Surge Current and Power in FETs

================== Bill Slowman = Demented

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** Mains rated PTCs consume a few watts at 240VAC. So only draws 30 milliamps or so. ** Only to reset the device and start again. ** No, it's a re-settable fuse.

..... Phil

If the circuit it's protecting draws enough current to do that.

It depends what the in-rush current is rushing in to do. If it is just charging reservoir capacitors, and the circuit draws a lot less current in normal operation, the PTC should cool down enough to let the circuit work.

A stalled motor is a different kind of problem.

Bill Slowman has go away

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** Bill has never seen one doing anything. ** Massive pile of FUCKING BULLSHIT !!!!! ---------------------------------------------------------

...... Phil

Not by anybody who knows how they work.

PTC thermistors are also used for in-rush current limiting, albeit in a somewhat different way.

Bizarre - and entirely fatuous - claim.

Phil doesn't agree, but he has some rather eccentric ideas about what words mean. I've snipped what he actually posted.

<snip>

Why on earth would I bother? They strike me as a cheap and nasty solution to a problem that can be solved with better characterised devices. Once you get into designing mass market products you do get stuck with exploiting the cheapest practical solution, but I've mostly worked on stuff destined for smaller, pickier markets .

Bill Slowman Has ZERO BRAIN LEFT

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** It's what the *makers* call them CUNTHAED !!

See the graphs??

Fuck you asshole .

** ROTFL - it was the plain truth - you LYING ASSHOLE ** No reason really - you might find out you are wrong though.

Best avoided.....

Would not want to confront your own insanity.

..... Phil

Yes, that's why the connector with the resistor in it works. 5.6 ohms, one size fits all. The connectors are made to handle some 50-60 amps. XT90-S Clear overkill for our design, but it works and is cheap.

What do you want to know? I'm happy to share any details I have. I've already provided a link to the schematic which one person was complaining I didn't provide. There it is. Make yourself happy.

I'm not looking for miracles. I'm just not interested in trying to shoehorn one solution into a design when it doesn't look to be a good fit. But yes, I'm lazy and don't want to spend a lot of time trying to analyze this approach to prove it will work when I think it is marginal at best.

A resistor with a bypass FET is easy in comparison.

Put an NTC in the V_MAIN feed from C13

I'm trying to explain to you why "stays out of the way" is not so simple.

If you aren't interested in helping with this, why are you here? Are you just being a drama queen?

Good question. I have not worried with that because it is not likely to be a problem. Once the battery is installed the machine is never 100% off much like most devices with backup battery. The power board designer decided to never shut down the higher voltage rail, only the mid level rail. Rather than cutting the battery off from the various power drains the switching regulator is shut down and everything remaining on the higher voltage rail is low power when not running. Funny that the spec on the caps indicates some 720 uA max and a paper I found indicates that can be derated to about 108 uA, roughly half the battery self drain current.

So they keep power on the 12V rail which keeps the caps charged up, which apparently is a good thing for the caps. Seems electrolytic caps degrade when sitting on the shelf. Leakage increases.

??? The goal would be to have the information, not to verify anything about the model.

Sometimes I don't think you are here for the hunting...

Scratching my head, I ask, isn't that when you get the inrush, when you turn power back on after being off???

I've provided all the info asked of me.

As I have explained very clearly, the 10 amps figure is the start current. This is probably the same as the stall current, but I don't care about that because we detect a stall and shut down the motor as it means something has likely broken.

As I have explained very clearly, the initial start of the cycle is to get the pressure up as quickly as possible. In fact, they had trouble getting the PID control software to respond adequately so the initial 100 ms or so slam the motor with 100% power regardless of any settings.

I believe the capacitors are to smooth the power fluctuations on the 12V rail. The motor is PWM and there are significant surges at various points.

When dealing with an issue of surge current to capacitors from the initial connection of the battery, why would you ask about the mechanical stress from the motor stopping and starting???

Yes, I can see how I didn't supply nearly enough information for this issue.

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** ROTFLMAO !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Rick C is so * f****ng* autistic, he can barely read.

Must think bottles of "Steak Sauce " contain actual steak.

Or that " King Kong " is the ruler of Hong Kong.

He must be a hoot at parties, with a few drinks in him.

...... Phil

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** But your OP claimed the issue it was mosfet current ?

** Plus you have been moving the goal posts about ever since.

FYI:

YOU are one f****ng arrogant asshole - Rick C

A pseudo intellectual CHILD in a man's world.

YOU FUCKING SUCK !!!!!!!!!!!!

...... Phil

I did not follow much of the thread, but it sounds similar to what we were trying to fix. Some of our equipments were burning out caps, switches and PCB with inrush current. So, we wired up two switches, with a power resistor for the first switch and put it in a switch box. Operator needs to open the switch box (which disconnect the second switch) in order to turn on the first switch.

Put an NTC in the V_MAIN feed from C13. If necessary, short it out with another component such as a FET when the inrush current has passed. That way it can do its inrush surge protection job at power up and be shorted out before the motor notices it's there. You might have the microcontroller turn the FET on a second or two before motor drive is needed.

Those are not Poly-fuses.

Littelfuse (the company that makes Poly-fuses, a brand name) actually calls them Polymeric Positive Temperature Coefficient (PPTC). But they also refer to them as PTC.

Yeah, the graphs on page 8 are exactly why I don't think these will do much for the capacitor surge. They take milliseconds to activate even at 100 times the rated current.

This document says PTCs are ceramic, so not the same as the polymeric fuses made by Poly-fuse.

Lol. Any particular NTC? That's my point. It's not where does the NTC go, although I would put it inline with the fuse rather than inline with V_main. No point in inserting it in the input power path when it is only needed for the battery.

So you didn't notice the power FETs U8A, U8B, U9A and U9B are connected incorrectly with drain and source swapped? lol

There's a note about the idle state power drain and he used a bogus number for the LTC4416 idle current. It should be 130 uA if I recall correctly.

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** " Polyfuse" is a generic word for any PTC using polymer or similar technology. ** Yawwwnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn...... ** You are a FUCKING HEE HAWING ASS !!!!!!!!!!!!!!!!!!!!!!!!

They do not need to TRIP and are f****ng useless if they do. You are DUMB as DOG SHIT !!!

....... Phil

Yeah, that's exactly what the XT90-S connector does. First it mates with the resistor ring, then when pushed together fully the resistor is bypassed. Good solution.

Maybe your device could be done with a single switch with three positions. The one in the middle connects the resistor. Or is your surge longer lasting than the brief time it would be connected in flipping the switch from position 1 to 3?

No but I did notice that the entire system topology is wrong as I've pointed out before.