This extremely small Tube Tester measures tube question: Calculate the ripple factor of a full wave 6 phase rectifier Calculate the ripple factor of a 6 p… in a pulsed mode. My current project in the form of a web-log. In contrast to most of these tube-lovers I have, at least up to now, little interest in tube amplifiers.
I simply do not have the ears for it! I am however fascinated by the technology of these fragile and romantic devices and I love to read and write about their history. It also applies to this project. Once the idea of a pulsed tube curve tracer was conceived, all the parts of the system seemed to fall into place as if they had been waiting to be put together. 1 Symbolic circuit diagram of the µTracer.
For clarity the processor and other digital parts have been omitted. In a normal curve tracer these would be by far the most difficult and by far the heaviest parts of the tester. 1 A few snapshots from a very early publication describing a curve-tracer mechano-electrical curve tracer . The center picture shows an example a set of Ia-Va curves. The right picture depicts the screen-grid current of the same tube under the identical measurement conditions. Philips Technical Review of 1938 .
The left figure depicts the circuit diagram of the curve tracer. The anode voltage is derived from a 500 Hz high voltage generator, most likely a mechanical alternator. The stepping voltage for the control grid was generated by a rotating commutator. 2 A description of this apparatus designed to measure transmitting triodes appeared in the Philips Technical Review of 1939 . A year later in the same periodical an article is published describing a curve-tracer especially designed to characterize transmitter triodes . During normal operation of transmitter triodes, it is quite common that the control grid becomes positive resulting in a significant grid current.
It is therefore important that these tubes can also be characterized in that biasing regime. 3 Click Here to view some more pictures of this amazing beast! After the war Philips emerged as the dominant tube manufacturer in Europe. The pentode patent in combination with its enormous production capacity all over Europe made its position as the foremost player in this field unchallenged. 4 As an example of what the curve-tracer of Fig.
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3 was capable of these measurements on an ECL80 are shown. Left, the pentode section, middle, the pentode switched as tetrode and right, the triode section. The whole monster comprised some 200 tubes. Assuming an average life-time expectancy of 10. 1 depicts the test circuit that was used in the first experiments to ascertain whether a voltage in the range of 300-400 V can be achieved with a simple boost converter. The heart of the boost converter is formed by inductor L1, MOSFET T3, and diode D2.
Q: OK, I understand the inputs and outputs, but where does the signing come in?
In the test circuit of Fig. 1 this pulse is generated by the differentiating network formed by C1 and R1, Diode D1 protects gate N1 against negative spikes caused by negative flanks of the square wave input signal. T2 form a buffer to drive T3 with as steep as flanks as possible. The charging time of the output capacitor strongly depended on the input frequency. MOSFET be driven directly from the processor? In the original plan I had, the anode current was measured my measuring the cathode current via the voltage drop over a cathode resistance.
This in reality measures the sum of the anode current and the screen current. The most left diagram in Fig. 2 depicts the pnp current mirror as I have intended it for this application. Note that T1 is switched as a diode, more precisely as the diode in the emitter-base junction. What is the maximum screen current that can be handled. The high-side voltage switch which pulses the high voltage to the anode and screen-grid of the tube is one of the key components of the circuit.
It should be able the switch on the voltage quickly enough, and should have a low voltage drop. Basically there are two options for this switch: a high voltage p-type MOSFET or a high voltage pnp transistor. I know that most people would choose for a MOSFET. They can easily handle the currents involved and can have a very low on-resistance.
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When the switch is pressed, they output of of N2 becomes high. The differentiating network C1, R3 in combination with N3 reduces the pulse length to ca. N7 buffer and invert this pulse. The high-side switch itself is formed around T1 and T2. 4 First pulsed measurements on an EL84.
I really could not resist trying the pulse circuit in combination with a tube. The first thing observed was that without C3 and C4 the tube immediately oscillated. 5 First pulsed measurements on an EL84. Having realized this, the left circuit of Fig. A much better solution is circuit B in Fig. Here T2 is used as a current source whose current is determined by the amplitude of the input pulse and the value of R1.
For the control-grid bias it was assumed that a range of 0 to -20 V would be sufficient to cover most tubes. This bias needs to be applied with respect to the cathode. Since the cathode current is measured by the voltage drop over a series resistance, some kind of circuitry was needed to correct for this voltage drop. A voltage drop over the cathode resistance was simulated with voltage source. An arbitrary voltage of 1 V was chosen. Resistors of 12k1 and 47k were used because there were readily available. 9 shows the circuit that was used to test the circuit.
