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Limiting inrush current without power loss?

Soapy

Mar 13, 2016
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Hi all,
Despite my GCSE B in electronics a, er, few years back, and various other learnings since then, I've got a trivial problem that surely has a trivial solution. But can I find it?

I've got a protected lithium ion battery with circuitry, & I'm running a motor that pulls about 3A at stall. The issue is that the in-rush current is about 9A and so it trips the short circuit protection.

This has been driving me nuts for years, especially as it is an "intermittent fault"!

It's a single pack of 2x18650 cells in parallel, so current isn't an issue once it gets going.

Things that "sort of work" include:
Adding a diode (but that gets red hot, dropping 0.7V & wasting lots of power)
Adding a resistor (likewise)
Adding a really long bit of wire (sometimes works, possibly as an inductor, possibly as a long resistor?)
Double tapping the button really fast (works about 50/50 - if it is still spinning as you get the second tap, it'll fire up just fine and run for hours)

What I really need is something that has a bit of resistance to start with, but drops to (near) zero after a few tenths of a second. Oh, and space is a bit tight, so smaller = better.

Help, please!
 

Gryd3

Jun 25, 2014
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What I really need is something that has a bit of resistance to start with, but drops to (near) zero after a few tenths of a second. Oh, and space is a bit tight, so smaller = better.

Help, please!
Well... that sounds like an inductor to me... the problem here is that they can be quite large depending on the value of inductor you need.
Alternatively, you had stated if you double tap the button you can get it going... perhaps a driver or other 'active' device would work for you. Something that controls the motor with PWM. The duty cycle starts at 0%, then ramps up to 100%. That should reduce the sudden current draw.
The 'active' circuit could be made with a few cheap parts... Although how 'small' do you need to keep things?
 

Soapy

Mar 13, 2016
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Going with a PWM circuit seems a bit extreme, and would be quite a step up from a motor with a switch and a battery. And I'd never fit it in the housing.

The ~6-9A limiting system is the on-board protection circuits attached to the Li-Ion batteries. I used to use bare cells, and it worked fine, but once I learned a bit more I decided that was a bad idea, and moved to protected cells.

An "In-rush limiting varistor" sounds good.
I've been looking at the various PTC and NTC thermistor options earlier today, but so far I'm none the wiser. These devices are outside my experience. As I understand it, they start off cold, then take a small amount of current to get warm, which either raises or lowers the resistance. Do they drop to the sub-ohm range? How fast do they go back to being high resistance? I've just had a read of http://en.tdk.eu/tdk-en/373562/tech...ations---cases/always-on-the-safe-side/761864 which says that the "reset" time is over 30 seconds, which means it isn't going to be any use. Is the varistor and the thermistor the same beast? If not, have you got a link so I can read up?

Ta.
 

duke37

Jan 9, 2011
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A resistor, shorted by a relay or fet after a short time would do the job with very little voltage drop.
 

AnalogKid

Jun 10, 2015
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If a PWM circuit won't "fit in the housing", I doubt a relay will, with or without its control circuit and coil driver.

Before soft-starting PFC circuits because so common in industrial power supplies over 1 kW, a common technique was a NTC limiter combined with the shorting relay. This has a better current profile than a fixed resistor, and the relay lets it cool off once the supply is up and running so if power bounces you get the full inrush protection again. A lot of parts, but the inrush for a 2 kW switcher was huge back then.

ak
 

cjdelphi

Oct 26, 2011
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Maybe not the best solution but adding a poly fuse should do the job
 

AnalogKid

Jun 10, 2015
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A polyfuse is the opposite technology - it starts out at a low resistance when cold, then snaps up to a high resistance above a thermal trip point (that is tied to a certain current level).

ak
 

cjdelphi

Oct 26, 2011
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Well yeah, precisely.... the fuse will heat up reducing current, once the inrush is over the polyfuse cools down supplying more current
 

AnalogKid

Jun 10, 2015
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By the time the poly fuse material heats up and the part changes state, the inrush might be over. Plus, the fuse doesn't just reduce the current, it shuts it off almost completely. The motor will stop dead after about 1/2 to 1 second.

ak
 

Gryd3

Jun 25, 2014
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By the time the poly fuse material heats up and the part changes state, the inrush might be over. Plus, the fuse doesn't just reduce the current, it shuts it off almost completely. The motor will stop dead after about 1/2 to 1 second.

ak
Yeah, we gotta axe the ploy-fuse idea. It operated in the opposite manner than what we require...
I guess you can let it 'trip' then rely on it 'recovering' and gradually applying current, but I would assume it would end up just staying 'popped'
 

Harald Kapp

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Nov 17, 2011
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I say duke's on the right path (post #7) when you dismiss the relay for the FET. At the voltages involed a power MOSFET can have a few mΩ at most which will not cause noticeable power loss at 3A current. Here's an example circuit.
 

AnalogKid

Jun 10, 2015
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Taking that one step farther, delete the resistor in the power path, add a resistor and capacitor to the gate, and let the FET be the current limiting element. This is a bit more complex than a single NTC resistor, but if you throw in a signal diode it has near-instant recovery and no hysteresis. Something not mentioned before is that NTC current limiters age a small amount with each power surge. And Polyfuses are not just worse, but much worse. They are not designed or intended for repeated trip cycles.

ak
 

cjdelphi

Oct 26, 2011
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Resistor / relay is how an amplifier works, but i like solid state options lol
 
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