Surge Current and Power in FETs

NTC is typically used as a power supply inrush current limiter. A quick Google found this example.

it's labelled NTCR1

I'd love to see this guy at the supermarket. He must be very entertaining.

I would ask him to explain exactly how he thinks the 1 ohm NTC would work in this context but he seems to be so upset he can't even type properly.

This ASSHOLE is a raving NUT Case !! ================================

** Don't bother, YOU are way too arrogant and f****ng stupid to have a hope.

..... Phil

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** It a good, one component solution used used in a billion PSUs.

Transformer and off line .

...... Phil

You might think this was designed in. But I didn't see anything in the LTC4416 about it. I'm not sure what circuit I posted. It may have been my idea which used transistors. The problem I had was similar, inrush current on switching between battery and line power without giving thought to connection of the battery. The guy designing the board didn't want to worry with that level of design so everything on the board is LT this or LT that at a healthy price tag.

The design needs a low power mode where the MCU only wakes every few minutes to check on the battery state and sound an alarm if it needs charging. It never occurred to me this would not involve shutting off the 12 volt power rail entirely and using a couple of diodes to steer power to the low power MCU circuit. That does at least mitigate dealing with charging the caps on power up, but it is still a problem when the battery is connected.

The board designer has just waved hands in the air and proclaimed having built boards with 10,000! uF of capacitance!!! without addressing any of the technical details. I made another post to the group explaining how adding a 250 mohm resistor mitigates the surge (just barely) but will drop 2.5 volts at peak power to the motor which is not acceptable.

For that to work there has to be enough current flowing to maintain the low resistance or it has to change rapidly enough to not interfere with spiky current demands. The range of current demand is rather wide in this design going from low mA to 10 amps when the motor kicks in. I just don't think the NTC will work without a lot of careful design work if then.

But you can't explain how it might work in this application, can you?

Didn't think so.

Bill Sloman is DAMN LAIR wrote: ==================================

** Complete bullshit. The inrush is all over in 50ms, well before the PTC reaches "switching" temp - peak current limiting is done by it's cold resistance. ** Utter bullshit.

** Or even seen one in use, like I f****ng have.

FYI f*****ad;

A few Crown power amplifiers ( with 800VA toroidals) have large 6 ohm, mains rated PTCs with relay bypassing on the incoming AC.

The devices barely get warm after switch on.

Their advantage is when a fault causes *high mains draw* the relay stays open and the PTC instantly switches high - effectively isolating the AC supply. IOW it acts like self resetting fuse.

..... Phil

========================== > >

** The one you have not described fully yet ??

Wot a f****ng, raving lunatic .

Would it have been better to have the amplifier enabled only when the relay closed?

piglet

** FYI s*****ad; 10 amps through 1 ohm NTC = 100watts dissipation. Time to drop below 0.1 ohms is about 2 seconds.

Bridge the NTC with a suitable mosfet, after DC power is applied and you have NO delay.

Thing is YOU don't f****ng know what is really needed.

....... Phil

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** Of course, Bose did not think to add a muting system at switch on.

The damn thing was fully on in 50ms.

..... Phil

<snipped Phil being as silly as ever>

Don't be a complete idiot. You are talking about an NTC current limiter.

They are all over the place, but it is a bit difficult to see it happening without something like a storage scope.

That's the whole point. It warms up only during switch-on

The PTC doesn't switch high "instantly". It has a thermal mass, you have to dissipate enough energy in the body of thermsitor to get it warm enough to push up the resistance which does further increase the rate of heat dissipation in the thermistor, until it warms up enough that the current starts going down.

It can be pretty quick - but not "instant".

Where?

How? It's not "howto_ntc-limiter".

Bill Slowman Fucking Retard = IDIOT wrote: ===============================

** It's a PTC *current* limiter !!!!

They put them in series with small motors to protect windings from overtemp when stalled.

Motor stalls, current jumps right up, PTC trips high and all is well.

..... Phil

<snip> <snipped Phil being unusually silly>

As I mentioned. I probably have seen quite a few, but not in circumstances where I could see what they were doing at start-up.

Not where I see what they were actually doing. They didn't push out clouds of smoke while they were coping with in-rush.

Not where you can see it. Seeing what's going on inside a PTC current limiter while it's doing it's job would take careful instrumentation, if anybody needed to do it.

That's scarcely instantly.

I'm not posting rude words, nor showing any other sign of agitation. I could check my blood-pressure - I've even got a machine that does the job - but it would be tediously normal. Phil doesn't seem to be doing as well.

Even more unlikely.

Seems unlikely too.

Not the sort of request that's likely to get a positive response. Phil does seem to be getting remarkably excited. I could pass on some of my blood pressure medications, but they probably wouldn't the of the kind he ought to be taking. Anti-pyschotic medication to deal with his disordered thinking does seem to be what he needs, and I haven't got any of that.

Which is about 1.99 seconds too late. I believe I've already explained this power supply is driving a motor that comes on about every two to four seconds. The inrush from the motor is 10 amps which needs to be supported, not blocked. The rest of the time the current will be well below 100 mA.

If I'm going to bridge the NTC with a mosfet, why use an NTC, a resistor will do fine which is what I've said.

Ok, can I borrow a gun and shoot myself?

If you weren't so rabid, maybe you could pay attention to what was written and understand what I'm describing.

You really should get your shots right away, but not for COVID, for RABIES!

It's one application.

The PTC doesn't "trip". It just gets warm enough to put enough resistance in series with the windings to stop them burning out.

Turn the power off and the PTC will cool down smoothly and it's resistance will drop as gets cooler. NTCs can do much odder stuff - once you have a "hot" channel all the current flows through that, and it takes a lot less current to sustain a hot channel than it does to create it.

Bill Slowman Has a Brain Tumor wrote: ===============================

** FFS check the data.

Polyswitches (TM) do exactly that and so do mains voltage PTCs

Resistance jumps by up to 1000 to 1

These are not ordinary PTC thermistors !!

YOU KNOW FUCKING NOTHING !!!!!

..... Phil