SPLat Logo

Hot Tips

NOTICE: We are updating our website. For product enquiries and orders please contact us directly.
NOTICE: SPLat Controls has moved. We are now at 1/85 Brunel Rd, Seaford, 3198. map

Coils ain't Coils!

I'm sitting at my desk late one afternoon, thinking I could get lucky and be out of here by 5 o'clock, when the phone rings. I pick up the phone and I say as follows: "Hello, SPLat support line".

On the 'phone this voice says: "Is 'dat 'de SPLat support line?".

Now, where I come from if someone answers the phone with "Mindy's Restaurant" I know I've gotten through to Mindy's. In fact, if I dial the number for Mindy's I'm going to give very long odds indeed against the phone being answered "White House Washington". So I decide to ignore a question being answered with a question, and I say: "How may I help you?"

At that the voice says: "Help? I'll tell you how you can help! I'm Dragger, Nuck L. Dragger, and my SPLat just made with 'de smoke big time!"

Now this Dragger character has a weird rasp in his voice, so maybe he's been inhaling all this SPLat smoke. Or maybe it's another kind of smoke he's been inhaling. Whatever he inhales or doesn't inhale, one thing's for sure, a future president of the US of A this guy ain't. "So what did you connect to the SPLat?" I ask.

"What did I connect?" he asks. "What did I connect? All I connected was one itty bitty little 24V contactor coil. And before you get smart and ask, the power supply is 24V".

Now I thinks to myself, coils ain't coils, but this Dragger guy probably ain't too wised up on anything much more than a bum tip for the 2:15 at Belmont Park. Knowing coils ain't coils I ask "AC or DC?".

This must have been the wrong thing to say, 'cause this guy yells down the 'phone at me and he yells as follows: "Hey, buddy, you questioning my lifestyle choices or somethin'? I've a mind to send some of 'de boys around there to see you about your lifestyle. After dey finished you ain't gonna have no lifestyle!"

Now I'm the kind of guy who's pretty attached to his lifestyle. For that matter so is my fiance Miss Peggy Mahonan, in fact she's very fond of my life style, if you know what I mean, and I want it to continue that way. So this attitude of Dragger's makes me quite upset. I say to him "Believe me Mr Dragger, your lifestyle I don't care about one iota. What I want to know about is this contactor coil. Is it an AC coil or a DC coil?"

When I say that the line goes all quiet, and I'm thinking maybe the guy just passed out from all the smoke. But then his voice comes on all kinda quiet "Ehr, I'm not sure".

"Well, can you check what it says on the contactor" I say, feeling maybe the odds on my lifestyle just got a bit better than 6 to 5 against.

"OK" he says, "let's have a look here ... 24V 12VA 2W. Don't say nothin' about AC or DC though.

This confirms my suspicion that he's used an AC coil when he should have used a DC coil. "The 12VA is the clue" I tell him. "If it's got a VA rating it's sure to be an AC coil, not a DC coil. VA means Volt-Amps and is the voltage multiplied by the current in an AC circuit. Because the coil is inductive the wattage will not be the same as the VA. The ratio between watts and VA is called power factor. If you want to know more there's an article about power factor tucked away on our website."

I hear this grunt over the phone, then he says as follows: "So why de smoke?"

"Because", I say, "an AC coil will draw much more current on DC than on AC. It's rated at 12VA, so on AC it will take 12VA divided by 24V equals 0.5A. But the label says it will dissipate only 2W of heat from that 0.5A." That means the ohmic resistance of the coil is," I do a quick sum on my calculator using the equation power = eye-squared-R, "eight ohms".

Silence on the line, then "Hang on, hang on, this I gotta check!"

I nearly pop an ear drum as he drops the receiver onto a concrete floor, then I hear frantic shuffling sounds, some clicking like a multimeter rotary switch, a beep like a continuity tester, a few words in the background that Miss Peggy Mahonan would not approve of (though I suspect she might know them) and then he comes back on the line and he says "OK Mr SPLat Man smarty pants, I got 8.2 ohms here. Howdya do it?".

I look at the clock on my office wall and see it's 5:07, and I think Miss Peggy Mahonan is going to be very upset indeed if I don't get out of here real quick. So I say into the 'phone "Just a fancy calculator I have. But with 24V DC that 8 ohm coil is going to draw 3A. Little wonder you smoked a SPLat output that's rated 0.5A tops!"

That seems to really get to him. "Holy smoke!"

It nearly brings tears to my eyes to think a customer could so highly regard the accidental bi-product of abusing a SPLat. "Quite so. So just remember, coils ain't coils. An AC rated coil is designed for its inductance to limit the current. Connect DC to such a coil and the inductance will do nothing. Only the resistance will limit the current, so you can get as much as 15 times more current than it will draw on AC. If you are at all unsure simply measure the coil resistance with a multimeter, and work out the DC current from Ohm's law".

That night Miss Peggy Mahonan makes me do the dishes. At least I still have my life style.

With apologies to Damon Runyon

More hot tips...