Power Relay

Even in the era of ever increasing electronic control systems, the working current relay has well defended it's place in the automobile. It has the ability to switch high working current using only low control current. In addition, it separates the working- from the controlling current much better than the transistor. Short circuits in one do not affect the circuits of the other.

To reach a relatively high magnetic flow, a thin wire coated with paint is very tightly coiled around an iron core. Thereby, a relatively low current in the controlling circuit is sufficient to move the spring-loaded contact (below on the right of the picture) to the left, thus closing the working circuit safely again. A relay can, in slightly altered construction form, switch 30 A and more, in the form of a magnetic switch (solenoid), even more than 100 A. In the above picture, a closer relay with 4 connections is shown. Opener/closer relays have 4 connections, change-over relays have 5 connections. Opener/closer relays, as the name indicates can, when power is applied to the terminals 85 and 86, only open or close a circuit.

One can check the function of a relay by simply laying on the control current and waiting for a clicking sound. If a click can be heard, then the component is probably OK. To check the working component, one must connect a circuit indicator to the two free terminals at the same time. In the case of a closer-relay, it will chirp when the control current is laid on and with a an opener-relay, when no current is laid on. In the case of the change-over-relay, 3 terminals are left over. Two of them always have contact, depending on the switch position.

If a power relay is switched off, the switch and the iron core remain in the working position. The spring brings them back into the starting position. Thereby, rcurent peaks can occur, which can damage sensitive electronic devices. For this reason, the magnetic coil is often given a resistance (picture, on the left) or a so-called catch-diode (picture, in the center) is switched parallel so that this is avoided. On the right you can see another circuit with a reverse polarity protection. 09/11