LEACH ® INTERNATIONAL
4. HOW TO PICK A RELAY
to heavy load
How to keep
a sub on the
up and up
There is no relay contact that can be used for switching all load levels. Each load level requires tailoring of the contacts for that specific application. Contacts rated as capable of handling "dry circuit to 2A" loads will do so. They cannot, however, handle loads that go from 2A to dry circuit levels. Knowing this might have prevented one misapplication that luckily had only embarrassing consequences.
DOWN TO THE SEA
In spite of the early hour, a small audience witnessed the departure of the submarine on its training voyage. Wives and friends along the channel waved their farewells to the crewmen still on deck. At last they were past the breakwater.
A klaxon signaled "dive" and the topside crew vanished. The last hatch cover clanged shut. Then silence except for the throb of the diesels as the sub sailed on. And on. And on. And on.
We won't describe the language that shattered this placidity once it was realized that the sub definitely refused to submerge.
WHERE DID IT ALL GO WRONG?
Back at the base, the failure was traced to a DPDT, 1/2 crystal can relay in the dive control circuitry. We received the relay and a 4-page, typed, single- spaced (but friendly) letter shortly after.
Analysis at the factory revealed that the relay, although rated dry circuit to 2A, no longer functioned satisfactorily at low load levels. It was still within its specifications at its maximum rated load. Cutting the casing off the relay, we found that the original gold alloy plating on the contacts had been burned off.
Checking again with the customer, we received a circuit diagram that showed the relay operating as shown in Figure 1. The solution was right there. Many relays are symmetrical physically and electrically, as was this one. At some time, probably during trouble-shooting before the exercise, the relay had been pulled from its socket, then replaced physically reversed. In terms of the poles and coil, it made no difference. In terms of operation, it meant a failure.
Why a failure when the relay was rated from dry circuit to 2A? Because any relay rated to cover the range of loads from dry circuit to heavy loads is a one-way device. Once it has been used for the larger loads, it no longer operates satisfactorily at the low levels. This is a limitation of materials, not design, and no immediate change in this situation is in sight.
SOME DEFINITIONS FIRST
Relays can be classified in terms of the duty or loading to which the contacts are subjected. Since this classification is often also the way to get into a fast argument, let's paraphrase the definition in the NARM "Engineers' Relay Handbook:"
Heavy loads: Current through the contacts is great enough that some arcing occurs under normal operation. Typically, the voltage must be greater than 12V and the current must be more than 400mA for arcing to occur with most metals. The arcing actually serves a useful function - it cleans the mating surfaces. Contact materials must be chosen, however, to minimize electrical erosion. A silver-cadmium oxide has been found to work well at these load levels.
Intermediate loads: Current is below the minimum level for even momentary arcing when the contacts are open. Loads of 50-400mA at 26V are typical for this range. Some arcing can occur during the "make" or "break" of the contacts, but extinguishes itself by the end of the contact transfer. This arcing is usually just enough to carbonize any organic vapors present. The carbonized material ends up as a deposit on the contacts and contact resistance goes up, often leading to failure. The difficulty is minimized by the appropriate choice of insulation, potting compounds and cleaning agents to reduce the amount of organic vapors present.
Low level loads: Currents are in the microamp or low milliamp range, with the voltage across the open contacts well below the melting point voltage of the contacts. No arcing occurs, but organic vapors still are a problem. The sliding of the contact surfaces causes polymerization of the organic compounds with the result that deposits with high, unstable resistance are left on the contacts. The problem is particularly bad with metals that are members of the platinum family. The solution is to use gold or gold alloy platings on the contacts.
Dry circuit loads: No current is switched. The contacts carry current only after they are closed or before they are opened. The currents may be high, as long as they are not switched. Since there is no arcing, contact resistance is kept low by using gold plating or gold alloy contacts.
The contact materials used are selected on the basis of the load they will be switching because materials for one load level are generally very unsuitable for the other end of the scale. Silver-cadmium oxide handles high currents easily, but fails at lower levels where there is no arcing. Gold alloys work well at low levels and eliminate the organic contamination problem, but at high levels erode very rapidly.
This explains why multi-load level contacts are one-way devices. The usual method for making them is to plate a heavy-load contact material with the gold alloy. At low levels, contact resistance is low and stays low. At high load levels, the plating is burned off, exposing the material with the high immunity to arcing. But, since the plating is gone, the contacts are no longer suitable for low levels.
Another method of making multi-load level contacts is to build projections on them. These provide a high unit area pressure at the point of contact, ensuring low resistance. These are one-way contacts, too, as the projection is designed to burn off at high loads to allow the mating of larger surfaces (necessary for heat dissipation and current distribution).
THE SUB AGAIN
Getting back to our example, the reason that the plating was gone from the contacts, even though the steady-state load on the B contact was only 100mA, is because a lamp is a non-linear load. The initial surge when the contacts first mated was probably over 1A. Obviously, the relay had been operated under one circuit condition for a period of time and then mechanically reversed so that the B contacts were later supposed to handle the dry circuit load of the logic circuit.
If you have an application that's a special problem, call our applications group now. They're interested in two-way communications to prevent relay failures. Use their assistance in your designs.
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