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Old 23rd October 2013, 07:07 PM   #1
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SjoerdSmits's Avatar
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Default Inrush Current on big capacitor bank


I have a problem:

I have a 500VA Toroid operating with 230V50Hz. Secondary is 2 x 25V/10A. The capacitor bank is pretty large, 176000uF.

Now I want to limit the inrush current but how? I already have a nice limiter installed designed by P/A Sjostrom. But I think the problem is the slow charging of the capacitor bank.

I guess this can be solved using thermistors in the secundary's. But whitch ones??? Can anyone help me on that?

Kind regards,
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Old 23rd October 2013, 07:22 PM   #2
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Place the PTC thermistor in the AC supply, arrange a 2Second timer to switch a relay contact across the thermistor. That will allow the thermistor to cool and ensure a soft start at next switch on.
Thermistors on the secondaries has too much load current to handle reliably.
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Old 23rd October 2013, 07:31 PM   #3
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Hi JonSnell,

I was thinking these ones in the secondary: THERMISTOR, NTC, 4R, 9.5A

But you say that's not reliable?

So what types do I need on the primary? I will need about 50 Ohms I guess?

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Old 23rd October 2013, 08:45 PM   #4
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Hi Sjoerd,

I've used 120 ohm thermistors with great succes in on the primary side. They're bypassed after 8 seconds to reduce heat buildup inside the enclosure and to let it do it's job again after a quick power cycle.

SL22 12103 Ametherm | Mouser

But you mention you already have a limiter installed? What's wrong with that one?

Last edited by funk1980; 23rd October 2013 at 08:51 PM.
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Old 5th November 2013, 04:40 PM   #5
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Hi Funk 1980,

My inrush current limiter was fine! But I changed it anyway for a smaller model(Hypex) And works perfect now.

The problem was not the limiter... It was wiring/combining two PSU's. Changed that and problem solved :-)

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