Tektronix TM 5006 manual Theory ο1 Operatio n-ΤΜ, Overvoltage and Overcurτent Detectors

Theory ο1 Operatio n-ΤΜ 5006

The positive going output pu lses from the clock oscillator charge C2150 to about 9.5 V through CR2042. When t he positive pulse at pi n 4 of U1 8408 drops to 0 V, C2150 discharges through R2050 causi ng α falling ramp waveform of about 50 mV peak-to- peak am plit ude to appea r at pi n 4 of U2140A.

The +8 V from the power supp ly out put is applied to voltage adjust potentiometer R2151 . The voltage on pi n 4 of U2140A is +7.15 V, the reference voltage ge nerated in 02250. Also, on pin 4 is α negative going 40 kHz ramp, as previously described. This ramp is ac coupled to pin 4 through C2140. On the rising edge of each cloc k pulse, the ramp goes positive rapidly. Pi n 2 of U2140A is low. At some point, duri ng the ramp decay, the ramp voltage and the feed back voltage at pin 5 are equal . At this point, pin 2 goes high, termi nating the drive pulse through the logic circuitry . The higherthe output voltage, the earlier i n the ramp cycle pi n 2 goes high.

Overvoltage and Overcurτent Detectors

Pi n 11 of U2140D, the negative over-voltage-detector, co nn ects to α voltage divider between the -26 V supply and the reference +7.15 V . Should pin 11 go more negative than pin 10, pi n 13 goes low shutting off the output. The input of U2140D is protected from α negative voltage by CR2130.

Primary curre in output transformer Τ1710 flows through T1750 1 . The secondary voltage of T1750 is proportional to (he primary current. The secondary voltage of T1750 is rectified by CR1860, CR1861, CR1862 and CR1863 and terminated in R1860. When the primary current i n Τ1750 exceeds the poi nt where the voltage at pi n 6 of U2140B exceeds t he 7.15 V reference at ρίη 7, pin 1 goes low turning off the output transistors via the wait mu ltivi brator.

The +26 V is applied through R2231 and CR2240to pi n 8 of U2140C, the positive over voltage detector. The +8 V is also applied through R 2130 and CR2230 to pi n 8. Pi n 9 of comparator U2140C connects to the +7 .15 V reference voltage. If pin 8 of U2140C goes more positive pi n 14 goes low. This action triggers U 1940A the wait multivi brato r, turni ng the supply off for about 3 seconds. The soft start cycle follows. This negative goi ng pulse is ti me delayed by R 2040 and C2052.

Wh en +10 V is app lied at power up, C1940 holds pin 3 (clear) of U1940A low for α short period. This over-rides the Α and Β inputs of U1940A, causi ng pin 6, the 0 output, to remai n low. Over-voltage or over-current causes α low at pin 4 of U1940A causi ng one high level pulse of about 3 seconds duration at pin 6. This 3-seco nd pulse duratio n time is determined by C1930 and R1931 . The clock pu lse

retriggers U 1940A if the fault persists . The purpose of CR1950 is to discharge C1940 when ac powe r is removed from t he supply . Noise from the li mit ci rcuitry is filtered by C2052.

Control Ci rc uit Regulator

The 16 Vac windi ng on Τ500 is applied through F2340 to rectifier diode CR2341, which charges filter capacitor C2260 to approxi mately 20 V. The +20 V is applied to voltage regulator U2250 . This reg ulator outputs two voltages: +10 V which is used throughout the enti re supply, and +7 .15 V, α reference voltage, at pi n 6.

The line detector ci rcu itry is composed of CR2340, C2350, 02240 and associated co mponents . Wh e n normal li ne voltage is app lied, the voltage across C2350 is approximately 20 V. Transistor 02240 is on and pi n 2 of U2250 is about 0.2 V above ground. If about two cycles of li ne voltage are missed or the li ne voltage goes low, 02240 no longer sat ur ates . The collector of 02240 rises, disabl- i ng the series pass transistor located i ntern ally i n 02250. This series pass transistor is effectively connected between pins 11 and 10 of U2250. The +10 V is removed from the power supply duri ng line drop out to prevent discharge of the mai n filter capacitors i n the output stage. Positive feedback is provided through R2241 tothe base of 02240 to improve the switchi ng action.

The PWR signal circ uitry (02160) pr ovides α sig nal to each compartment i n the power module to give power supply stat us informatio n to the plug-ins. See the rear interface information part of the Mai ntenance section (Section 5) of this manual fo r timing i nformation.

Pi n 7 of U2160 goes low when the rising voltage at pins 2and 6 reaches 2!3 of the value of the voltage connected to pin 4 (+1 V) . Pin 7 of U2160 connects to the base of

01525 . Th is transistor i nverts the signal from pi n 7 to the ρΙυ ιη co mpartments .

When the line power goes low or off, pin 13 of U2250 goes low. This action raises pin 7 of U2160 t urning off the PWR signal . Pi n 7 of U1940A is also low duri ng the 3 second wait state. The cathode of CR2150 is pulled low which turns off the PWR sig nal .

The soft start feat ure also co ntrolst he PWR signal . This is accomplished throug h R2061 .

Wh en α fault occurs, pin 6 of U 1940A goes high. When the fault is removed pin 6 of U1940A goes low causing pi n 8 of U1840D to go high. As the voltage at the j unction of R 2061 and R 1953 goes high pins 6 and 2 of U2160 also go high causing t he PWR signal to go high.