IRL (ae IR ) Ae aie 
GES ae (B1) 
Pie pl eae 
Ge We b 
Kaufman* described a circuit called the asymmetric half- 
bridge, which consists of the thermistor, # , a series resistor, 
r, and two batteries, 7, and#; (fig. B3). For a given thermistor 
and series resistor, the sum of F, + Hz adjusts the sensitivity 
while the ratio ae Ue #, adjusts the balance point of the bridge. 
Thus for zero output at temperature 7, with corresponding 
thermistor resistance Rf, : 
male 
Sabie 
a, (B2) 
x 
The half-bridge offers some advantages over the full 
Wheatstone bridge. Half-bridges are capable of greater power 
output for use with noninfinite loads because of their lower out- 
put impedance. For multiple bridges in parallel, used with one 
common dual power supply, a common ground at the center tap 
of the dual supply, may be used. Kaufman** mentions the im- 
portance of this advantage in the application of telemetering out- 
put signals. 
In both of the above mentioned articles by Kaufman, he 
discusses half-bridges for use only with essentially linear tem- 
perature elements, such as platinum resistance thermometers. 
For best linearity in this case, a very large value of series 
resistor, compared to the temperature element resistance, is 
needed. Then the bridge becomes essentially a constant current 
device and the /? drop as a function of temperature across the 
temperature element is nearly linear, since the element resist- 
ance itself is nearly linear with temperature. 
Beakley*** worked with thermistors in describing a bridge 
circuit consisting of a thermistor, , a series resistor, 7, and 
only one battery, # (fig. B4). He used the following method to 
obtain an equation for the value of series resistor,7 , which pro- 
vides optimum linearity. Using the empirically derived 
*Reference Bl, page 55 
**Reference B2, page 50 
***Reference B3, page 55 
