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diode for collapsing magnetic field

Discussion in 'General Electronics Discussion' started by Blank Stare, Oct 10, 2012.

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  1. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    That is of course true, but we can't know how often the motor will be switched on and off.

    You certainly need a diode capable of at least 20A peak current (preferably more).

    The 1N5400 sounds ideal because it is unlikely that the 20A over 30 sec will ever be exceeded even with near continuous switching on and off.

    You (in the unquoted portion) also speak of transorbs. I was thinking of MOV's too, but I believe that they degrade over time.
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    I think the OP said it wouldn't be often. Less than once every few minutes or maybe longer.
     
  3. Blank Stare

    Blank Stare

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    Kris,
    Yes, I did not indicate it, but that is what I intended the drawing of the diode from hell to indicate.

    I ordered the parts last night, so that I have them on hand, when I am ready to install them. If I get a sudden burst of energy, and have some other, higher priority chore, I am going to take your suggestion, and make the modification - that is, of course, as long as the relay does not fail before that. (You know, when push turns to shove...)

    Warm diode? - good tip, thanks.

    Yes, that is exactly correct. If the remote switch is closed, and tank pressure is below the lower limit, (in this case about 80 psi) the pressure switch closes, and the relay is energized, and the pump gets power. Therefore the relay is closed, and stays closed until the pressure switch opens.

    If the remote switch is turned off, the relay can not send power to the pump, and of course, nothing happens.

    By the same token the pressure switch opens when the tank pressure reaches an upper limit (in this case, about 130 psi).

    The idea is that the pump only comes on when needed to keep the pressure above the lower limit, in situations where I might, for example be using the air to blow debris off something I am working on in the field, such as pressurizing my air shocks, when carrying a heavy load, or in the woods repairing a chainsaw, or otherwise using the air for something.

    Occasionally, I need the pump to come on, when the pressure is higher than the lower limit, but lower than the upper limit (like filling high pressure tires). In this case (assuming that the remote switch has energized, the coil,) I press the manual button, which bypasses the pressure switch, and turns on the pump.

    Left diode, brown wire out to solid green LED #3 = automatic.
    Right diode, blue wire out to red side of two color LED #2 = manual over-ride.

    All of the other other diodes and bi-color diodes are there for trouble shooting the system, should I have a failure somewhere - basically just to help me isolate the problem, so I know where to look for a failed component, or fuse burnt, or broken wire, etc..

    ~~~~~~~~~~~~~~in case you wondered~~~~~~~~~~~~~​

    If the system fails with the relay and the pressure switch in the closed positions, there is a safety valve, that vents the pressure, when the tank reaches about 150 psi, which is well below the tank's maximum safe operating pressure.

    I also have a pressure regulator for controlling pressure between the tank, and the air line that I use for keeping the air shocks at the pressure I select, depending on the situation, as well as a dump valve, if I need to lower the air pressure. All of this is for convenience, and not relevant for the purpose of this discussion, but I thought someone might be interested.

    OK, I think I have all of the changes to my schematic, and ready for install, when the time comes. How did I do, this time?

    Again - You guys rock!!
     

    Attached Files:

  4. Blank Stare

    Blank Stare

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    Off to the Dentist shortly, so I'll keep this short...

    Replacing the relay has always solved the problem...in the short term.

    Since the relay is under the hood of the truck, and is a little bigger than and inch on all edges of a cube, I have never heard it make any noise, even with the engine off, hood up, someone else in the driver;s seat, turning the remote switch on and off (with a working relay, at least.)

    I have assumed that the relay failed in the closed position, since the pump ran continuously, even after the pressure reached 150psi, and the mechanical safety valve vented pressure. Even turning the remote switch off didn't turn off the pump, if I remember correctly. I had to pop the hood, and pull the fuses. Perhaps the coil failed?...but that does not make sense to me.

    I'll be more responsive when I am finished with the Dentist, and errands...
     
  5. Blank Stare

    Blank Stare

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    Motor rarely turns on or off less than a minute two or between switching. More frequently, it runs 3 or 4 minutes, before shut down, and about a minute (or longer) before coming on again.
     
  6. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Yes, this indicates welded closed contacts because one contact is supplied directly from the 30A fuse..

    BTW, This topic is starting to come full circle. I said at the outset that I'm not convinced that BEMF is the culprit. Electrician's posts reinforces my my suspicions that the issue is elsewhere. Unless I've missed it all these years there has never been BEMF protection across the contacts of an automotive starter solenoid.

    Chris
     
  7. The Electrician

    The Electrician

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    In post #5 you said; "Sometimes they fail open, sometimes they fail closed." Perhaps I misunderstood; what did you mean by "fail open"? I took that to mean that when your circuit attempted to energize the relay it didn't send power to the pump.
     