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The working of the circuit is simple. The BC548 npn is driven with the same 15-20 us pulse that is used for the high-voltage converters. I did have a whole bunch of BD138 transistors. Agreed,a power PMOS transistor would perform better in this circuit, but the performance is not that critical so that a simple BD138 performs good enough. The voltage dividers reduce the high voltages so that they can be measured with the on-chip AD converter which has an input voltage range of 0-5V. They consist of a simple resistive voltage divider and a protection diode. The protection diode ensures that when, for any reason, the input voltage is too high, the voltage on the input of the AD converter is clamped to Vdd.
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Here Vin is the high voltage input, Vout the input voltage of the AD converter, and n the output value of the converter. To ensure the in the datasheet of the AD converter specified acquisition time, the impedance of the circuit connected to the AD converter should not be much higher than 2k. 1 One of the best parts of any project: bread board testing! In this case the thyristor protection circuit is being tested.
Some people are of the opinion that instead of actually building and testing a circuit, you could just as well simulate them! To a certain degree I agree with them. If you have good component models, simulation is a valuable and powerful tool. Most simulators and device models do not, or at best very poorly, include thermal effects.
The same holds for junction breakdown behavior. 2 When all the difficult circuit parts have been tested on breadboard, everything comes together on perfboard. When you build up the circuit on a perfboard, do it step by step! The larger the circuit, the bigger the chance that you will make a mistake, and the more difficult it becomes to trace that mistake. In all projects that use a microcontroller, I almost always start at that end of the circuit. Once you have the mico-controller up and running, testing of the rest of the circuit becomes easier.
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The digital part, basically only the microcontroller and RS232 driver. The software for the microcontroller, which controls the analog electronics, performs the measurements and communicates with the PC. However, at this moment the analog part of the circuit as I have it in mind is shown in Fig. I you have read the previous section, the circuit will look pretty familiar and straightforward. The digital part of the circuit could hardly by more straightforward. For maximum speed and accurate serial communication timing an external 20 MHz Xtal was used. MAX232 taking care of the necessary level translations.
There really is not anything more to tell about it. I decided to let the PC do the entire user interface so that the tasks for the microcontroller are rather limited. The microcontroller in the first place has to receive the settings from the PC. When the settings are received, it will charge the buffer capacitors in the boost converters and set the grid bias.
When the anode, screen and grid voltages have reached their set-point, all boost converters are switched off to reduce noise, and the electronic switches are closed. 5 Example of the command and reply strings. The communication protocol between PC and microcontroller is very simple. The PC always sends a command string with fixed length of 18 ASCII characters to the microcontroller, and the controller in return always returns with a reply string of 34 ASCII characters. 5 gives an example of the syntax of both strings. For me, the user interface software is one of the more challenging parts of this project.
In the end I want to have a nice windows interface a nice Ia-Vgk graph and all the other features one would expect from a windows interface. QBASIC and running from a dos command window. A Screen dump of the user interface of the program is shown in Fig. On the top of the screen the adjustable voltages can be found. By pressing F1, F2, F3 or F4, the setting for the anode voltage, the screen voltage, the grid voltage and the filament voltage can be changed. 7 This picture has absolutely nothing to do with this page, it is just here for my pleasure. I knew from the beginning of this project that the high-voltage switch would be the most difficult part of the circuit.
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It has to switch voltages up to 400 V with currents up to 200 mA. 1 A SEM micro-photograph of the BF487 transistor die that failed during testing of the high-voltage switch. Before diving into the circuit, I was very curious to see what actually happened to the BF487 high voltage transistor when the full power of a 100 uF capacitor charged to 400 V was dumped into the rest of the circuit through this tiny device. Unfortunately I was so stupid only to test this circuit only at 40 V because I was too lazy to build-up the high voltage circuit! The point I totally overlooked was that a resistor, because of its larger mass compared to a transistor, is much easier capable of dissipating high power pulses.
The test at 400V and 0. The simplest way to implement an overcurrent protection is shown in Fig. R6 is the current sense transistor which usually has a small value. When the voltage drop over R6 reaches 0. 4 Dynamic behavior of the overcurrent protection circuit at high voltages. For voltages higher than 200 V, the dissipation in the pnp transistor apparently reduces the breakdown to such a value that after some time the transistor breaksdown.