ELECTRICAL REBUILDER’S ASSOCIATION—Technical Document MI201411A www.electricalrebuilders.org Using Relays to Soive Problems Reversing bidirectional motor using two SPDT Bosch relays BY BOB THOMAS AND WES GRUENINGER, SR. AN is simply an electrical device designed to switch a large amount of current using a smaller amount of current. Some, like the fender-mounted starter relays on Ford vehicles of the 60s through 80s, were made to carry several hundred amps for a short duration. Others, like the small mini-relays (see Figures 1 and 2), manufactured by Bosch and Tyco, are designed to carry smaller loads of up to 50 amps for unlimited lengths of time and are known as continuous-duty relays. Cole-Hersee even makes a small continuous-duty relay rated at 70 amps. Se Figure 1—Mini Relay SPDT RELAY 85 86 Pinouts Schematic WSG Figure 2—Single-Pole-Double-Throw PROBLEM: You have a_ small bidirectional, permanent magnet, DC motor that must be momentarily operated in different directions by reversing current flow through the motor. This motor could be powering a small pump or a gear driven mechanism. SOLUTION: Use two SPTD relays (see Figure 3) and a momentary three-position toggle or rocker switch so that each direction of the motor is activated by one relay. This is accomplished by applying power to one motor lead while the inactive relay provides the ground path (see Figures 4 and 5). Note that the number 30 terminals that are normally connected to B+ in most relay applications are connected to the two motor terminals in this circuit. B+ is connected to relay terminal 87 on both relays here. Terminal 87a is grounded on both relays. Both motor terminals are normally grounded in this circuit until one of the relays is activated. When either relay is activated, power is supplied to one of the motor terminals while the other remains grounded by the inactive relay. CAUTION: Maximum current draw of the motor must not exceed the amperage rating of the relay. Ratings available in this size relay now go up as high as 70 amps. Ensure that the wire sizes you use are sufficient to carry the maximum amperage load. LET RIA) MOMENTARY CONTACT SWITCH BATTERY 30 87a ? 86 85 RELAY 1 Figure 3—Wiring diagram using two SPDT relays RELAY 2 An ERA Publication. ©2014, Electrical Rebuilder’s Association. All rights reserved. No portion of the guide may be reproduced in any way, or stored in any electronic retrieval system without the prior written consent of the Electrical Rebuilder’s Association. ELECTRICAL REBUILDER’S ASSOCIATION—Technical Document MI201411A www.electricalrebuilders.org + MOMENTARY + MOMENTARY CONTACT CONTACT OOO0000 SWITCH OO0O000O SWITCH y y BATTERY BATTERY RELAY 1 RELAY 1 RELAY 2 ENERGIZED 0 . AT REST ENERGIZED 87aQ Figure 4—Current flow in forward rotation Figure 5—Current flow in reverse rotation An ERA Publication. ©2014, Electrical Rebuilder’s Association. All rights reserved. No portion of the guide may be reproduced in any way, or stored in any electronic retrieval system without the prior written consent of the Electrical Rebuilder’s Association. Page 2