Selecting an inrush limiting resistor

Discussion in 'The Projects Forum' started by tom66, Apr 1, 2012.

  1. tom66

    Thread Starter Senior Member

    May 9, 2009
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    I have a 15A, 0-265VAC Variac I got a while ago. This is a 4000VA device; and naturally, it trips the 32A CB whenever I turn it on because of the inrush. Usually, after a few flips of the breaker, I can turn it on again, however it worries me the breaker will be damaged by this repeated tripping (the breaker is only rated for something like 500 trip cycles.)

    So, I've designed a circuit to solve this problem. This is a similar circuit I've seen for charging up large capacitor banks in audio equipment and plasma TVs which typically have >500µF of bulk PFC capacitors.

    Essentially it consists of connecting the load in series with a resistor, and then after a pre-determined time, the resistor is shorted or removed from the circuit (typically a relay is used) and the device powers up as normal. I believe this is called pre-charging. It's needed for inductors because the core has no magnetic flux at that point in time and inductance is nearly zero, so it looks like a short circuit to the breaker.

    The problem I'm having is determining the value of this resistor. My circuit can pre-charge any time from 100-500ms (for now, 200ms is achieved with 220µF cap.) Too low resistance, and it still trips the CB. Too high, and insufficient pre-charge is provided, so it trips the CB too. I can't measure the inductance of the variac unfortunately, but I can provide a datasheet if necessary.

    Does anyone know what the calculation might be for this inrush resistor? I've attached a schematic of my design. The circuit is powered from a mains transformer ripped from a cheap wall wart (there should be no safety concerns with capacitive based power supplies, etc.)
     
  2. wayneh

    Expert

    Sep 9, 2010
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    I think you just need to use the full voltage across the variac at the instant it's switched on, and use that to calculate the resistance needed to keep the peak current below the breaker's trip limit. In other words, treat the variac as a dead short. This approach at least gives you the upper limit on the resistance you need.

    BTW, why do you need 3 transistors in the delay circuit? I understand one, and maybe another to invert the signal, but 3?
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    I'd replace Q1-Q3 with a single Darlington. Wayneh's suggestion should work OK.
     
  4. tom66

    Thread Starter Senior Member

    May 9, 2009
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    Okay, let's say I want a maximum current of 20A.

    Maximum AC voltage: 253V in UK (358V peak.)
    Maximum current: 20A.
    Resistance: 17.9 ohms ~~ 18 ohms.

    The resistor dissipates a power of 2.93kW for 300ms. So, do I need a 3kW resistor? I hope I can use just a 25W or 50W resistor for a brief period of time. Maybe reduce the time to 100ms.

    Because the 10k wouldn't provide adequate base drive to turn on the relay. If I increased it, I would then need a much larger capacitor.

    Also I accidentally ordered 100 of the wrong transistor, gotta use them up :).
     
  5. tom66

    Thread Starter Senior Member

    May 9, 2009
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    I've got some TIP122's, but I've got more 2N2222's than I need.
     
  6. wayneh

    Expert

    Sep 9, 2010
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    I suppose the variac must have some DC resistance that contributes to the 18Ω total?

    Not sure about power resistors and instantaneous currents. At some point they become a fuse instead of a resistor. Datasheets for the resistors must address this?
     
  7. pilko

    Senior Member

    Dec 8, 2008
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    Where is the 32A breaker. Is it attached to the variac? It sounds like the breaker is tripping too fast.
    Is it a thermal (slow) or a solenoid (fast acting) breaker?

    pilko
     
  8. tom66

    Thread Starter Senior Member

    May 9, 2009
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    I checked the resistance, it's 3.2 ohms.

    What I would like to know is how much current I should let flow through the variac to ensure it doesn't then cause inrush when the relay shorts out the resistor.

    I shall check the datasheet of the resistor before I order it.
     
  9. tom66

    Thread Starter Senior Member

    May 9, 2009
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    It is an MCB for the garage (household), which is shared with the utility area sockets. Not sure on the trip time, but it seems to go ping as soon as it is plugged in. I can use the variac if I throw the breaker enough times but the breaker makes a nice blue flash if I do this.
     
  10. pilko

    Senior Member

    Dec 8, 2008
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    I don't know if they are the same as N American breakers. The breakers in N A tend to trip easier and faster with age and use. Sometime I have to replace a breaker because of this.
    Try a new breaker or switch one from another slot.

    pilko
     
  11. tom66

    Thread Starter Senior Member

    May 9, 2009
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    If possible I would like to avoid this. The breaker is new, it was changed only a few months ago in a revamp of the lower electrical system.
     
  12. DigitalReaper

    Member

    Aug 7, 2010
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    You could probably use a lamp or heating element/heater in series with it instead of a power resistor. The DIY audio crowd does that all the time for smoke testing their new amplifiers :)

    It would need to be a fairly powerful lamp though, maybe one of those big halogen floodlights. How much current does it need with no load anyway?
     
  13. tom66

    Thread Starter Senior Member

    May 9, 2009
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    This is something I'd need to measure, but I'll offer one data point: It barely gets warm with no load. If it's got 3.3 ohms of resistance, then that resistance can't be dissipating much power, so the inductive current is probably low. I'll need to check it with a clamp meter, which is a toy on my wish list.

    I'd rather use a resistor, because the device should be self-contained, if possible.
     
    Last edited: Apr 1, 2012
  14. paulktreg

    Distinguished Member

    Jun 2, 2008
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    Try running it through a reel extension lead. Worked for me!
     
  15. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Do away with the circuit you want to build and buy the correct size PTC thermistor for the circuit you want to test. Here is a link to explain them - http://www.ge-mcs.com/download/appnotes/ptcnotes.pdf What your trying to do is one of the main applications for a PTC thermistor, no other components needed.
     
  16. tom66

    Thread Starter Senior Member

    May 9, 2009
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    I could use an NTC. I want resistance to fall as it heats up. But it's difficult to find an NTC that will sustain a 15A surge, and they are very expensive.

    I checked the datasheet of a few power resistor manufacturers, it looks like 25x overload can be tolerated for 1 second for most of these, possibly longer for shorter times, but that's not always specified.

    So I could probably use a 120 ohm resistor (dissipate 440W). But I think I could put two or three 120 ohm in parallel for extra pre charge current and peak dissipation would still be okay. I would use a 25W clad aluminium resistor. There should be no need for a heatsink, as the dissipation will be peak only.

    I still wonder how much current I need to ensure the transformer is adequately energised before connecting to mains.
     
    Last edited: Apr 2, 2012
  17. pilko

    Senior Member

    Dec 8, 2008
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