At first sight the circuit diagram of the uTracer may seem rather intimidating. It is true that there are quite a few components, but most of them are standard components like OpAmps, resistors, capacitors and transistors. Furthermore, the circuit consists of a number of blocks which to a large extend operate independently, and what is more, can be tested in dependently. On this page these blocks will be discussed in some detail. To keep the explanation comprehensible a lot of details will be skipped. These details can be found in the project blog of the uTracer 3 tube tester project. For the convenience of the readers a [MORE] link is included which directly points to the relevant part in the project blog text.
Figure 1. Building blocks of the uTracer V3 tube tester. The complete circuit can be found at the bottom of this page.
The heart of the uTracer is a 16F874 PIC micro-controller from MicroChip. The controller is used in a standard configuration with a 20 MHz external X-tal. The firmware in the 16F874 was developed using “in-circuit” programming [MORE]. For in-circuit programming a number of connections between the controller and the programmer are required. Alhough the micro-controller is available with the firmware code already programmed into its flash memory, a program connector was included on the PCB anyway. So in normal use this connector is not used. A special point of attention is Jumper J3. This jumper should be placed to avoid erratic resetting when the circuit is not connected to a programmer. Almost all of the I/O’s are connected to different parts of the circuit. These points are indicated in the circuit by a closed connection triangle. Almost all of the analog inputs are protected for voltages outside of the supply voltage range by two Schottky diodes which clamp the input voltage to the power supply lines [MORE]. Some of these diodes are drawn near the controller (D22-D25), while the others are distributed over the circuit. The high voltage switches for the anode and screen voltages are operated through opto couplers [MORE], the LED part of which is drawn near the controller (OC1, OC2). The microcontroller communicates with the PC via a serial RS232 link. A very standard MAX232 is used to translate the TTL levels used by the controller to the levels used by the RS232 protocol. Jumper J4 is only used during the construction of the circuit to allow for testing of the serial link and the MAX232.
The Anode and Screen Supplies.
As explained on the previous page, the unique feature of the uTracer is that it works in pulsed mode. This means that the anode and screen voltages are only supplied to the tube for a very short time, one millisecond to be precise, and that during this short interval the anode and screen currents are measured. This greatly simplifies the design of the high voltage supplies [MORE].
Figure 2. Detail of the total circuit showing one of the high voltage supplies.
Figure 2 shows the part of the circuit which is associated with the anode voltage supply and the switch which connects the supply to the anode of the tube. The circuit for the screen is identical to this circuit. The heart of the circuit is electrolytic capacitor C18. This 100 uF / 450V capacitor supplies the energy to the anode during the measurement. The capacitor is connected to basically five circuits. In the first place it is charged by a very simple boost converter consisting of L4, T14 and D13. When a pulse is applied to the gate of T14, the current through L4 will start to increase. When T14 is opened again, the energy stored in L4 will be dumped into C18, so that the voltage of C18 will increase a bit. In this way the voltage over C18 can be increased to any value by just pulsing T14. This process is controlled by the micro controller which measures the voltage of C18 though a resistive voltage divider formed by R32 and R33 [MORE]. In this way the voltage of C18 can only be increased. It can of course sometimes also be necessary to decrease the voltage of C18. This is done with the circuit around T15 [MORE]. When T15 is closed, C18 will be discharged though R35. The discharge current will at the same time charge capacitor C19 to a maximum voltage of 10 V which is limited by zener diode D15. The purpose of C19 will be explained later.
Resistor R45 is the heart of the current sensor [MORE]. At first sight its position - in the ground lead of the reservoir capacitor - may seem a bit strange because one would expect the current sense resistor to be placed in the anode lead. The choice for this position becomes clear when we realize that the circuit has been designed in such a way that during a measurement pulse all the anode current also passes though the C18. This is achieved in three ways: the boost converter is switched off during the pulse (D13 blocks), T15 is off, and the high voltage switch is operated via an opto-coupler so that it is isolated from the rest of the circuit. In this way the current flowing through R45 is exactly equal to the anode current, and is it possible to measure the voltage drop over R45 with respect to ground which makes the current sense circuit simple and robust. Finally, diode D14 conducts the charge current during charging of the reservoir capacitor, and the low side of the voltage divider R32/R33 was connected to the “hot-side” of R45 so that the current in the divider does not add to the measured current [MORE].
The last part of this part of the circuit is the high-voltage switch which is the part of the circuit around T16, T17, T18 and OC2 [MORE]. As explained in the previous section, one of the important design considerations of this switch is that, in order for the current sense circuit to function correctly, the high voltage switch has to be completely isolated from the rest of the circuit so that no current can leak away. This is achieved by controlling the switch with an opto-coupler. This has the additional advantages that it protects the micro-controller from being damages should the switch fail for one reason or the other. The switch itself consists of pnp darlington pair T17/T18. When the switch is closed (conducting) the base current for T18 is supplied by C19 through the conducting transistor in the opto-coupler. This means that C19 is always charged prior to a measurement by shortly pulsing T15 [MORE].
