Thermistor help needed.

Discussion in 'Tech/Mech General' started by olley, Oct 27, 2009.

  1. olley

    olley Funster

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    Hi I sometimes trip the EHU due to inrush current in the stepdown tranny, its not a major problem but one I thought I would fix. Following a link from scotjimland to here: SL32 Series Inrush Current Limiters - bigAmp

    I bought the SL32 2R025, now after a trial fix, it doesn't trip a 16amp Mcb anymore but still trips a 5amp, can anybody suggest a better thermistor from that list, or from an alternative supplier? It must have a continuous rating of at least 16amp.

    Ian
     
  2. Geo

    Geo Trader - Funster

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    Olley
    before spending any more dosh I know you love experimenting.
    Would you try an experiment for me and it may solve your problem and save a few ££
    Imagine the problem of tripping being caused by a difference in earth potentials between the RV and the electricity source
    one way to balance this and i may be totally off the target here, would be to sink an earth rod near the post or even the RV and connect up the RV chassis to it. thereby attempting to even out the earth potentials caused by the RV sitting on rubber boots and the post in effect being earthed out to the local ground
    I have a concrete base so cant sink a rod
    if it the becomes stable on your 5 amp were on a winner
    Geo
    Ok start laughing at me:Doh:
     
  3. olley

    olley Funster

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    Hi Geo, good try but no cigar. :BigGrin: The EHU trips because of inrush current, and it depends where the AC cycle is at the moment you switch on as to the amount you get.

    Nowt to do with bashing stakes in the ground, unless you just fancy the exercise. :BigGrin:

    This from wiki

    When a transformer is first energized a transient current up to 10 to 50 times larger than the rated transformer current can flow for several cycles. This happens when the primary winding is connected around the zero-crossing of the primary voltage. For large transformers, inrush current can last for several seconds. Toroid transformers can have up to 80 times larger inrush, because the remanent magnetism is nearly as high as the saturation magnetism at the "knee" of the hysteresis loop. This is caused because the transformer will always have some residual flux density and when the transformer is reenergized the incoming flux will add to the already existing flux which will cause the transformer to move into saturation. Then only the resistance of the primary side windings and the power line are limiting the current

    Ian
     
    Last edited: Oct 27, 2009
  4. scotjimland

    scotjimland Funster Life Member

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    Last edited: Oct 27, 2009
  5. Geo

    Geo Trader - Funster

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    Yup I know all about that crap:Rofl1:
    Thats why I termed my suggestion an Experiment :cry:
    What a shock you would get if it worked or even made a difference
    If I was on a field and had the time to play I would give it a go
    Remember no one thought about gravity till an apple hit wots it on the ed:RollEyes:
    Isac Geo:thumb:
     
  6. redpis

    redpis Read Only Funster

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    Might be worth considering changing the 5A MCB to a type 'C' or if type 'C' already, 'D', these are designed to cope with larger in rush currents,
     
  7. olley

    olley Funster

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    hi redpis, they get annoyed if you do that on site. :BigGrin:

    Hi Jim seems my test procedure might be wrong :Blush: after a bit more research on the web I found a more detailed explanation of how they work, as the current goes through them they heat up and their internal resistance drops to near on zero, you should allow at least 60 seconds between tests to allow them to cool down, :Doh:

    So it may have been working, as it didn't trip the first time, so I am going to fit it and forget it for the time being, and see what happens. :BigGrin:

    Ian
     
    Last edited: Oct 27, 2009
  8. Knine

    Knine Read Only Funster

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    Hi Ian

    If it hasn't cured the problem, you could try putting another SL32 2R025 in series with the first one (to double the overall cold resistance) or even put three in series (to treble the cold resistance). Once you sort the problem and mount it permanently, remember that at 16 amps the thermistors will run a surface temperature well over 100C so put them in a ventilated enclosure with good airflow around them.

    Kees
     
  9. olley

    olley Funster

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    Hi kees, sounds as unlike myself who's just bodging in the dark, you know what your talking about. :BigGrin:

    Thanks I didn't realise they got that hot, maybe a 230v relay to bypass it would be a good idea then, I could energise it from a push button, after I plugged in. Going to refit it today and and run some appliances, and see what happens.

