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Thread: Ohm's Law discussion

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  1. #1
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    In the past week, I have encountered two examples of people having a misconception of Ohm's Law and how it works. The theme is the same in both instances, but I wanted to discuss the issue I saw today at work.

    About a month ago, a piece of lab equipment we call a "hotplate" had a switch failure. It is a "rocker" type of switch rated for 20 amps. The hotplate, which is basically a coil type heater inside a ceramic top plate designed to warm a fliud in a beaker or other container, is rated for 220 Volts at 15 Amps. When this failure occured a month ago, we replaced the switch, and lo and behold, the failure occured again. What essentially is occuring is the contact points of the switch are not making at rest position. One has to apply pressure to the switch to get the circuit to close and maintain it. Once you let up, the contacts again relax and break the circuit. I suspect that the contacts are getting dirty due to arching or too hot and warping.

    In the past month, we replaced the switch 4 times.

    Today, my coworker, a man with 30 years experience, and the apprentice we are training, a completely inexperienced young fellow, went about replacing the factory switch with a beefier 30 amp toggle switch. In passing, I mentioned that this might be a "band-aid" fix but might mask the real problem. Certainly, the 30 amp switch will probably last forever, but why were the factory switched going bad when they are rated 5 amps above the operating rating of the device? Possibly, a bad batch of switches from the factory, but that isn't likely.

    Now, here is where the fun begins. I mentioned to the senior fellow of our three man group that we may want to put an amp-probe on the circuit to check if there is a short somewhere, even a partial one. He flatly told me "Nah, its the switch...The wire terminals are dirty an adding resistance to the circuit, and the added resistance is making it get hot.". I'm in a predicament here, because I see this as an opportunity to train the new guy on the use of the test equipment and the theory of Ohm's Law. I also know that the preceding statement "the added resistance is making it get hot" was dead WRONG and totally demonstrated a lack of understanding of Ohm's Law. (Some of you guys might even argue the same thought as he, but the real Elmers here will know that this statement is 180 degrees wrong.

    Now, The first thing I said to my coworkers was, "Indeed, the switch most likely IS the problem, but it might also be a short somewhere". I said this in as polite a way as possible to prevent the appearance of argiung with the other technician in front of the trainee. AND INDEED, I MEANT WHAT I SAID. I know that the switch may be the problem, but not for the REASON that he stated, which is the point!

    Again, he said "No, you are wrong, its the switch, dirty connections will cause resistance and the resistance will cause a voltage drop and make it get hot. Resistors consume voltage and get hot." Again, I politely said that the switch may be causing the problem, and it might be the dirty contacts, but NOT because of resistance. At this point, I was more determined to properly educate the "young skywalker" on Ohm's Law rather than be diplomatic. You see, my coworker is one who flies on autopilot. He knows how to fix a lot of things, but he doesn't always understand the reasons for the failure. He has never been interested in learning anything past the no-brainer approach, and relies totally on past experience. When confronted with a problem that lies outside of this "box", he has trouble. My coworker is my friend, and I always try to be polite and nonconfrontational, but on occasion, we agree to disagree. However, I do not want the new guy to learn things incorrectly.

    So, here is what I contend. The connections on the switch were kind of dull, but not extremely so. They were not particularly pitted, just discolored. What I contend is that arcing is occuring at one or more of those contacts. They are spade type connections and you know how they get loose over time. But of this, I am not totally sure. I also mentioned that the heater element might be warping and thus changing its operating impedance or partially shorting to ground. I also mentioned that the solid state relay may be intermitently shorting to "full on" instead of pulsing the heat as its supposed to do, and thus, the avarage current and power it has to dissapate is more at times. This is not likely, but a possibility to consider. Also, the control circuit might have a component that is faulty and creating a partial short.

    Bottom line, Ohm's law requires us to understand that LOW SERIES RESISTANCES allow higher current draw, THUS, more total power and heat. High resistances cause less current draw by the load and thus less heat and power to be dissapated. A potentiometer, if one suitable to handle 15 amps were put in series with the heater, would be HOTTER WHEN THE RESISTANCE IS NEAR ZERO OHMS and COOLER WHEN THE RESITANCE IS IN THE HIGH HUNDREDS OR THOUSANDS OF OHMS.