  8. The Electrician

    The Electrician

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    I'm thinking that he needs a bigger relay; something like a starter solenoid, actually. I can't imagine how erosion of the relay contacts due to switching off an inductive could lead to failing open unless the process went on for a very long time. I had this happen to the starter solenoid in my 1977 Honda Accord. It got so that it wouldn't start with the first turn of the key. I had to try repeatedly. When I took the solenoid apart, the contacts were eroded so much that they wouldn't make contact well. As it happened, the contacts could be bought as separate items. But, this took years to happen.

    I can imagine how a 100+ amp surge might melt a small wire connecting to the armature contact of a small relay. Such a surge could also account for welding the contacts. Taking apart a failed open relay would provide invaluable diagnostic information. :)
     
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Electrician, I like your in the field approach to diagnostic troubleshooting. I don't see myself approaching this any differently. By the way, earlier in this thread Blank Stare said he also fried a tractor starter solenoid on this pump. It's a bit hard to dismiss this.

    Chris
     
  10. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    This is a long thread so forgive me if I've missed it. Has the question been asked of what gauge wire was used for the motor circuit and the relay coil circuit? Have we asked how long these runs are? If either circuit is undersized it's going to cause problems. Photos would be useful.

    Chris
     
  11. Blank Stare

    Blank Stare

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    The power across the relay and to the pump, is supplied by a 12 or 14 gauge wire. I don't remember which, but I can look. (When it stops raining...)

    I am pretty sure that the gauge wires I used were comparable to the size of the "lamp cord" that came with the pump, which was intended to plug into a cigarette lighter socket.

    I usually over size wires, to avoid the kinds of problems that come with under sizing. I do not specifically remember if I did that in this case, but I am sure I did not under size it - I just don't do that - ever.

    The pressure switch run is from the firewall in front of the steering column, to the front right (passenger) side - about 8 feet, but after snaking them around to avoid potential damage from other parts of the truck. maybe 10 feet. it's about a foot, (maybe less) to the pump.

    It occurs to me, that I have some extra positive wire coiled up, between the relay and the pressure switch - could that be causing BEMF? (see pictures)

    The pump is grounded to the body of the truck,through the air tank, The relay, and any other ground connections outside of the cab are also grounded to the body. I do not remember needing to ground any wires in the cab, except for the display LEDs, which I ran back out through the firewall, and grounded to the body.

    It's a few of feet (let's say 4 for good measure, from the fuse block under the hood, to the relay.

    The coil is on a much lighter gauge wire - 16 or 18? (I'll look when it stops raining...)

    It runs from the battery block at the (nearly) front of the engine compartment, left (driver's) side, about maybe 5 feet into the cab, and connects to the remote switch. It then returns maybe 5 feet, to the coil side of the relay. The relay grounds to the body.

    None of the wires have ever shown any discoloration on the insulation, nor have they ever had to be replaced.

    I hope that wire gauge is not an issue - it's a serious pain in the rear to work upside down under the dash. to snake wires, not to mention tearing the dash apart, to access the switches, which are mounted directly through it.

    I included the pictures that I took a few years ago, when I installed the system. There is a pot and a switch on the dash panel that are no longer being used. (I bought crappy stuff, and replaced it, later.) These are in the circle labeled 2.

    The air horn is not connected to the system at all, except that it shares air from the tank
     

    Attached Files:

    Last edited: Oct 23, 2012
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    From the outset I've never bit the BEMF hook of this thread. I'm less convinced now than ever. I don't think that all the diodes in the world or MOVs will cure your problem. If we knew that your pump was a cheap plug in emergency pump I don't think this thread would have approached this length. I would dump that pump and buy one designed for constant duty. Products designed to plug into cigarette lighter jacks don't fit this criteria. The temperature rise on that motor must be in the ozone.

    Chris
     
  13. The Electrician

    The Electrician

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    Blank Stare, in your first post, you said that the motor load is 20 amps. How do you know this? Did you measure it? The reason I ask is that 20 amps seems high for a device intended to plug into a cigarette lighter outlet.
     
  14. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Exactly. I think they're fused @ 10A, or used to be.

    Chris
     
  15. Blank Stare

    Blank Stare

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    That's what was spec'd in the paperwork that came with the pump. It may have been 30 amps, but I found what I think is the correct pump on the Viair website, and it states 19 amp maximum, at 100 psi.

    http://www.viaircorp.com/450P-A.html

    I specifically bought this one, because it was rated 100% duty rating - I wanted to make sure I could fill large tires without having to "rest" the pump.

    I don't know what it would draw at the higher psi ratings, but that's why I put it on a larger fuse, and relay.

    I stand corrected. - my "proto-type" was a cheapo throw away air pump. While it did the job, it got very warm when filling the tank from low pressures. Once I had a working proto-type, I discarded it (Really - in the trash.)

    The pump in service now is about 300 bucks, and actually had battery clamps - NOT a cigarette plug - my bad. While this pump has a power cord that resembles a cigarette plug cord, it is actually much heavier gauge.