The uTracer tube tester employs several defense mechanisms to protect the uTracer should a direct short circuit (flash-over) between the anode / screen and one of the other electrodes or ground occur. One of these mechanisms is the current limiting circuit formed by T16 and R38. When the current exceeds 250 mA, the voltage drop over R38 will switch on T16 which will sink just so much of the base current of T18 that the current will be stabilized at 250 mA [MORE]. However, in case of a full short circuit at maximum voltage this still means an instantaneous dissipation of 0.25*300 = 75 Watt which, even if it is just for a millisecond, will destroy the circuit. For this reason there is a second defense mechanism to protect the circuit which will be discussed in the next section.
The Current Amplifiers.
In the previous section the basic working of the current sense circuit was explained: the anode current supplied by the reservoir capacitor is measured at “the corld side” of this capacitor by a current sense resistor against ground. This provides a simple and drift free method to accurately measure both the anode as well as the screen currents.
Figure 3. The high voltage supply and the current amplifier.
In Fig. 3 the current amplifier circuit has been added to the high voltage supply circuit already depicted in Fig. 2. In the strictest sense the term “current amplifier” is not correct, because it is not the current which is amplified and measured, but the voltage drop over the current sense resistor R45. Unfortunately this voltage drop is negative with respect to ground, so before we can process it further the signal has to be inverted. This is done by OpAmp IC7 which provides a -1X amplification. Capacitor C20 was added to limit the measurement bandwidth and thus the noise. The positive signal at the output of IC7 is limited to ground and the 5 V supply voltage by R42, D17 and D18 and fed into the input of IC8. IC8 is a Programmable Gain Amplifier (PGA). Under software control the gain of the PGA can be set to 1,2,5,10,20,50,100, and 200; a set of gains which is sometimes also called “a scope range.” It is this PGA which really turned the utracer into a valuable instrument which can accurately measure any current between several uA and 200 mA [MORE] [MORE]. As mentioned the gain is set under software control whereby the user has two options: 1. Select a gain manually, 2. Use an auto-gain algorithm so that always the most optimal gain is selected.
Additionally the input signal of the PGA, basically the inverted voltage drop over the current sense resistor, is connected to one of the analog inputs of the controller. It is a part of the second defense line against short circuits. In the micro controller the signal is compared to the output voltage of an internal programmable voltage reference. When the voltage drop over the current sense resistor exceeds a certain pre-programmed value an interrupt is generated which immediately switches off the high voltage supplies. In practice this means that the high voltage is switched off within 20 us after an over current situation has been detected. To summarize what happens during a short circuit: first the current is limited by the hardware circuit around T16 and R38. This limits the current, but can still causes excessive heating of the switch even though the measurement pulse only lasts 1 millisecond. So 20 us after a short circuit is detected by the controller, the high voltage switch is opened to limit the total amount of dissipation. Despite these precautions care should be taken to avoid a short circuit since this is at any rate a very violent event which stretches the circuit to the limits [MORE] [MORE] [MORE].
The Heater Supply.
The heater supply circuit is very simple (Fig. 4). Since the heater of a tube is basically nothing more than a resistive load, a simple Pulse Width Modulation circuit can be used to control the amount of dissipated power. Heaters, especially heaters of power tubes, consume a lot of power. An EL34, a popular audio output pentode, dissipates about 1.5 Amp at 6.3 V. To keep the costs of the project low the uTracer was designed to operate from an (old) laptop power chord. These power supplies are small, cheap can supply a lot of power, usually many tens of watts. The output voltage is usually something between 17 and 22 V. Since the heater voltage is directly derived from the power supply it is important to know the exact value. That is why the uTracer measures the supply voltage immediately after start-up through voltage divider R43/R44 (see complete circuit diagram). It also implies that the maximum filament voltage is limited to the power supply voltage. When a higher heater voltage is required an external power supply has to be used.
Figure 4. Heater power supply.
Back to the heater supply. The input signal of the circuit is one of the two PWM outputs the micro controller has on board. This output provides a 19 kHz PWM signal with 10 bits resolution. The PWM output of the controller is first buffered by a totem-pole buffer formed bij T4/T5 so that the capacitive gate of T3 can be driven with short rise and fall times to minimise the switching losses. Mosfet T3 is a modern low voltage transistor with an Ron as low as 7 mohm. The peak currents surged by T3 can be very high, especially when the heater is cold. The filter L2/C6 smoothes out the very high current peaks to provide a somewhat nicer load to the power supply [MORE].
