U640B

The U640B has the following features including Voltage stabilization;Relay output with integrated Z-diode limitation;Digital time sequence across one pin-RC-oscillator;Low voltage effects a reset pulse;Relay is switched ON by trigger pulses;Relay is switched OFF with an external adjustable delay after the last trigger pulse;Interference protected according to VDE 0839 and load dump protection.

The relay output U640B consists of a Darlington output stage with an integrated Z-diode (Fig. 3) and is designed for a pulse current of approximately 1.5A.The Z-diode lirr}its the relay winding's inductive switch-off voltage (Fig. 4).The relay winding has a typical resistance of 80 .With the circuitry of Fig. 1, the U640B can be supplied with 24 V for two minutes without damage.The principle of operation is identical for both types. When the first ignition pulse appears at pin 5,the relay output becomes conductive and the petrol  pump is fed with current. The internal monoflop is retriggered by each further ignition pulse (refer to the pulse diagram).Each ignition pulse at pin 5 triggers a monoflop with ts=2.5 ms. The monoflop pulses can be observed at pin 6 when this pin of U640B is not connected.The ignitionpulsesare synchronized by the internal clockpulse,visible on an oscilloscope as jitter of the monoflop pulses.The integration capacitor is connected between pin 6 and ground and charged via an integrated 210 kf2 resistor during the active monoflop time ts,and U640B is discharged during the pulse pauses. A DC voltage proportional to the ignition pulses frequency with superimposed residual ripple is produced at the capacitor.

If at a specific speed this voltage at C1 should ever exceed  an  upper reference voltage which  is adjusstable by means of a voltage divider at pin7,the relayoutputisdisabled,i.e.theelectricpetrolpump is switched OFF and thus limits the speed. As the result of this, the speed and the voltage at C1 also drops accordingly until it falls below a lower reference voltage, U640B can be adjusted bymeans of a voltage divider at pin 8. The relay is activated again and the control pump then delivers fuel again.