LEACH ® INTERNATIONAL
CORPORATION
9. HOW TO PICK A RELAY
3-Minute
Time Delays,
Transients & You
or :30, :29, :28, Zap!
(Oops!)
A common time delay relay application is the delaying of high voltage to klystrons and magnetrons. And the trick is to avoid applying full voltage for at least 180 seconds or suffer the loss of an expensive tube. Yet, along these lines, even the most noble of intentions can run awry. For example:
THE MAN WHO KNEW NO TRANSIENTS
Some time back, an engineer came to us and asked for a special three-minute time delay. Special because he wanted it to be very small. As we discussed his particular requirements with him, the question of transient protection was raised. "No problem," said the engineer. "I've got good, solid power, so I don't need any transient protection. I don't need any recycling protection, just make it small." So we made it small. 1" x 1.2" x .5" to be exact. And he was quite happy. His relay worked perfectly during initial system testing. But later, during more extensive testing, things got pretty hot. A technician switched the power off for a very short period, then switched it on again. The cycle was thrown off. Time-out was less than 180 seconds. Bam! A $3,000.00 magnetron went up in smoke. And so did another before the problem was pinpointed.
UNFORTUNATELY, HINDSIGHT IS NOT FORESIGHT
This particular incident points out a problem which is all too common in time delay relay specification. In this case, it was the engineer's misunderstanding (or lack of complete information) as to the transients he could expect on his supply line. He had thought in terms of normal system operation. He had not explored the peculiar ramifications of what power interruption during check-out and test might mean to him. And so he got burned.
TRANSIENT PROTECTION, PLUS
Solving transient problems is only one consideration in the selection of the right time delay. Maximizing equipment availability and "on-the-air" time is another. The following examples show how two particular customers accomplished both these things. First, by thoroughly thinking out their applications. Then, by specifying a relay to meet their exact needs.
ANTS-IN-THE-PANTS APPROACH
This customer wanted a time delay with a built-in immunity to "short" power interrupt. He said: "If I have a power interrupt for up to five seconds, then power comes back on, turn me back on immediately. Don't make me have to start from zero." He had a good point. So we designed a special time delay that did exactly what he wanted.
ANOTHER TIME, ANOTHER DELAY
Then there was the engineer who approached it a different way. He reasoned (rightly) that you heat up on a gradual curve for three minutes. Then you're hot enough to apply full power. When you lose power, you're coming down on the same kind of gradual curve. "Give me a recycle time," he said, "that's proportional to the time off. If I'm off for a minute, make me time for a minute only. If I'm off for thirty seconds, make me time for thirty seconds, and so on." The relay we designed (Figure 1, bottom) gave our customer two important things: It allowed for interrupts of longer durations than the minimum without hurting the tube. It didn't keep him "off-the-air" for three minutes each time he had an interrupt.
WE INTERRUPT THIS INSERT TO BRING YOU A WORD FROM OUR SPONSOR
We can design the exact time delay relay for your needs. But we've got to know what they are. And that's where you come in. Literally. After you've designed the system that'll get the job done. After you've looked at it carefully to determine the kind of transients you're most likely (or least likely) to encounter during check-out, actual operation and all other conditions. After you know when power's going to be lost, how frequently, for how long, and so on. If you hold up your end, we'll hold up ours. We'll put together the time delay that best meets the sum total of your requirements. A unit that's been 100% tested over the voltage range at temperature extremes. Then you can go back and do good radar, without having to worry about a time delay zapping an expensive tube, or being off the air when you need to be on. Our applications group stands ready, willing and able to help you.
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