298 lines
16 KiB
Plaintext
298 lines
16 KiB
Plaintext
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| File Name : UJTOSC.ASC | Online Date : 12/12/94 |
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| Contributed by : Darrell Moffitt | Dir Category : ENERGY |
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| From : KeelyNet BBS | DataLine : (214) 324-3501 |
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| KeelyNet * PO BOX 870716 * Mesquite, Texas * USA * 75187 |
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| A FREE Alternative Sciences BBS sponsored by Vanguard Sciences |
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The following is based on experiments done by Darrell in his work with high
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density charge clusters and how they could be generated. Darrell is an active
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researcher who has freely shared his ideas with KeelyNet. This is an
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excellent file with numerous applications for the experimenter.
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Thanks Darell!
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------------------------------------------------------------------------------
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UJT Relaxation/Pulse Oscillators
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by Darrell Moffitt
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(with little thanks to inaccurate references,
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and many thanks to those which aren't)
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These following circuits depict unijunction transistor based oscillators.
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They are easily modified to vary pulse width and repetition frequency over a
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broad range, and are recommended for applications requiring outputs between
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1/8 and 3 watts, at current levels between 5 and 250 milliamps.
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For example, an npn - coupled circuit will yield 100 milliamps at 7 - 7.5 VDC
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(app. 10 VAC) from a fresh 9 V battery. Source input may be varied from 3 V
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up to approximately 30 V, though greater care is necessary at higher voltages
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and frequencies. This will be explained in greater detail in the section
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describing circuit operation.
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For those unfamiliar with unijunction transistors (ujt's), a bit of background
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is in order. The ujt is among the oldest of active semiconductor components,
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dating from the early 1950's. It is also the most simple in construction and
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use. Its primary function is switching and timing, with a frequency range from
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several minutes to several hundred kilohertz.
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Some varieties go up to app. 1 megahertz, notably the 2N4947. The more common
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varieties are the 2N2604 and the 2N4892, which sell on average between $.30
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and $.50 apiece. Both are rated for up to 30 V source input and 50 milliamps
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emitter output.
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Ujt's consist of a single bar of n-doped silicon, with a positive region which
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acts as a gate and control junction. The positive junction is termed the
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emitter, while the ends of the negative bar are referred to simply as base-1
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and base-2. The positive junction is an ohmic connection, which indicates that
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the positive region is attached, rather than diffused. It is this property
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which gives the ujt its special characteristics.
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The technical details of ujt function will be addressed later, but it suffices
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to note that ujt's function much like the better known silicon controlled
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rectifier, with current controlled by voltage.
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______
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| N |---base-2
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|__ |
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emitter --- |p_| | Unijunction transistor
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|_N__|---base-1
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The crudest form of ujt oscillator requires just three parts, and will output
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close to 50 milliamps, at a voltage controlled by the source and the timing
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resistor. The waveform is that of a typical relaxation oscillator, roughly a
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bent sawtooth. By adding extra bias resistors across the two bases, positive
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or negative pulses may be drawn off as well.
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This circuit yields ONLY the relaxation waveform, while the following circuits
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yield either weak or amplified pulses. Here now is the simplest:
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---- ---- (+)
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R1 () | b2 |
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<---- ---- Û ujt # battery
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output C1 [] | b1 |
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<---- ---- ---- (-)
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The timing resistor, R1, MUST be set at a minimum of 2K, though specific apps
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may require higher values. This is necessary to maintain stable oscillation.
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R1 controls both power and frequency SIMULTANEOUSLY, so adjustments of the
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timing capacitor, C1, are necessary to achieve a desired frequency and power
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output. Oscillation rate is approximately .8/R1*C1.
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In these circuits, base-1 is connected to the negative ground line and C1,
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while base-2 is connected to the positive rail and R1. The ujt emitter
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connects to both C1 and R1 at the junction, and C1 takes any value between
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several hundred microfarads and about 500 picofarads.
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C1 may be either polar or non-polar - note the polarity! The negative pole of
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C1 should always be attached along the negative ground line (base-1) if polar
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capacitors are used.
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Some circuits require sensitive setting of R1. For most applications, it is
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not recommended that one go above 10K, as this reduces power output to
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unacceptable levels.
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You can usually identify base-1 and base-2 by given specs in metal-can TO-18
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models, while the TO-92 plastic case (2N4892, 2N2604) models place base-1 to
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the left side of the flat face, and base-2 to the right, as the ujt is seen
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head-on.