Since the writing of these pages it has become clear that the circuit has a problem driving tubes with low voltage / high current heaters. The problem is caused by inductances in the heater circuit. For these tubes it is possible to use an external DC power supply [MORE]
The Tube Circuit.
One of the problems with using a boost converter to generate the anode and screen voltages is that the output voltage of a boost converter can never be lower than its input voltage, in this case the 17 .. 19 V supply voltage. In order still to be able to measure the tubes to anode and screen voltages as low as a few volt a trick is used: the cathode of the tube is not connected to ground, but to the positive supply voltage. So suppose that the anode of the tube is to be biased to 10 V, then the boost converter output voltage has to be set to 10 + Vsupl. The supply voltage thus has to be accurately known to the GUI. This is also one of the reasons why it is measured at startup. A disadvantage of this “trick” is that it makes the grid bias circuit slightly complicate since the grid bias now has to be referenced with respect to Vsupl instead of ground.
Figure 5. The tube circuit
A problem with all tube testers is unwanted oscillations in the tube circuit. Tubes are made for amplifying, and especially in tube testers where they are often connected with long leads, conditions may occurre which cause them to oscillate. To suppress these oscillations the electrodes of the tube are provided with RFI chokes. These chokes are made of a lossy ferrite. So at DC or low frequencies they are basically a short, but at higher frequency the resistive part of their impedance rapidly increases so that they tend to suppress unwanted oscillations.
Another disadvantage of having the cathode connected to the supply voltage rather than to ground is that the current which flows through the tube during a measurement is now “dumped” into the positive supply line rather than to ground. This may cause problems for the power supply. Power supplies are in general made for supplying current, not for sinking them! The problem is solved by isolating the cathode from the power supply by diode D11, which is a Schottky diode so that there only is a very small voltage drop between Vsup and the cathode. Now the current through the tube during a measurement pulse is dumped in C9. This causes the voltage at the cathode side of D11 to increase slightly so that D11 blocks, isolating the tube circuit from the power supply. R30 quickly drains the excess charge in C9 to ground so that the original situation is restored.
D11 in combination with D12 and C9 also form another defense line against Short circuits and flash-overs. When a short circuit occurs the energy stored in the high voltage electrolytic capacitors is dumped in C9. This causes a voltage increase which isolated that part of the circuit from the power supply and the rest of the circuit. When the voltage reaches 24V, D12 starts to conduct and surges the remaining power [MORE] [MORE].
The Grid Bias Circuit.
The grid bias circuit generates a variable negative grid bias of 0 to -50V with respect to the cathode = the supply voltage. The input to the grid bias circuit is a 19 kHz PWM modulated square wave generated by the second PWM generator on board of the 16f874. A second order low pass filter around OpAmp IC4 converts the 0 – 100% duty-cycle of the PWM signal to a 0 - 5 V DC voltage [MORE].
Figure 6. The Grid Bias Circuit of the uTracer tube tester.
The circuit around IC5, T6 and T7 converts the 0 – 5 V signal to a 0 to -50 V signal with respect to Vsupl. It basically consists of a subtraction circuit with a 10x gain which has an additional output stage included in the feedback loop consisting of the current mirror formed by T6 and T7. The current generated by the OpAmp in the left branch of the current mirror (T6) is copied by the right branch (T7). The voltage drop over R9 in the right branch, in combination with the -40 V supply voltage connected to that branch allows the output voltage to swing between (almost) Vsupl and -40 V, so almost 60 V. To keep a margin the real wing is limited to 50 V allowing for a 10 V reserve. These kinds of circuits always have a tendency to oscillate, it is therefore important to use an OpAmp for IC5 which is not too fast. A good old 741 is ideal for the job. C12 finally reduces the tendency to oscillate even further [MORE].
The Negative Power Supply.
The negative power supply generates the negative supply voltage for the OpAmps as well as the -40 V negative supply voltage for the circuit which generates the control grid bias [MORE].
Figure 5. The Negative Power Supply.
The negative supply voltage is generated by an inverting boost converter formed by T1/T2, D1, L1 and C1. T1 supplies the base current for T2 which is driven heavily into saturation. Just like the two high voltage boost converters, also this boost converter is controlled by the micro-controller with a feedback loop. The negative output voltage is measured via a resistive voltage divider against the +5 V supply so that the 0 to -40 V output voltage of the boost converter is translated into a positive voltage between 0 – 5 V. The -15 V supply voltage for the OpAmps is generated with an LM337 since the -40 V input voltage is a bit too much for a standard LM7915 [MORE].
Figure 6. Total circuit of the uTracer V3.
| previous page | next page |
Cartoon bij Hans Evers PAoCX