Thermistor help needed. (1 Viewer)

[olley]

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

 

[Geo]

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 meoh:

 

[olley]

Hi Geo, good try but no cigar. 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.

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

^{_{Subscribers  do not see these advertisements}}

 

[scotjimland]

Hi Ian

I had a look at the calculation on the site and still none the wiser oh:

Why not contact them with your requirements.. save guesswork

jim

edit


BTW...

they have a UK distributor

Link Removed

 

[Geo]

Hi Geo, good try but no cigar. 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.

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

Yup I know all about that crap
Thats why I termed my suggestion an Experiment
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
Isac Geo:thumb:

 

[redpis]

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,

^{_{Subscribers  do not see these advertisements}}

 

[olley]

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,

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

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, oh:

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.

Ian

 

[Knine]

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

 

[olley]

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

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

Ian

^{_{Subscribers  do not see these advertisements}}

 

[olley]

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

 

[Knine]

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!)

 

[Jaws]

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:

^{_{Subscribers  do not see these advertisements}}

 

[olley]

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!)

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.

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

Ian

 

[Jaws]

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

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 !

 

[Knine]

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

^{_{Subscribers  do not see these advertisements}}

 

[Geo]

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
Geo

 

[olley]

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
Geo

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

Ian

 

[pappajohn]

Yup I know all about that crap
Thats why I termed my suggestion an Experiment
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
Isac Geo:thumb:

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

^{_{Subscribers  do not see these advertisements}}

 

[Geo]

Nice try John
It was Woman that discoverd Torque and they aint stopped since
Geo

 

[Geo]

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

Ian

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
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:
Geo

 

[Jaws]

Ouch ! Yes of course.. In that case why not a choke with a cap across it ?
That would def take out A surges as the choke would act as a ramp and slow it down completely.. Just thinking out loud here

^{_{Subscribers  do not see these advertisements}}

 

[Knine]

Hi JawsMC

You're getting closer! A choke in series with the mains input would solve the surge problem. The trouble is, it would have to be a 50Hz choke rated at 16 amps and would physically be about half the weight of the transformer ! The capacitor across it would still do more harm than good.

The thermistor does a much better job because it has high(ish) resistance when you turn on, and then automatically goes low(er) resistance as you run current through it, then returns to high resistance as it cools down when you switch off. 99% of computer power supplies and televisions use a thermistor to reduce start-up surges.

Sorry to be so boring, but you did ask

Knine

 

[Jaws]

AGrred re the choke but I simply hate thermistors..
old school you see,,,, A lot of the old rank bush murphy and thorn chassis used them for all sorts way back in the bad old days..
Prone to unexpected combustion would be a nice way to put it I think !!

 

[olley]

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 where :thumb:
Geo

Funny you should say that, but at the moment my "mass displacer" does exactly that, I think its to do with the mini black hole that is supposed to suck the RV in and spit it out over Spain somewhere, been meaning to ask on here if anybody had any ideas? should it be a stationary black hole, or a spinning one?

Cause it could just be the old dilithium crystals, got them cheap of ebay, from some guy in Mogadishu, he seemed honest. oh:

On a more serious note, its amazing, a week ago I thought a thermistor was something to do with your central heating, but now I know different, I intend to drop it in to my conversations down the pub. yeah its your thermistor mate, yeah know all about them I do, tricky little blighter's they are.

Olley

^{_{Subscribers  do not see these advertisements}}

 

[olley]

Hi all done and fitted, taking knine's advice I bought a 230v switch of Maplins, it took a day longer to arrive than the thermistor from the states.

It fits nicely in a 4x4 junction box, and the knock outs make it easy to ventilate. Even fitted a little green telltale LED.

 

[scotjimland]

Hi all done and fitted, taking knine's advice I bought a 230v switch of Maplins, it took a day longer to arrive than the thermistor from the states.

It fits nicely in a 4x4 junction box, and the knock outs make it easy to ventilate. Even fitted a little green telltale LED.

Nice job.. but no cable gland in the box.. ???? tisk tisk Olley ..

jim

 

[olley]

Hi Jim I thought with 5 big holes in it already, a gland wouldn't do much.

Olley

^{_{Subscribers  do not see these advertisements}}

 

[scotjimland]

Hi Jim I thought with 5 big holes in it already, a gland wouldn't do much.

Olley

never heard of grommets ?

Link Removed


guess it will help to keep the thermistor cool ... :Cool:

 

[hilldweller]

Hi all done and fitted, taking knine's advice I bought a 230v switch of Maplins, it took a day longer to arrive than the thermistor from the states.

It fits nicely in a 4x4 junction box, and the knock outs make it easy to ventilate. Even fitted a little green telltale LED.

Where is the CE mark ?

 

[olley]

Where is the CE mark ?

Hi Brian, do thermistors have CE marks??

Olley

^{_{Subscribers  do not see these advertisements}}