Taming Relays for Real World Integration
The scene is one you know well: As you pull up to the home you say to yourself, “This is the second service call I’ve made on this system this year.” The problem is an automatic timer control designed to switch a bank of external floodlights. “I have had it with this panel,” says the voice in your head. “On the first repair call, I had to replace the main circuit board. On the second call, I had to replace the external control relay. What can I do in the future to help reduce service expenses on these types of applications?”
None of the equipment was defective. However, there are several design precautions that could have been taken to make this system operate more reliably. Let’s take a closer look.
Find the Root of the Problem to Find a Solution
The output control is commonly known as an “open collector” transistor output. In this case, the switched, 12VDC output was limited to handling 50mA current maximum. A 12VDC/5A relay that draws only 30mA was specified. The relay appears to be in the specified load range of the transistor output. What could be the problem?
Inductive devices, such as the coil of a relay, a solenoid in a door strike, or a motor winding, create a large “inductive kick-back” voltage spike when voltage has been applied and then quickly removed. This voltage spike can often be 10 times to 20 times that of the original supply voltage, and can destroy a transistor output. An add-on semiconductor device can help suppress this kick-back voltage spike and protect the transistor output.
One of the most common and popular suppression configurations is a clamping diode (1N4xxx) connected in reverse across the relay coil. However, a less-known configuration of using a zener diode and diode-in-series across the coil may work better.
Other suppression devices can be specified; they are a metal-oxide varistor (MOV), and a Tranzsorb(r). When using an MOV, one must remember that this device weakens during a period of hits and eventually could fail. If selected, a protection voltage of 1.414 times the source voltage should be used. The Tranzsorb could be used, as it is basically a bilateral zener diode, and is good for AC circuit suppression.
Make Sure the Current Is Correct for the Circuit
The second problem with replacing the relay may have been caused by not originally specifying the correct current handling capacity. A better understanding of various types of electrical loads and how to derate the relay current ratings will help.
An incandescent-type load has a very low resistance due to the cold lamp filament. The turn-on current can be 10 times to 15 times higher than normal. This can cause arcing and welding on the relay contacts if the correct relay type is not selected.
Some final tips and comments:
—Solid State Relays (SSR) can help when dealing with AC-type loads. They must have good heatsinks and can fail when shorted. Circuit fuse protection is suggested.
—Measure the current inrush with a fast data hold clamp-on ammeter or oscilloscope before specifying a relay.
—Don’t use diodes to suppress an AC load. They only work on half the cycle.
—Never make parallel contact connections to increase current capacity.
—Explore other suppression configurations, such as resistor/capacitor (R/C) circuits or combinations of R/C, MOV, Transzorb, zener diodes and diodes to better fine-tune the circuits. The metal type in the relay contact can affect overall performance.
Touring Show Aisles Proves Worthwhile, Even Fascinating
Recently, I had the opportunity to attend two major trade shows: the ISC West in Las Vegas and the EHX Home Automation in Orlando, Fla. I will report, after each show I attend, on the products and services I find particularly interesting, unusual, or just plain handy.
Window Candles: Yes, window candles. This has to be the most unusual product I have seen displayed at ISC. We run wire to windows for alarm contacts; why not run an extra pair so customers can have low-voltage decorator candles in their windows? You might laugh, but your competition may be putting them in as we speak. Go to www.windowcandles.com for more information.
Handy Installation Aids: Are you tired of having to prepare the end of a wire for pulling? Check out the Fiberfish screw tip (FIB25) that allows you to simply push the end of jacketed cable onto your pull rod tip. Contact Service Warehouse at (800) 822-6004 for more information. Also, ask about its shoe covers (SHOE-50P) for cleaner residential installations.
Here are a couple of items from Roy Bowling’s bag of tricks at Labor Saving Devices Inc. (LSD). A pulling sock tip (works like the old Chinese finger puzzle) called the PullSleeve is used for pulling preconnected cables such as BNC and Cat-5. Do you find yourself having to drill out a hole for a larger diameter contact that already has the wire pulled? Take a look at the LSD Rebore-Zit drill bit. Go to www.lsdinc.com.
Easy RJ-45 Connections: Do you ever pull your hair out trying to make a really tight, short RJ-45 connection on Cat-5 type twisted cable? Make the leads as long as you want, then push them up tight before crimping. Consider the EZ-RJ45 connector from Platinum Tools at www.calcentron.com. Overhead (OH) Door Latch Switch: This supervises any metallic overhead door-rail latch. It is called the Quickswitch and you can find more information at www.quick-switch.com. The company also offers items to help reduce the detection gaps often found in OH alarm contact magnets.
Disclaimer: The information and advice presented in this article is made available in good faith, and is derived from sources believed to be reliable and accurate at the time of release. Before using such information, make sure codes, safety issues, and manufacturer’s specifications have been fully adhered to. The writers of this article and Security Sales & Integration magazine disclaim any liability from the use of this or any other information.
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