BNC 130 - Manual (Page 26)

26
THEORY OF OPERATION
Since it is desired to stabilize the Baseline level only, a signal proportional to the duty factor is required.
This signal is obtained from Z3-2 (via R163). Both signals are sent to the auxiliary feedback loop. The
IMPULSE COMPENSATION and IMPULSE FEEDBACK signals (sheet 1) are combined in Z6 along with
a dc level from R37 and are sent back (via Z11-3 and Z8-1) to the Baseline current source (Q1, sheet 2).
R37 adjusts for the small (approximately 10 µW) amount of residual light that is emitted between
impulses. R42 adjusts the balance between the optical sample (from D51-7) and the digital sample (Z3-
2) so that there is no change with varying rep rates.
Laser Protection (Schematic Sheets 1 and 3)
The laser is protected from excessive drive currents by a group of circuits referred to as LASER GUARD.
These circuits ensure that whenever an unusual situation occurs the laser is promptly shunted by a low
impedance. This action is called "crowbar." The path around the laser is from DS1-10 through LI, Q2,
and R79 to ground. Even if both sets of driver transistors (Q6, Q9 and Q1) were to short circuit, the
resulting current would be shunted around the laser by Q2.
If there is excessive current (20 mA or more) flowing in the crowbar transistor, Q2, it is sensed by Z7.
This causes a positive voltage (via CR14) to reinforce the triggering of Z12. The circuit remains in the
crowbar state until the over current condition is removed. Although the control circuits generally turn on
"gracefully" without excessive current transients, a time constant is provided (C59, R101) that turns Q2
on when the power is initially applied.
Another input to Z12-4 is from the internal monitor diode DSI-7. This produces a voltage across R102
and R103 that is set to trigger Z12 when the optical output becomes too high.
When a change of Mode is made, the ENABLE signal (Z9-9, sheet 1) triggers the crowbar action. This is
applied to a timer circuit, Z12, that acts to turn on the crowbar transistor Q2 (via 012) It also is triggered
at initial power-on by CR-23 and R130 A CROWBAR signal is sent to Z17-2 (sheet 1). This is another
timer that acts to further hold off the control circuitry and permit a smooth return to normal operation.
While Z17 is in its timing cycle, the LASER ACTIVE LED is turned off and a positive level from Z17-3 via
Z15-1 turns on the LASER GUARD indicator. After several seconds the Z17 timer will recover (if no over
current condition is present) and allow the control circuits to recover.