TLH8709 – 12

FIGURE 9

PROBE CURRENT REFERENCE CIRCUIT

The circuit defining the probe circuit is given in

Figure 10 A

reference voltage is obtained from a bandgap regulator de-

rived current flowing in a diode resistor chain to set up a

voltage 2 volts below the supply This is applied to an ampli-

fier driving an external PNP transistor to maintain pin 3 at 2V

below supply The emitter resistance from pin 3 to supply

defines the current which less the base current flows in the

probe Because of the sensitivity of the measurement to

probe current evident in

Figure 3 the current should be ad-

justed by means of a variable resistor to the desired value

This adjustment may also be used to take out probe toler-

ances

TLH8709 – 13

FIGURE 10

TYPICAL APPLICATIONS CIRCUIT

A typical automotive application circuit is shown in

Figure 11

where the probe selection signal is obtained from the oil

pressure switch At power up (ignition on) the oil pressure

switch is closed and pin 8 is held low by R4 causing a probe

1 (oil level) measurement to be made Once the engine has

started the oil pressure switch opens and D1 pulls pin 8 high

changing over to the second auxiliary probe input The ca-

so that a second probe 1 measurement can not occur in

disturbed oil Non-automotive applications may drive pin 8

directly with a logic signal

TLH8709 – 14

FIGURE 11 Typical Application Circuit

7