Circuit Digital Inductimeter Transform
This Inductimeter measures inductors between 3µH and 7mH, in two measurement scales. The reading of the inductance is made in the scale of miliVolts of a multimeter. The precision of this Inductimeter is around + / – 10%, what is adapted perfectly to the experimental character of the radio amateurism. The multimeter should be digital. Measure of reels is a thing that we hardly heard to speak, until it seems that how many is the value of reels you exhale she has, but inductors has your unit of measure Henry.
The circuit of the meter is composed by the following blocks: Feeding source, Oscillator, Differentiating, Integrator and Adjustment of Offset. The feeding source is composed by a pile of 9V and a circuit integrated tension regulator (U2). To increase the useful life of the pile we chose the regulator 78L05 that has a current quiescent around 3mA. A regulator of the type 7805 already has a current quiescent around 10mA (some manufacturers’ regulators get to present even to 25mA of current quiescent!).
The oscillator (U1:D), that it supplies the pulses for the differentiating, it generates pulses in two fixed frequencies. These frequencies determine the measurement scales. The first scale, with the oscillator in 60KHz, is going from 3µH to 500µH. In the second it climbs, from 100µH to 7mH, the oscillator works with a frequency of 6KHz. The selection of scales is given by the key, S1, of the type 2 poles 2 positions. This oscillator is of the type RC with a gate INVERTERS (U1:D) and with a buffer (U1:C) to isolate him of the load RL.
The differentiating is formed by a load resistive R and for the inductor to be measured, L. In this differentiating one pulses will be generated whose width depends of the constant of time RL, like R it is fixed, as larger the inductor L,
The pulses of the differentiating are applied to the entrance of the following logical door (U1:B), that it transforms these analogical pulses and level pulses TTL, e of this proceeds for the last door logic (U1:A) and RC formed by R3 and C3 – the integrator that transforms these pulses in a tension proportional dc the width of these pulses.
Last, the circuit of offset Adjustment generates a small tension to zero the instrument in the scale from 3µH to 500µH.