Overcurrent Output Shutdown - HP LOGIC PULSER 546A Operating And Service Manual

Model 546A
Page 19
OUTPUT STAGE.
(Pins 2,1,16,15.) Capacitor C2 is a charge storage capacitor that is charged through
an internal resistor to the supply voltage (V+), the "1" output level. This capacitor allows the Pulser
to delivery high output current without causing heavy surges on the power lead. Breakdown diode
CR1 protects U1(1) from high voltages. Transistor Q1 is the drive transistor for the logic "1" pulse and
Q2 is the drive transitor for the logic "0" pulse. Capacitor C1 and resistor R1 are coupling elements to
the tip of the probe. Capacitor C1 decouples high de levels from the tip of the probe to the output cir-
cuitry and also acts as an overload sensing element, activating the shutdown circuitry in U1 (pin 16).
Between output cycles resistor R1 discharges any residual charge on capacitor C1. Resistor R21imits the
current into U1 on this sense line, and capacitor C3 is a speedup capacitor. Capacitor C4 on pin 14
controls the normal output pulse width of both the "1" and "0" through a one-shot in U1, typically
10
J..tS
(may vary between 5-15 p.s). Resistor R5 stabilizes the output stage.
Overcurrent Output Shutdown
If the circuit under test presents an overload to the Pulser's output, a sensing circuit within U1 causes
the overcurrent shutdown cycle to occur and turn off the output transistor. If the overload presents a
low impedance to V+, the "0" overcurrent cycle occurs. If the load presents a low impedance to
ground, the "1" overcurrent cycle occurs. These cycles are described in the following paragraphs.
Waveforms of the overcurrent conditions are shown under test procedure paragraphs.
Immediately prior to any output pulse cycle, capacitor C2 is charged to V+, capacitor C1 is discharged
by resistor R1 to 0 volts, transistors Q1 and Q2 are off and U1 pin 16 is at the tip voltage.