AVO CT 160 Meter Protection

Transcription

1 AVO CT 160 Meter Protection The following oscillograms show that with a real valve the voltage across the standard CT 160 meter (3250 Ohm, 30 ua f.s.) does not become flat zero when the Anode current is backed off to zero mean DC. The resulting voltage trace has pulses. Their amplitudes must not be affected by the forward voltage of antiparallel clamping diodes, otherwise the meter reading will be upset. The pulse amplitudes can be lowered by a parallel damping capacitor and they decrease with increased capacitance. Thus a relationship exists between minimum allowable forward voltage of the clamping diodes and the minimum capacitance required for unaltered meter reading. Voltages across CT 160 Meter All oscillograms were taken with a Fluke 97 Scope Meter and a 6L6GC valve load. Anode and screen voltages = 250 V. Tester connected to standard 50 Hz mains. gm scale in "set zero" position. AVO CT 160 Valve Tester modified as proposed by Martin Forsberg: 4x BYW96E Silicon rectifier replacement of EB91 valve rectifiers plus 1x BYW96E for Anode voltage rectification. Karsten Simon Meter at zero No damping capacitor Max = 1960 mv Min = mv With a purely resistive load the voltage would be flat zero. A real valve however begins conducting only after electrical zero degrees and stops conducting before 180 degrees whereas the backing off voltage is present during full 180 degrees. This leads to the positive and negative voltage pulses with a mean value of zero, i.e. area under the positive pulse = sum of areas under the two negative pulses. Meter at full deflection (approx. 97 mv mean DC) No damping capacitor Max = 2200 mv Min = mv Amplitudes grow with Anode current. Pulses are shifted upwards (positive) such that the result is approx. 97 mv mean DC. Therefore, the critical voltage which must not be clipped results at maximum current and full meter deflection. 1

2 Meter at zero Damping capacitor 13.6 uf (2*6.8 uf 50 Volt WIMA MKS2 Polyester parallel) Max = 432 mv Min = -392 mv Capacitor attenuates all harmonics of order n by 1/n. With sufficiently large damping capacitance this eventually leads to a sinusoidal wave form of frequency 2*f at every half cycle. Meter at full deflection (approx. 97 mv mean DC) Damping capacitor 13.6 uf (2*6.8 uf 50 Volt WIMA MKS2 Polyester parallel) Max = 584 mv Min = -328 mv Again pulses are shifted upwards (positive) such that the result is approx. 97 mv mean DC. Meter at full deflection (approx. 97 mv mean DC) before insertion of clamping diodes Damping capacitor 13.6 uf (2*6.8 uf 50 Volt WIMA MKS2 Polyester parallel) Silicon clamping diodes 1N4148 Max = 560 mv Min = -336 mv Meter drops to approx. 72 mv mean DC after insertion of clamping diodes. This upsets meter reading. Meter at zero Max = 96 mv Min = -96 mv Pulse amplitudes decrease with increased damping capacitance. Meter at full deflection (approx. 97 mv mean DC) Max = 204 mv Min = -4 mv Increasing damping capacitance further will eventually lead to a flat trace of mean DC volts. 2

3 Interdependence between Damping Capacitance and Clamping Diode Forward Voltage The following graph shows the measured positive and negative voltages as a function of damping capacitance. It may be used to select the proper combination of damping capacitance and forward voltage of clamping diodes. A capacitance of 100 uf is not only sufficient for the use of low forward voltage Schottky diodes but also greatly smoothes the meter needle motion thus rendering an efficient meter protection. AVO CT 160 Meter Peak Voltages vs. Damping Capacitance 2500 measured at maximum meter deflection with 6L6GC valve at Anode Volts = 250 and Screen Volts = Peak Voltage [mv] Current values as set with Anode Current backing off controls Capacitance [uf] 100 ma 85 ma 50 ma Clamping Diode Schottky (200 mv) Clamping Diode Silicon (600 mv) Karsten Simon

