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11. HOW TO PICK A RELAY
a time delay relay
specification
or
Man Does Not Live By
MIL-STD-704 Alone
There's a lot more to specifying a time delay relay than meets the eye. Any manufacturer's data sheet is a good start, but you can't always stop there. You've got to dig beneath the surface of each spec and ferret out the things which really relate to your specific application. Otherwise, you're liable to spend a lot more than you bargained for. And end up with a lot less. Or something worse.
THE REAL WORLD AND WELCOME TO IT
Whether you want to admit it or not, the time delay relay you specify is going to be used out there in the real world. Which means it's got to be done right. Unfortunately, "close" doesn't make it in Relayland.
GETTING STARTED
As a systems designer, you can't be expected to know all the ins and outs of a time delay relay specification. You may want somebody to guide you through the various nuances of meaning which lie hidden beneath the surface of the parameters on the printed page. We've found that there are seven "major considerations" which, if properly understood, can lead you to specify a better, less costly time delay relay. We won't attempt to detail each of these seven parameters in this insert. We will, however, take a quick look at them and share some of the insights we've gathered over many years' experience as a time delay relay manufacturer:
FUNCTION
In our last insert (How To Pick A Relay #11, Clearing The Multi-Definitioned Air of Time Delay Relay Terminology), we discussed the different functions which time delays perform. Basically, this covers what you want the time delay to do for your system. Do you want a delay-on-operate? Delay-on-release? Or what? How long a delay do you need to get the job done? And, is this the minimum time only or the maximum time only? There are a lot of different combinations, so it's always best to check with the manufacturer to make sure you're both on the same base.
POWER SUPPLIES
Specification of the nominal input voltage (28 VDC or 115V, 400 cycle AC) is not a complete answer. Beyond that, the time delay manufacturer must know your power supply specifications with particular regard to voltage limits, transients, power interrupts and so on. (A graphic example of a mis-specified power supply can be found in How to Pick a Relay #10, High Voltage Three-Minute time Delays, Transients and You.) Typically, system voltage supply is specified "per MIL-STD-704." So, let's take another look at MIL-STD-704. It's not a magic number or a cure-all in itself. You've got to be more specific. Is it Category A, B or C? And, does the unit just have to survive the transients that will be imposed upon it by the specification; or does it have to perform its intended function while being subjected to all those bad things? As it turns out, about 80% of what we supply is meant for MIL-STD-704. Nevertheless, it's always a good practice to let the manufacturer in on what you've got cooked up. Especially if your specs are "just a little bit different" than the mil spec.
OUTPUT SPECIFICATIONS
These are usually pretty easy for you to specify because they're controlling your load. The thing to remember here is that the time delay manufacturer who also manufactures electromechanical relays is a better source than one who doesn't. There are several advantages to such a company. A big one is knowing how to make the time delay and the relay work together. Even a seemingly simple thing like turning a relay off and on can be costly if not handled in the smartest way. (For more on this, see How to Pick a Relay #7, Relay Coil Suppression.)
ENVIRONMENTAL CONDITIONS
Specification of the environmental conditions often poses a problem because of the variety of possible conditions found lurking on the manufacturer's data sheet. Just remember that your overall systems specifications provide basic guidelines for environmental conditions. And, when you specify the output relay, you automatically impose the relay specification limits on the entire time delay relay. Obviously, a hermetically sealed output relay gets you around most of the environmental problems you're apt to face. But there may be a few things you have not thought about.
For instance, if you mount a relay on a wing strut, you've got to figure out how much additional vibration you're going to come up against. Finally, it's a good idea to specify temperature two ways; the temperature the relay has to be able to survive at (ie, storage); and the temperature range over which it has to operate. If these are different, you can save money by letting the manufacturer know beforehand.
TOLERANCES
It all boils down to this: the tighter the tolerance, the more money the time delay relay costs. What you have to decide, then, is what you can get away with and still get the job done. An important thing to remember is that a reputable manufacturer will put some kind of a "pad" into the tolerances he lists. So, if you specify +/- 5% in order to make sure you're going to get +/- 10%, you've overcompensated. And needlessly increased the cost of the time delay. Tolerance, temperature and money are all interrelated. A wide temperature range by itself is easy to make. A narrow tolerance by itself is easy to make. But mix these two things together and it can cost you a lot of extra money. A final tip: always verify with the manufacturer that his tolerance is specified over all temperature and voltage variations.
BASIC PACKAGE REQUIREMENTS
Here, the manufacturer wants to know if you have any size limitations. Or if you want the unit mounted in some special way. In regard to size, there are a couple of good rules of thumb to follow: First, the volume of the output relay plus 50% is needed to package the output relay. Second, it is desirable to have an additional 1 1/2 cubic inches for the electronic timing function. Obviously, you can specify any size and shape you want. It's only sensible, though, to use the cans, headers and mounting styles the manufacturer is already tooled for and save the nice retooling charges that can be accrued if you want it "just like that only 1/20,000th of an inch less in some dimension." Besides checking manufacturers for their particular size and shape cans, it's also good to look at an established relay mil spec for other package configurations.
ELECTRONMECHANIC INTERFERENCE (EMI) SPECS
Once again, merely saying you'd like "EMI protection as per MIL-STD-461" is not always adequate. You've got to look beyond the surface of the mil spec and define the exact performance requirements under your particular EMI environment. A little extra thought early can save you a lot of money later on. We don't pretend to know everything there is to know about time delay relays. But we've lived through enough mistakes along the line to be able to solve 99.9% of the problems you're most likely to come up against. So, if you're about to specify a time delay relay and you've got any kind of question, just give us a call. Our applications group is waiting to give you some experience-nurtured, realistic answers.
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