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Allowable inrush current

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Dr. Dark Current

Thu Sept 18 2008, 06:01PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I was asked to make a demo electronic Jacobs ladder. I want to keep everything as simple as possible for better reliability.
I don't know how to design the inrush current limiting. The thing will be drawing at most 2 amps from 230V. I could make a relay delay circuit which shorts inrush limiting resistor after the caps are charged, but there will be another relay to turn the power part of the circuit on after the control electronics voltage has come up, and I don't want another mechanical component.

The simplest way is obviously a resistor, but the problem is its power dissipation so its desirable to make it as small as possible.
So what is the allowable inrush current? How small can I go with the resistor? Ideally I would want to use 2.2ohm 10W resistor but I don't know if that isn't too small...

guitardude012

Thu Sept 18 2008, 09:14PM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

You could probably replace that relay delay circuit with a transistor triggered by a voltage divider.

edit: Actually if you are trying to minimize power dissipation you would maximize resistance right?

Billybobjoe

Fri Sept 19 2008, 01:20AM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

Just out of curiosity what is the circuit? From what I understand this is going to be flyback based?

Inrush current is usually limited so as to not destroy your diodes/rectifier from the filter capacitors presenting nearly a short circuit before they are charged - unless you're talking about some other type . . .

So what are your filter cap. values and what kind of rectifier are you using?

There are types of thermistors especially designed for this purpose - see

for an example.

Dr. Dark Current

Fri Sept 19 2008, 05:11AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

It is half-bridge driving a "twinflyback" (2 secondaries on 1 core), filter caps 2x680uF in series. I don't like thermistors because when they heat up and you turn it off and on, the surge is much bigger as they are still hot.

I got another idea, what about choke damped by diode, after the rectifier?

Firefox

Fri Sept 19 2008, 05:18AM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

Could you use a HOT from a TV and slowly bring it to saturation to bring up the current? They generally have current ratings around 15A, so they should work for your application. Unfortunently, that does allow for user error by leaving the potentiometer at 0, but I am sure there is a way to put a failsafe on it.

Dr. Dark Current

Fri Sept 19 2008, 06:04AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Well I was thinking about one component or a few of them at most. If I was to implement the HOT softstart, there are much more things to go wrong, and I could at that point implement the "relay delay" circuit.

I think the inductor wins for now, I just don't have an idea on the inductance and air gap (it will be wound on a small laminated EI core, possibly with around 1.5 by 1.5cm cross section)

Coronafix

Fri Sept 19 2008, 09:03AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

I don't get it. You are talking about a Jacobs ladder, right?
Why not just use a NST?

uzzors2k

Fri Sept 19 2008, 09:21AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

NSTs are lame, that's why.

Seriously, have you seen Jan's work? I bet the bipolar flyback will have a max arc distance of 30cm.

I your application Jan, I think the inrush current limit will be set by your mains rectifier. I can't think of anything else besides the breaker which would suffer from high inrush currents. In the datasheet they've often specified some maximum current for a certain amount of time, probably on a graph. These currents are in the 150 - 250A range for a 15A bridge rectifier, probably enough that you won't need to worry limiting the current, but at least you'll have some numbers to go from.

Dr. Dark Current

Fri Sept 19 2008, 09:45AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Uzzors wrote ...

NSTs are lame, that's why.

I don't have a NST and I'm not buying one, not saying my ladder will be nearly free and have bigger arcs. I won't push the twinflyback much, the thing needs to be RELIABLE so I'm aiming for 15cm arcs at most (~300-400W).

"I can't think of anything else besides the breaker which would suffer from high inrush currents. "
The switch, welding its contacts together?

GeordieBoy

Fri Sept 19 2008, 12:07PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

NTC thermistors are the standard way of solving inrush problems on small off-line SMPSUs up to a kW or so.

If the power dissipation, elevated running temperature, or relatively long cool-down time really bothers you, then you can switch out the NTC once the DC bus comes up to voltage and the downstream supply starts running. It improves efficiency, reduces operating temperature, and gives the NTCs plenty of time to cool before being hit by the next inrush event. Either a relay or an arangement of SCR's can achieve this. (They are often driven off the auxilliary winding of the power transformer in SMPSUs so that the relay shorts out the inrush limiter as soon as the supply has done it's soft-start and begins to output real power.)

For even higher powers, multiple NTCs, and NTC+power resistor combinations are used to limit inrush. And for very high powers the DC bus capacitors can be pre-charged before connecting to the AC supply.

-Richie,

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