CS289 Datasheet, PDF (5/8 Pages) Cherry Semiconductor Corporation – 20mA Air-Core Tachometer Drive Circuit

Tachometer Application
RPM _x # OF CYL.= _Frequency
60
2
V_F/VOUT = 2.1 + Frequency x C_T x R_T (V_REG -0 .7)
The above equations were used in calculating the follow-
ing values, where V_F/VOUT = 7.1V at =270° and C_T = 0.01 F.
4 cylinder: Freq = 200Hz, R_T = 320kΩ
6 cylinder: Freq = 300Hz, R_T = 220kΩ
8 cylinder: Freq = 400Hz, R_T = 150kΩ
Typical values shown above apply to a nominal value of
V_REG of 8.5 volts. It must be realized that trimming of R_T
will be necessary to compensate for variations in regula-
tor voltage from one unit to another.
An alternative to this adjustment is to replace R₂ with a
potentiometer, as shown in Figure 2.
Partial schematic shown in Figure 3 represents one met-
hod for use with DC applications instead of frequency.
TRIM
10kΩ
I_OUT/EΘ
R_T
C_P
C_T
1kΩ
Figure 2: Alternate Trimming Method
R_T
C_P
0.7V
I_IN
Figure 3: DC Application
2.1V
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