Introduction

INTRODUCTION

RELAYS

A relay, in its simplest version, consists of coil and a contact: when an "energizing" or "pilot" voltage is applied to the coil ends, the contact closes. Usually the coil terminals are identified as pins 85 and 86, whereas the terminals of the contact are pin 30 (from the power supply) and pin 87 (toward the load).

The output pins to the load may be two(double relay): 87 and 87a when the contact closes simultaneously on both; 87 and 87b when closing separately. The contact may be a shunt, i.e. closing on one pin (87a) and at the same time opening the other(87).

To check a relay, connect pins 85 and 86 with a 12 V source(the coil does not have polarity, therefore the two pins can be changed over) and connect the ohmmeter tester to terminals 30 and 87: when the coil is "energized" (between its ends there is a -12 V voltage difference ) the tester must read continuity( 0 Ohm) between pin 30 and pin 87; when the coil is not energized, the tester must read an open loop ("infinite"resistance ).

For a shunt relay the test consists of the passage from continuity to open loop on one pin, and the other way round on the other.

One of the most frequent relay faults is the contact "locking": this is immediately found since the ohmmeter signal does not change when energizing and de-energizing the coil (always 0 Ohm or "infinite" resistance). Some relays have a fuse incorporated: this, however, should be checked separately, before checking the relay.

RHEOSTATS

Rheostats are variable resistors used for some types of regulations: When a voltage (12 V)is applied to the main terminals (A and B), the output signal from the third terminal(C) is changed by a mechanical action (for example rotating a regulating knob ).

To check correct operation connect an ohmmeter tester tip to one of the main terminals(A) and the other to the third terminal(C) : acting on the regulating knob, the resistance measured should change considerably.