® ® DMS APPLICATION NOTE Implementing a Line Frequency Meter Introduction The circuits illustrated below can be used to measure an ac line frequency using the LM2917 frequency-to-voltage converter and DATEL's DMS Series digital voltmeters. The circuit in Figure 1 is used to provide isolation from the ac line. The transformer is a stepdown type with a secondary voltage of 6.3Vrms. This voltage is rectified and applied to the resistive voltage divider consisting of R7 and R8. Figure 2 depicts the resulting waveform seen at the junction of R7 and R8. This voltage (FIN), which will have a peak of approximately 1 Volt, is then applied to the input (pin 1) of the LM2917. Theory of Operation The following equation is used to determine the output voltage (pin 3) of the LM2917: The circuit in Figure 3 will have a display reading of 60.0 for an input frequency of 60Hz. The 0.047µF capacitor (C1) should be a polyester type. The 47µF capacitor (C2) is used to reduce the output ripple of the LM2917. If the value of C2 is made too large, the ripple will be further reduced, however, the response time of the output for a given change in FIN will take longer to reach its final value. With the meter configured as shown, the display reading for an input of 0.6V will be 60.0. R2 is adjusted as necessary until the desired display reading is obtained for a given input frequency to the LM2917. The 5kΩ potentiometer is a multi-turn type that allows for calibration between 50 to 60Hz. All other resistors should be precision, metal-film types with good temperature stability. VOUT = (R1 + R2) (VCC) (C1) (FIN) VOUT = (25500 + 2850) (7.5) (47 x 10 –9) (60) VOUT = 0.6 Volts The value of VCC used in the above equation is +7.5 Volts. This is due to the zener diode internal to the LM2917. The value for R2 is its adjusted value. It is used in the above equation for simplicity. The 0.6 Volts is applied to pin 11 ((+) INPUT HI) of the DMS meter. Further Reading The LM2917 ia an extremely versatile frequency-to-voltage converter IC. Its recommended source is National Semiconductor. Many more useful applications using the LM2917 can be found in National Semiconductor's Linear Applications Handbook, specifically in Application Note 162 (AN-162). DMS-30PC-1-RS 1 +5V 120Vac 6.3Vac R7 100k 8 REF OUT 3 GND F IN 7 REF IN R8 20k 4 DP3 Figure 1. Input Isolation and Step Down F IN 12 11 (–) IN LO (+) IN HI 1 14 2 13 4 R1 25.5k R2 5k LM2917 7 11 10 5 6 Vpeak 0 Volts C1 0.047µF +12V 12 3 C2 + 47µF R3 470 7.5V 9 8 + C3 R4 R5 10k 3k 10µF 16V Figure 2. FIN Waveform Figure 3. Line Frequency Meter www.murata-ps.com Technical enquiries email: [email protected], tel: +1 508 339 3000 DATEL, Inc., Mansfield, MA 02048 (USA) • Tel: (508)339-3000, (800)233-2765 Fax: (508)339-6356 • E-mail: [email protected] • Internet: www.datel.com DATEL makes no representation that the use of its products in the circuits described herein, or the use of other technical information contained herein, will not infringe upon existing or future patent rights. The descriptions contained herein do not imply the granting of licenses to make, use, or sell equipment constructed in accordance therewith. Specifications are subject to change without notice. The DATEL logo is a registered DATEL, Inc. trademark.