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Inrush power pulse into 12R 5W resistor

cupoftea

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Hi,
Do you agree that the 5W, 12R Vishay AC05 series resistor is not clearly defined in the datasheet as to whether it can handle the following inrush power pulse. (attached)
(its used as an inrush resistor in an offline SMPS with a 33uF, 400V input capacitor)

Do you believe that an NTC of 12R and a similar mass as the 5W,12R, would far better be able to handle this inrush?
 

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Easy peasy

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~ 7kW for 200uS, ( 1.4J ) what is the thermal capacity of the resistor? probably 1/10 of water - also - what is the pk current? - fusing can be an issue, as all the energy is initially in the wire and takes time to spread to the resistor core ...
 

cupoftea

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Thanks, this relates to the Mornsun LS15-13B12 15W, 12Vout offline SMPS

..On page 5, they recomend a "12R/3W" inrysh resistor, but i cant find one that could manage the inrush pulse, especially at 265VAC and mains peak at switch on.
Attached is the corresponding inrush current pulse for "on at mains peak" and 265vac
 

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  • Inrush current in 12R.jpg
    Inrush current in 12R.jpg
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KlausST

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Hi,

The AC03 is specified to handle 10J pulses in 1.2/50us, so I see no problem for a 1.4J pulse in 200us.
It is relaxed in time as well as in energy.
And it can handle up to 2kV in 1.2/50us. So even the dielectric strength is within specification.

AC03 resistors are wire wound resistors, they have a lot of series inductance. So on a high speed voltage pulse ... the current will rise more slowly.

Klaus
 

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Klaus has the answer, the AC05, 12 OHM can handle 50 J supposedly, on the 1.2/50uS test impulse curve (see Vishay data sheet p5 ), 10 pulses @ 30sec between each, so 1.4J over 200uS should be fine - these R's are designed to be fusing for larger over currents / powers ....
 

cupoftea

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Thanks, I see what you mean.

It seems strange that they are saying it can handle “10” of those 1.2/50us pulses with 30 seconds in between…it makes you wonder…. if they doubled it to 20 such pulses, then the resistor may fail on the 11th, or the 20th pulse, say?

And in the same vain, regarding the power pulse of the top post, ...after 4 years of those happening every day……would the cumulative damage then be enough to blow it? (the AC05)
 

FvM

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The 10 pulse spec is probably reflecting specific surge test definitions. It doesn't tell you anything about cumulative damage. To understand the long term behaviour, you need either to refer to manufacturer specs (if any exist) or analyze the basic device properties. Wire wound resistors are rugged, much more than e.g. polycrystalline NTC resistors. If the pulses don't cause exceedance of a certain wire temperature, the resistor will live very long.
 

cupoftea

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Thanks,
I was wanting to compare the Vishay AC05 with an “Inrush limiting NTC”, eg
https://www.ametherm.com/datasheetspdf/SL2210008.pdf

I see what you mean about NTC’s , though It seems automatic to think that an actual “inrush limiter” will be best…though the NTC datasheet gives no real way of being able to tell what kind of inrush it can withstand and says nothing of longevity.

With NTC’s, its tempting to think that even though they are weaker, they may (even over 100us) reduce their resistance somewhat during the inrush pulse and so reduce the stress on themselves like that……..
 

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FvM has a good point, NTC's only limit current when cold, e.g. 25 deg C, if they are hot and you cycle the mains power - you will get a larger surge current than for cold start .....
 

cupoftea

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Thanks yes, as you say, a hot NTC is no good at current limiting. But how about this…..use 20mm diameter NTC of value 12R…..the mains current in it will be just 111mA RMS max (for 20W SMPS). So the NTC will be dissipating just 148mW……so the NTC will never warm up, so even if there is an ON/OFF/ON…..then the NTC will still offer up the 12R.

There are NTC’s which are specifically called “inrush NTC’s”…..it seems counter intuitive to think that a simple wire wound resistor could be better than them? (ie higher pulse rating)
__________________________--------------------------------
We think that inrush limiters will come of age with the total movement to solar/wind power…because this power is non-continuous, and I believe we will see regular mains outages during the day time, every where….so equipment that would normally be connected to the mains 24/7, and so only suffer inrush once in its life (at installation), will now suffer inrush every day, as the mains returns from its outages (even many outages per day). Do you agree?
 

KlausST

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Hi,

If you want the NTC to stay cold, then why not just use a simple 12R resistor cold.
Where do you see the benefit of an NTC then?

There are NTC’s which are specifically called “inrush NTC’s”
You need to add the context.
Inrush limiter NTCs are used for NF transformers for example. Transformers, especially toroid ones - may easily draw 10 fold rated current when powered_ON due to remanece and saturation effects.
Here the cold NTC limits the current, the remanence and saturation effects are vey reduced and then relative slowly the NTC becomes hot and low impedance to drive the full rated current.

Klaus
 

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