    I explained all of this to the two guys I am working with, and I kept getting arguments about how resistors consume power...which is true, but not in the way this fellow is trying to fabricate in his mind. He also tried to go into the nature of wire gauges, and said the thinner (higher resistance) wires will get hotter than fatter because of the resistance. Again, another misconception...we all know it is the surface area and heat dissipation properties of smaller gauge wires that make them unsuitable for high currents.

    Anyway, I could not detect a short with my amp-probe in the circuit, which is a shame, because it may reinforce my friends misunderstanding and also make me lose credibility with the younger guy. As I said, I never argued that the switch wasn't the problem, and even described HOW it might be causing it. There may even be another explanation that I can't think of, and someone smarter than I can provide.

    But I think this is a very common misunderstanding of Ohm's Law, and I hope that the nature of my posting will help clear some things up with others who may not understand.
    73, Heath/KE5FRF
    CWOps#776/SKCC#1940/NAQCC#1712/WAS#52445
    EchoLink Node#268023
    W5YI-VE
    My favorite mode? Morse, of course.

  2. #2
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    AS much as you try to explain to people, sometimes they are just right, even when they're wrong.

    And I'm afraid electrical ignorance goes way beyond ohm's law.

    For example, one guy tried to tell me once that when you're wiring additional outlets on the same circuit in your house, that they're wired in series.

    That's right, in series.

    He claims that the additional connector on the outlet to wire additional outlets together makes it a series circuit.

    Now, I can understand for GFI's, where the additional outlets are actually in series with the GFI outlet (and the GFI interrupts them all when a fault is detected). But he wasn't talking about GFI's.

    What a maroon, but I'm afraid they're everywhere.

  3. #3
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    What you need to do is check the current AT THE MOMENT OF SWITCH CLOSURE. Heating elements draw a lot more current cold than when they're up to temperature. (A standard lightbulb has 10x the resistance hot as it does cold&#33 Heating elements aren't as extreme, but certainly DO draw larger currents at startup.

    I'll bet almost anything you're torching your switch during turn-on.

    eric
    "The more you know, the less you don't know."

  4. #4
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    2MH

    ... and I'll be he don't know Jack.




    http://forums.qrz.com/group.php?groupid=89 ←The QRZ Railfans group

  5. #5
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    Not to burst you bubble Heath, but dirty/corroded connections at the switch COULD indeed be the problem.

    The added series resistance would reduce the over-all current flow in the circuit, but the heat developed in and around the switch by the higher than normal series resistance of the switch (and dirty/corroded connections from the wires to the switch) could warp the internal contacts of the switch causing it to open and fail prematurely.

    The idea of checking the total current flow is a sound idea, and may indicate that the element has a higher than normal current, but cleaning the contacts between the conductors and the switch contact points would probably be the first step I would take.

    If there were a problem in the heating element, you probably would have noticed a change in the heating characteristics before the switch failed. Either too much heat, one particular hot spot, or an open breaker on the mains.

    Lots of different ways to skin the cat !


    edit:
    I just saw Eric's post above, cold surge current could do the same thing.

    Lots of different ways to skin the cat !

    That is why troubleshooting is an art



    Politicians...Round em up...Drown them all and start over !

  6. #6

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    Quote Originally Posted by [b
    Quote[/b] ]I also know that the preceding statement "the added resistance is making it get hot" was dead WRONG and totally demonstrated a lack of understanding of Ohm's Law
    Actually he is not.


    ----------\/\/\/\/---------/\/\/\/\/---------
    R1 R2
    Scenario 1
    R1) 1 ohm switch resistance
    R2) 99 ohm heating element
    Apply 100volts, total power consumption 100 watts.
    1 watt dissipated by R1, 99 watts by R2.

    Scenario 2
    R1) 100ohm switch resistance
    R2) 100ohm heating element
    Apply 100 volts. total power consumption 50 watts.
    25 watts dissipated by R1, 25 watts dissipated by R2.


    Which switch is hotter? The one dissipating the most power.

  7. #7
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    Quote Originally Posted by [b
    Quote[/b] (KB3LIX @ April 26 2007,13:03)]Not to burst you bubble Heath, but dirty/corroded connections at the switch COULD indeed be the problem.

    The added series resistance would reduce the over-all current flow in the circuit, but the heat developed in and around the switch by the higher than normal series resistance of the switch (and dirty/corroded connections from the wires to the switch) could warp the internal contacts of the switch causing it to open and fail prematurely.

    The idea of checking the total current flow is a sound idea, and may indicate that the element has a higher than normal current, but cleaning the contacts between the conductors and the switch contact points would probably be the first step I would take.