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A pulse circuit merely adds two resistors, typically 47 ohms on base-1, and
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100 to 180 ohms on base-2. 100 ohms on each base is typical. A weak negative
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pulse is taken from base-2, while a high (positive) amplitude pulse is taken
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from base-1. Pulse forms are rounded or "spiked" unipolar, and output is
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generally less than that of relaxation circuits.
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You will no doubt wish to try low value potentiometers to optimize
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performance. Adding a diode between the ujt emitter and R1/C1 junction will
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increase the standoff voltage of the device and yield roughly square pulses.
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(The anode of the diode connects to the emitter.)
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You may also couple R2 through a small coil to C1 on the negative ground rail,
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thereby achieving more complex waveforms, or, construct a small auto-
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transformer connected such that base-1 connects to the tap, while the two ends
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are connected from the end of R2 to the negative pole of C1. This will boost
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the output voltage slightly and, by raising the voltage to C1, decreases the
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time of the C1 discharge cycle.
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For higher performance square pulses, the circuit below adds an npn transistor
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inverter, which takes the output of base-1 (+), feeds it to the base of the
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npn (direct coupled) and outputs a negative pulse of higher amplitude. In this
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circuit, it is MOST important to optimize the base-1 resistor (R2) at a value
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between 100 and 1000 ohms, depending upon your choice of npn transistor.
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This is due to the tendency of direct coupled circuits to overheat, AND the
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necessity to protect the base-emitter junction of the npn from overvoltage.
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This will be explained in more detail shortly. Here now is the square pulse
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oscillator:
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(+)
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|-----------|
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| --------- .---> (+) output
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| R1 () | .---> (-) output
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battery # ---- Û ujt | < Collector
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| C1 [] () R2 --² npn < Base
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| --------- | < Emitter
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|-----------| (npn orientation)
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(-)
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Note again that R2 is connected between the base and emitter of the npn. That
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is, R2 is connected in such a way that one end is attached to base-1 of the
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ujt, and the other end is attached to the negative ground line AND the lead
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from the emitter of the npn. Another lead goes from base-1 of the ujt to the
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base of the npn.
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The optimal value of R2 depends upon the setting of R1. In general, R1 is
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inversely proportional to R2. Also, high frequency operation (above 10 KHz)
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appears to require higher R2 values due to power characteristics which require
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more stabilization. It is recommended that you use a 5K-10K pot for R1, and a
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1K pot for R2. This allows you to "dial up" desired outputs with different
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varieties of npn. When R1 is at a minimum, 220 ohms or higher is suggested for
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R2.
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This circuit has been tested with several kinds of npn. All appear to work,
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but R2 will vary widely. 330-470 ohms is a good general range. When tested for
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extended runs (2 hrs.) with a 9-18 V input, and R2 set at 470 ohms, good power
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output was achieved with no shutdown.
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The npn types tested included common (TO-92 case) 2N3904's and 2N2222's, but
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better results were achieved with metal-can 2N2222A's and .8 A, 1.8 W B5J 7539
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medium-power transistors (available in Radio Shack "grab bag" packages). SK-
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3010 transistors (used in TV's) and similar were successful, but their results
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were weak in comparison to those designed for low power supplies.
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Again, one should experiment with potentiometers for given applications. The
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output, taken from the positive rail and collector of the npn, may be filtered
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through an RC differentiator and diode shunt to isolate the negative pulse.
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You may observe while testing that this circuit generates substantial radio
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frequency output. A common receiver will detect the output at its RC frequency
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- that is, one may place the receiver close (6" or so) to the circuit, and
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detect a tone at the RC frequency.
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When R1 is set around 3,000 ohm, and C1 is set at 1 nF (giving an RC around
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200 kHz), an audio tone may still be detected with the receiver. This is
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likely due to stray harmonics, and may be used or shielded as desired.
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(Detection is through the AM band from 550 kHz to 1600 kHz...).
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This circuit may be modified to control pulse width by replacing R2 with an
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inductor, typically of 100 microhenries or more. The LC constant then
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determines pulse width, while the RC constant controls repetition rate. In
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this case, a resistor of 330-470 ohms is connected between the positive rail
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and base-2 of the ujt.
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Another resistor of 220-330 ohms is connected from the positive rail of the
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ujt to the positive rail of the npn, while a resistor of 47-220 ohm is
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connected between base-1 of the ujt and the base of the npn (coupling
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resistor). By setting C1 low (1 nF) and the inductor at 100-500 microhenries,
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you may obtain pulse widths below one microsecond. One circuit of this sort,
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with 27 VDC input, output large sparks when the leads were shorted.
