage,
is a small fraction of a volt.
No battery with such low voltage is generally available, so a divider is needed.
Fig. 3 illustrates the crystal oscillator. Note the low value for R3 (which determines
circuit resistance). Oscillations occur when the voltage across the diode is about
0.2. Start with a 100-ohm pot for R1. To set it, vary R1 (and R2 for fine adjustment)
and listen for the signal on a nearby receiver tuned to the crystal frequency. Tune
C2 for maximum output. For 27-30 mc, L is 16 turns of No. 18 wire, 14-inch diameter,
close-wound.
You may also adjust this oscillator and observe circuit operation by measuring
voltages. Start with maximum resistance at R1 and R2, then raise the voltage across
the diode. (R2, the 20-ohm pot, is rather critical. Use 10 ohms for ease of control.)
At about 0.17 volt (as measured with a 20,000-ohms-per-volt meter) you will note
a sudden jump (to about 0.2 volt) and oscillations will begin.
Fig. 1 - Tunnel diode characteristic in an idealized - or cubist-form.
This circuit will work with any overtone crystal from 27 to 30 mc without retuning.
This is hardly the upper frequency range of a tunnel diode. With smaller tanks you
should have no trouble pushing the range higher. If you remove the crystal, the
circuit will self-oscillate with a rough and unstable signal.
Once R1 has been determined for your particular diode and circuit, it may be
replaced with a fixed resistor. A 47-ohm unit is just right for my oscillator.
For safety, limit diode current to a maximum of about 6 ma till you determine
the current value for R1.
Construction is uncritical. In my unit, a battery and voltage-divider unit already
on hand was used. That portion of the circuit could, of course, be mounted on the
board with the other components.
Trigger switch
When the load line intersects the diode characteristic in three places (as load
1 does in Fig. 4) we have a switch. As an example, suppose the circuit voltage is
E2 about 1.2 volts. Load line 1 resistance is about 250 ohms. When the
circuit is switched on, it stabilizes at point A.
Now if a weak positive pulse is applied to the diode, the operating point will
rise over the peak. It will move along the downward slope quickly (due to negative
resistance) and arrive at C. (Point B is an unstable position, and is of little
importance in this explanation). All this takes place instantaneously. Thus even
a very weak pulse will trigger the circuit from A to C. In a 1N653 these points
correspond to about 5 and 2 ma, respectively. (This is terrific amplification, if
you want to look at it that way.)
Fig. 2 - Operating point for tunnel diode oscillator.
The triggering push may be an externally applied voltage or it may be just a
reduction in circuit resistance. To show this, refer to Fig. 5. Starting with maximum
R1, lower the resistance. At about 5 ma the meter will suddenly deflect downward
to 2 ma, although we are reducing R1. The operating point (Fig. 4) has moved up
the curve till it reached the peak of the characteristic. At that instant (when
it reaches the peak) it snaps to C. R2 (Fig. 5) is a limiting resistor.
Now increase R1. The slope of the load line becomes more horizontal until it
coincides with load 2, at which instant the current will snap upward. For the 1N653
this occurs when the total series resistance is about 1,250 ohms. The current will
snap from a low of 0.6 ma to 1.2 ma.
The circuit may be triggered by light falling on a photocell (Fig. 6). A low-resistance
Hoffman S1-A solar cell is shown in series with the winding of a relay RY to be
energized. Light falling on the solar cell increases the circuit voltage and triggers
the circuit. The higher voltage corresponds to changing the load line from 2 to
3 (Fig. 4). When the new line touches the peak point on the characteristic, the
circuit triggers and the relay drops out.
R1 - 47 ohms, see text
R2 - 20 ohms. pot (see text)
R3 - 10 ohms
All resistors 1/2-watt 10%
C1-0.001, ceramic
C2 - 25-μμf trimmer
L - 16 turns No. 18 enameled, 1/4-inch diameter, closewound
XTAL - see text
S - push-button, normally open
D - tunnel diode, 1N653 or equivalent
BATT - 1.2-volt, nickel-cadmium rechargeable. (The one shown was mounted in a
metal screw cap with a phone plug.)
Miscellaneous hardware, jacks, boards or other housing as desired
Fig. 3 - The tunnel-diode oscillator on the right; power supply on the
left.
A look inside the oscillator.
Switching circuit uses a tunnel diode.
One resistor sits under the chassis of the switching circuit.
The photo shows an actual switching circuit based on the above experiment. The
relay must have a low-resistance winding. Being unable to find a suitable unit,
I modified a Sigma 5F-1000, which has two coils in series, for a total of 1,000
ohms. They are reconnected in parallel. to cut resistance to 250 ohms. Observe polarity
of windings. The yellow lead of each coil is soldered to the black of the other.
A 470-ohm resistor is shunted across the windings. This lowers the resistance and
the sensitivity of the relay. Now the relay should be adjusted to pull in at 4.6
ma, release at 2.3 ma, approximately. Prime the circuit by pressing the RESET button,
then lower R3 gradually until you are just below the trigger point. If triggering
occurs, note the point and start over again, this time not advancing the control
so far.
Fig. 4 - How the tunnel-diode characteristic is used in switching.
Fig. 5 - Tunnel-diode switching circuit.
When properly set, the illumination from a 40-watt lamp at about 18 inches is
sufficient to trigger the circuit and release the relay. Note that the solar cell
is in series aiding with the battery.
Fig. 6 - Practical tunnel-diode switch.
R1 - 470 ohms
R2 - 10 ohms
R3 - 20-ohm pot
RY - Relay, Sigma SF-1000, modified, see text
S - push-button, normally closed
D - tunnel diode, 1N653 or equivalent
V - photocell, Hoffman S1-A or equivalent
Miscellaneous hardware, perforated board, jacks, etc., as desired
This circuit may be made so sensitive that it can be triggered by switching on
or off a nearby soldering iron or other appliance. The magnetic field of the ac
does it.
If you reverse the polarity of the solar cell, you can prime the circuit with
illumination on the cell. Then you can trigger the circuit by interrupting the light
falling on the cell, even for a brief instant.
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