    If there were a problem in the heating element, you probably would have noticed a change in the heating characteristics before the switch failed. Either too much heat, one particular hot spot, or an open breaker on the mains.

    Lots of different ways to skin the cat !


    edit:
    I just saw Eric's post above, cold surge current could do the same thing.

    Lots of different ways to skin the cat !

    That is why troubleshooting is an art
    Go back and read my post. I made it clear that yes indeed, the switch connections could cause the problem, BUT NOT BECAUSE OF RESISTANCE. That's the key here. Its NOT RESISTANCE THAT CAUSES HEAT. Arcing or some other abnormal situation, maybe, but not ADDED SERIES RESISTANCE. See what I mean now?

    BTW, the cold switching of the device is not the problem either. WE SELDOM TURN THE DEVICE OFF. And as a troubleshooting mechanism, we instructed the lab technicians not to turn it off at all.
    73, Heath/KE5FRF
    CWOps#776/SKCC#1940/NAQCC#1712/WAS#52445
    EchoLink Node#268023
    W5YI-VE
    My favorite mode? Morse, of course.

  8. #8
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    Current flowing through switch contacts that are clean, smooth and tight would produce very little heat because of very low contact resistance.

    However, if the switch contacts are dirty, rough and/or loose, resistance increases, and current flowing through a resistance DOES produce heat.

    "Twinkle twinkle little star,
    Power equals I squared R."
    Semper ubi sub ubi. 73
    K7KBN CWO4 USNR Ret.

  9. #9
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    Quote Originally Posted by [b
    Quote[/b] (k7kbn @ April 26 2007,11:38)]"Twinkle twinkle little star,
    Power equals I squared R."
    Oh, how clever. I thought I'd heard them all.
    "The more you know, the less you don't know."

  10. #10
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    I don't even think your tech even cared about ohms law or anything, and thats the unfortunate thing.

    He only cared about fixing it.

    Now, that may not be the truth, but in most situations like this nowadays, the fix seems to be whatever makes it work, and whatever is cheapest to do said fix.

    Honestly if this problem is continuing, I would question the safety of that hot plate. Not that I think its gonna explode or anything, but anything is possible, and with the possibility of that, comes the possibility of injury. I would question its integrity.

    But I know if I had been the tech in the situation, I would have been told to replace the switch. Git R Done.

    Now, in this situation it may not apply. But what I'm speaking of here was something I noticed when I was in technical school for computers. The goal was no longer to troubleshoot the actual problem, the goal was to find out what part was bad, yank that board, and put a new one in, and more often than not, toss the old-maybe send it back for credit, but in most cases, toss and carry on. Not that its a bad system per se, but I've seen situations in real life where a tech came out, "fixed" the problem and then a bigger problem arose because the part he fixed had taken the heat for the bigger problem, and after replacing the part that had went bad because of the other part, the problem manifested itself.

    In this situation, the fact that the tech was closed minded to it doesn't tell me he didn't want to deal with it. I just honestly have a feeling thats how he was trained. With something like that, I would have taken the measurement, but I wouldn't have told my boss. But since its a simple diagnostic to take, I would have done it anyway.

    Its just that in these days of lore, it appears to be quicker to just tackle the obvious problem, and handle other problems later when they crop up-instead of checking a little further to make sure other stuff didn't get damaged, etc.

    Case in point-a new building is going in next door where I work. They just hooked up the electrical feed last week. Right after they did it, (the next day in fact) one of the "safeties" (on the pole where the feed goes into the building) blew out. The power company came out and was there and gone in 10 minutes-I'm guessing they didn't check anything, they just replaced the thing that blows on the pole and left.

    My suspicions were confirmed yesterday when I heard another pop again. This time though, the safety didn't blow-it was arcing right where the wires went into the conduit onto the pole (they then go under the street to the building) and the pole was on fire. We had to call the fire dept out to check it out.

    I'm just hoping the guy didn't put a larger safety on the pole and that caused it to do this, but it was arcing nicely, you could hear it popping and stuff down here, and it left a nice scorch mark on the pole. The only lights I see the building using right now are the parking lot ones, so something is up.

    I'm just surprised that they didn't check more, but I guess it works the same across the board. Get it fixed and working.

    They were out there for about 7 hours yesterday. Dunno what it was, but I guess the fire made them take a better look......

    Sorry for my rambling, but just trying to make my point.
    Joshua, KD6NIG
    Since 1992. General July 2014.

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