Circuit Timing Diagram

t1

25 ms

07V

Threshold

t2

35 ms

10V

1st Measurement

t3

10V

2nd Measurement

t4

08V

Measurement Latched

t5

07V

Probe Current Off

TLH5699 – 4

Circuit Operation

A measurement is initiated when the supply is applied pro-

vided the control input pin 7 is low Once a measurement is

commenced pin 7 is latched low and the ramp capacitor on

pin 12 begins to charge After 25 ms when switch-on tran-

sients have subsided a constant current is applied to the

thermo-resistive probe The value of probe current which is

supplied by an external PNP transistor is set by an external

resistor across an internally generated 21V reference The

lamp current is applied at the start of probe current

35 ms after switch-on the voltage across the probe is sam-

pled and held on external capacitor C1 (leakage current at

pin 1 less than 1 nA) After a further 15 seconds the differ-

ence between the present probe voltage and the initial

probe voltage is measured multiplied by 3 and compared

with a reference voltage of 850 mV (externally adjustable

via pin 16) If the amplified voltage difference is less than

the reference voltage the lamp is switched off otherwise

the lamp commences flashing at 1 Hz to 2 Hz 10 ms later

the measurement latch operates to store the result and af-

ter a further 8 ms the probe current is switched off

A second measurement can only be initiated by interrupting

the supply An external CR can be arranged on pin 7 to

prevent a second measurement attempt for 1 minute The

measurement condition stored in the latch will control the

lamp

PROBES

The circuit effectively measures the thermal resistance of

the probe This varies depending on the surrounding medi-

um (

Figure 1 ) It is necessary to be able to heat the probe

with the current applied and for there to be sufficient

change in resistance with the temperature change to pro-

vide the voltage to be measured

Probes require resistance wire with a high resistivity and

temperature coefficient Nickel cobalt alloy resistance wires

are available with resistivity of 50 mXcm and temperature

coefficient of 3300 ppm which can be made into suitable

probes Wires used in probes for use in liquids must be de-

signed to drain freely to avoid clogging A possible arrange-

ment is shown in

Figure 2

The probe voltage has to be greater than 07V to prevent

short circuit probe detection less than 5V to avoid open

circuit detection With a 200 mA probe current this gives a

probe resistance range of 4X to 25X This low value makes

it possible to use the probe in partially conducting fluids

Using resistance wire of 50 mXcm resistivity 8 cm of 008

mm (40 AWG) give approximately 8X at 25 C Such a probe

will give about 500 mV change between first and second

measurements in air and 100 mV change with oil hydraulic

fluid etc in the application circuit With an alarm threshold

of 280 mV (typ) lack of fluid can readily be detected As the

probe current measurement reference and measurement

period are all externally adjustable there is freedom to use

different probes and fluids

Another possibility is the use of high temperature coefficient

resistors made for special applications and positive temper-

ature coefficient thermistors The encapsulation must have

a sufficiently low thermal resistance so as not to mask the

change due to the different surrounding mediums and the

thermal time constant must be quick enough to enable the

temperature change to take place between the two mea-

surements The ramp timing could be adjusted to assist this

Probes in liquids must be able to drain freely

FIGURE 1 Typical Thermo-Resistive Probe

TLH5699 – 5

FIGURE 2

4