    Just looked at that link of Jims to the UK distributor, I paid £1.32 from the states, they want £5.20 :Eek!:

    Ian
     
    Last edited: Oct 28, 2009
  10. olley

    olley Funster

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    Hi Kees just fitted it again and loaded it with 12amps, about a 1v drop across its terminals, and as you said, its flippin hot, to hot to touch, defiantly needs to be in small enclosure. Going to let it cool down and try it again, I will do that a few times and see if it trips again.

    Ian
     
  11. Knine

    Knine Read Only Funster

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    Hi Olley

    Yes, the relay across the thermistor once it has done its job is a good idea (16 amp relay contacts needed of course). Or you could just use a 16A rated mains switch to short it out.

    If you wanted to be really flash you could use a mains relay with a short time delay driving its coil. Then it would automatically open and close at the right times.

    Knine (slip of the brain - thought I was Kees last night!)
     
  12. Jaws

    Jaws Funster Life Member

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    Ummm.. might seem a bit obvious but why not just slap a 1000v .1 uF across the line ?

    Or maybe even a non electrolitic 10 uF ! Bet that would kill any transient spikes stone dead :thumb:
     
  13. olley

    olley Funster

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    Hi Knine switch is ok, but I would forget to turn it off :Blush: and that would be the time it tripped the EHU. A 230v relay is olleyproof, bell push to close the contacts and then feed it through one set of the contacts, disconnect the EHU and it drops out. Found an 11 pin 10amp relay and base in the workshop, so I will use two contacts for the power and one for the relay, even found a neon to stick on the outside of the enclosure. :BigGrin:

    Hi Jaws, while I know what a capacitor is, I don't have a clue whether your idea would work. :Doh:

    Ian
     
  14. Jaws

    Jaws Funster Life Member

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    Well in thoery it should take out any nasty spikes..
    In the purest sense it allows a path for AC but not DC.
    Of course we are dealing with a wholly AC signal but putting a .1 ( or even a couple in parrallel which ( unlike resistors which would need to be in series ) giving .2uF ) across the feed and return should just kill of the nasty great big spikes that are giving you gyp :Smile:

    Do make sure you use ones with a very high working voltage.. The P / P of 250v AC is a tad over 350v and spikes could easily double that !
    Big advantage is the cost would be about 70p !
     
  15. Knine

    Knine Read Only Funster

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    Hi JawsMC

    The problem is a surge of current when the transformer is energised. Putting a capacitor across the supply would actually make the problem slightly worse by adding another spike of current to charge the capacitor when mains is applied.

    Olley

    I like the idea of running the coil from one pole of the relay. Don't use a bell push to operate it though as these are usually only rated for low voltage. Maplin should have a mains rated push button switch at a reasonable price.

    Knine
     
  16. Geo

    Geo Trader - Funster

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    I can see Olley now with his pre flight check just to plug in the hook up
    Button in
    Relays Energised
    Capacitors Fluxed
    OK Mildred plug it in:Rofl1::Rofl1::Rofl1:
    Geo
     
  17. olley

    olley Funster

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    Hi Geo, do you think it would be alright to do that in sandles and tee shirt? or should I wear a uniform?

    Ian
     
  18. pappajohn

    pappajohn Funster Life Member

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    ok Geo, why did they name a measurement of torque after a man who discovered gravity ??........Sir Issac Newton/Meters :Rofl1:
     
  19. Geo

    Geo Trader - Funster

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    Nice try John
    It was Woman that discoverd Torque:Wink: and they aint stopped since:Rofl1::Rofl1:
    Geo
     
  20. Geo

    Geo Trader - Funster

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    Sandles and shorts will be fine Olley
    Just remember No Alcohol in the preceeding 24Hrs before engaging or disengaging shore lines, less the sequence be lost:Eeek:
    Do you know, I am 100% convinced that if time travel is at all ever possible, one of your gizzmos will be at the bottom of it, one of these days you will throw a switch and find yourself God knows when:thumb::Rofl1::Rofl1::Rofl1:
    Geo
     
    Last edited: Oct 28, 2009
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