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------------------------------------------------------------------------------
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Ujt Theory
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The ujt is a voltage controlled switch, similar to the silicon controlled
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rectifier or thyristor. When voltage is applied to the ujt, current flows from
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base-1 to base-2 with a resistance of 7-10K. The positive emitter is
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controlled by voltage increase across C1, so that when C1 reaches a sufficient
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value, the emitter conducts and discharges C1.
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The most important values in ujt's are the intrinsic stand-off voltage, î, and
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the interbase resistance, R-Bb. R-Bb represents the sum of Rb1 and Rb2, the
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base-1 and base-2 resistance:
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R-Bb = Rb1 + Rb2.
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With the emitter open, Rb1 is larger than Rb2, so the total voltage across Rb1
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and Rb2, V-Bb, is divided proportionally.
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The fraction of V-Bb appearing at the emitter junction (between Rb1 and Rb2)
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is the stand-off voltage, î:
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î = Rb1/R-Bb
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which may also be written:
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Rb1 = î * R-Bb.
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Thus,
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î = Rb1/(Rb1 + Rb2), and Vb1 (voltage across base-1) =
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î * V-Bb.
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î, V-Bb, Rb1 and Rb2 are typically given by manufacturers for specific models
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of ujt.
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The voltage at which the emitter conducts, Vp, or peak voltage, is
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proportional to the product of î, and V-Bb plus the diode potential, which
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is about .7 V for silicon. Thus,
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Vp = î * (V-Bb + .7 V).
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For V-Bb = 9 V, î = .78, the emitter conducts at .78 * (9 V + .7 V), or 7.57V.
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More extensive theory is available in texts, but a word of caution is in
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order. While some texts indicate that the timing resistor of the circuit, R1,
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may be set as low as a few hundred ohms, based upon detailed calculations,
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this is rarely the case. More to the point, it is often FALSE. For reasons not
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immediately apparent, setting R1 below 2k almost inevitably causes unstable
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operation. Many thanks to that rare reference which states this explicitly!
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The exception to this rule may be those oscillators which employ capacitors
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larger than 10 microfarads. In this case, the longer time constants, (and
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stabilizing characteristics of large capacitors), may allow one to go down to
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700 ohms or so. You will probably find that quick potentiometer testing is
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more reliable than theory, in this case. This is especially true when one is
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working with "off the shelf" parts of dubious origin.
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The most ubiquitous ujt is known to this experimenter simply as the "mu 10",
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so called because it is marked with the Motorola logo, resembling the Greek
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letter "æ", and an "I O" along the base. It is probably the 2N2604, and sells
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for $.25-.30.
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Applications
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For those desiring a cheap, powerful oscillator to step up voltage with
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transformers, this is one of the cheapest and most versatile of all circuits.
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With a 9 V battery source, this circuit easily lights NE-2 bulbs from a 12.6-
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120 VAC @ 300mA transformer (Radio Shack 273-1385B) and other low frequency
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tranformers. It continues to do so when C1 is varied from 47 microfarads down
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to 1 nanofarad, with R1 set between 2-5K, and R2 set at 470 ohms.
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Furthermore, it continues to light the NE-2 when a second stage, 5-230 VAC @
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.4 A transformer is added, giving several kilovolts AC at a minimum 1/25 W.
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(CAUTION!)
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Another application is in ultrasonics. One may purchase small transducers for
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$2.00 or so, and power them from one or more of the circuits described.
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Suggested experiments include sonoluminescence and levitation. Both require
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the use of opposed transducers.
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For sonoluminescence, one merely directs opposed transducers through a fluid
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medium, such as water, and observes the light discharge which originates from
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cavitation.
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Levitation may be observed by opposing tranducers in a small cylindrical
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chamber filled with small particles. When vapor or smoke is introduced to the
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chamber, one may observe the active interference patterns created by sound.
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You should note in both sonoluminescent and levitation experiments that the
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effects are due to the properties of longitudinal wave action.
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In ultrasonics experiments, current is typically the more important quantity.
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For a given power output, one way wish to use lower voltage, high current
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sources, such as D cell batteries.
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This set of circuits compares well with more sophisticated oscillator types,
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such as astable (dual npn/pnp form) multivibrators or Miller, Colpitts and
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similar LC oscillators. In particular, it is cheaper and more versatile than
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most op-amp or general transistor oscillators.
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With an average cost around $1.00, it serves many needs for simple, powerful
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oscillators. The ujt, though old and simplistic, will likely continue to serve
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in some form for years to come.
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Darrell Moffitt, 1993
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