    I believe the highest gauge I used anywhere was 12 gauge. I keep that size around for various projects, such as rewiring my trailer, that has internal lighting, and it's own battery for emergency brakes. That battery charges when the trailer is connected to the truck. I don't keep any 10 gauge around, and I don't remember buying any...but that does't rule out the possibility.

    What's an acceptable length to add, for a service loop? I suppose I could remove all the extra, but I'm always worried about something tearing up the wires, under the hood. I can get over that, real fast, if it's a concern. I was just trying to avoid replacing wires.

    The wire I am referring to is the coil side of the relay, on the 10 amp fuse. That's a tough one to replace, but again, if it's a concern, I'll do it in a heart beat.

    The power side of the relays I have used have been spec'd at 30 and 40 amps, mostly. I also tried a 50 amp, that fried after one cycle. These are all automotive relays, specifically designed to work on cars, trucks, etc.. That does not mean, however, that they are designed to handle the sort of situation that I I am using them for. I used them, because it made sense at the time.

    It finally got dry outside. I have a list of things to answer, that have been asked - let me know if you think I have missed anything..

    1. Wire gauge to coil side of relay.
    2. Wire gauge to power side of relay.
    3. Wire gauge to pressure switch.
    4. Wire gauge to pump.
    5. Wire gauge to pump ground.
    6. Wire gauges to instruments in cab. (Those will be the "fun" ones...)

    As always, thanks so much.

    ~ Blank Stare
     
    Last edited: Oct 25, 2012
  16. BobK

    BobK

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    This discussion has been going on for weeks now. Did you ever try the diode(s)?

    Bob
     
  17. The Electrician

    The Electrician

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    You should have given us this web page on your very first post. :D

    As far as what it would draw at higher pressures, the chart on that web page gives all the numbers up to 150 PSI.

    Knowing all the data on the pump is good. For example, they say that it comes with a 40 amp inline fuse. They don't say if it's a slow blow fuse, but nonetheless, it suggests to me that the start up surge can't be much over 40 amps; at least not for long. Since you have used 40 amp rated relays, the possibility I mentioned of melting the wire connecting to the armature is pretty well ruled out.

    It's too bad you don't have all the test equipment some of who are helping you have, darn it! I hate this kind of thing, where we're trying to help someone by guesswork! :mad:

    Here's what I would do. I'd open up a relay so I could watch the sparking at the contacts. Take a moto tool, or a jewelers saw, or a pair of diagonal cutters, or something and cut the case off a relay so you can see the contacts. Operate the relay and watch the contacts, noting the amount of sparking when you turn the relay off. Then add the diodes for absorbing the BEMF; see if the sparking is greatly reduced.

    And, if you have some more relay failures, save the failed relays for post mortem examination.

    I'm going to do some experimenting. I have a lot of diodes, some transorbs and a bunch of MOVs. I'll see what kind of sparking reduction I can get with an inductor for a load. I'll report back.
     
  18. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Observation: This motor has an inexpensive rocker switch on it. I can't fathom how this was supposed to survive while heavy duty relays, including a tractor starter solenoid, failed consistently. To dismiss this would be illogical.

    Chris
     
  19. The Electrician

    The Electrician

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    I did some experiments using the primary of a 200 VA power transformer for an inductor and a 5 amp bench supply, with a small relay to apply 5 amps to the inductor and then turn it off. I needed to apply 22 volts to get 5 amps in the inductor.

    I used a 100x probe on my scope and captured the turn off transient across the inductor.

    [​IMG]

    With nothing to suppress the transient, a substantial arc was visible at the relay contacts. The voltage across the inductor can be seen in the first image. Note the voltage scale is 100 volts/cm and zero volts is not at the middle of the screen--it's displaced two divisions up. The arcing begins at the middle of the screen, lasts for about 2.3 divisions, and the voltage reaches -600 volts.

    [​IMG]

    The second image shows the result of connecting a 1N5400 diode across the inductor terminals. The negative going excursion is clamped to one diode drop, a little less than a volt. The sweep speed is decreased and the voltage scale is now 10 V/cm, with zero volts at the middle of the screen. There is still arcing visible at the relay contacts, and it can be seen on the scope capture, lasting about 1.3 divisions.

    [​IMG]

    Next, I added a series combination of a 4 uF capacitor in series with a 1.2 ohm resistor across the inductor terminals, in addition to the diode. Now I can't see any sparking at the relay contacts, and the third image shows the transient voltage across the inductor terminals.

    This combination of suppression devices should eliminate the sparking of your relay contacts, Blank Stare. I would still sacrifice a relay and cut one open so you could verify that the arcing at the contacts is suppressed by these components.
     

    Attached Files:

    Last edited by a moderator: Oct 26, 2012
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Very impressive.

    It would have been interesting to see the effect of the snubber on its own. (Considering the price of a diode, it's not a huge saving, but I'm curious)
     
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