4 Load of Clamping Diodes The 1N5711 is a small signal Schottky diode with a maximum mean forward current of 15 ma. Peak forward current is not specified. Some data sheets state a 2A pulse maximum = 40 (A^2)*us load limit integral. This value is defined for the us time range and cannot be directly applied to ms pulse events. However, the load limit integrals calculated from the oscillograms below are so much smaller than the allowable 2 us maximum that they may be considered to be on the safe side. This has also been confirmed by the fact that the 1N5711 diodes did survive many repetitions of these worst case load tests. Worst case load of clamping diodes (case 1): Anode current = 100 ma Backing off controls at 0 Clamping diodes 1N5711 Upper trace = voltage across damping capacitor = 1500 mv Lower trace = current through clamping diodes = 23 ma (measured as voltage across 2 Ohm shunt resistor) Actual load limit integral I^2*t = 3 (A^2)*us Allowable load limit integral I^2*t = 40 (A^2)*us No overload of 1N5711 diodes Worst case load of clamping diodes (case 2): No valve Backing off controls at max. (90 ma + 10 ma) Clamping diodes 1N5711 Upper trace = voltage across damping capacitor = mv Lower trace = current through clamping diodes = -37 ma (measured as voltage across 1 Ohm shunt resistor) Actual load limit integral I^2*t = 7,6 (A^2)*us Allowable load limit integral I^2*t = 40 (A^2)*us No overload of 1N5711 diodes 4

5 Improved Meter Protection In the following cases 1A and 2A a pair of 1N4001 standard Silicon rectifier diodes has been connected in parallel to the 1N5711 Schottky pair. A combination of Schottky and standard Silicon diodes is superior to Silicon diodes alone: The maximum current through the two Schottky diodes is greatly reduced. The Schottky diodes have a much higher differential resistance than the Silicon diodes but they begin conducting at a lower voltage. This additionally reduces meter overshoot during normal operation compared to Silicon diodes alone. Due to their small differential resistance the Silicon diodes safely carry even high currents and thus protect the entire meter circuit also in cases of valve inter electrode shorts or erroneous electrode selector switch settings. Worst case load of clamping diodes (case 1A): Additional pair of diodes 1N4001 parallel to 1N5711 Anode current 100 ma Backing off controls at 0 Clamping diodes 1N5711 // 1N4001 Upper trace = voltage across damping capacitor = 750 mv Lower trace = current through clamping diodes = 9 ma (measured as voltage across 2 Ohm shunt resistor) Worst case load of clamping diodes (case 2A): Additional pair of diodes 1N4001 parallel to 1N5711 No valve Backing off controls at max. (90 ma + 10 ma) Clamping diodes 1N5711 // 1N4001 Upper trace = voltage across damping capacitor = -770 mv Lower trace = current through clamping diodes = -10 ma (measured as voltage across 2 Ohm shunt resistor) The graph below explains how both diode types complement each other. It plots the meter overload factor vs. the Anode current backing off setting with no valve inserted. It has been shown above that this is the worst case meter overload situation. Without any diodes the meter overload goes up to 74x full scale deflection (blue line). With Schottky diodes alone the overload is limited to 10x at 100 ma and 4x in the most common current range up to 60 ma (magenta line). With Silicon diodes alone the overload is limited to 6x and 5x respectively. The black dashed curve shows the combination of both diode types. It reduces the overload to about 70 % of what can be effected with a single diode pair (red curve). 5

6 AVO CT Effect of Clamping Diodes max. 110% Meter Overload Factor (worst case: no valve and ma/v dial at or beyond "set zero") with 100 uf smoothing capacitor 100% 90% 80% 70% 60% Overload Attenuation: (1N5711 // 1N4001) / (single Diode pair) Without Diodes 1N5711 Schottky alone 1N4001 Silicon alone 1N5711 // 1N % Anode Current Backoff Setting [ma] Overload Factor defined as DC mean multiple of f.s. voltage (= 97,5 mv) Karsten Simon The combination of small signal Schottky and standard Silicon rectifier diodes cannot simply be replaced by a single pair of "bigger" Schottky diodes because these tend to have a higher reverse current which will alter the meter reading. (Ende) 6