One solution is to use a low cost thermistor in conjunction

with some low-TC resistors (see

Figure 8 )

There are three fixed resistors and one thermistor This is

an NTC thermistor since it has a negative temperature co-

efficient This is what is required in order to have Iset in-

crease

as the temperature rises The data sheet with the

thermistor described a number of ways to establish different

final TC’s The thermistor itself has a very large TC which is

somewhat difficult to describe mathematically But if it is

used with some other fixed resistors such as Rmin and

Rmax definite end point limits can be established and an

approximate staight line TC generated See

Figure 9 for a

graphic representation of the ideal calculated values of Iset

and the actual measured values generated Notice that

there is very close agreement between the two graphs The

circuit actually creates an S-shaped curve around the ideal

The low-cost thermistor is available from Keystone and is

listed as follows RL2008-523K-155-D1

OVER-CURRENT LIMIT DETECTOR

Other applications include an over-current detector as

shown in

Figure 10 The load represented here can be ei-

ther a single component or an entire system Resistors R3

TLH8707 – 11

Thermistor

Keystone

RL2008-523K-155-D1

100k

25 C

FIGURE 8 ThermistorResistor Network

TLH8707 – 12

FIGURE 9 Iset vs Temperature with

Figure 8 Circuit

7