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Old 25th November 2004, 01:53 PM   #1
k1jroth is offline k1jroth  Finland
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Default load current

I`m trying to design soft start circuit for 2x40V 1000VA toroid and
filter capasitors (63V 144000uF).

I think that resistors in primary coil with relay should do it.
Secondary coil is the problem because of the capasitor load current.

I`m thinking resistor in series with capasitors and when they are loaded Zenerdiode will turn the relay on. (relay shortcuts resistor)
Same relay to capasitors and primary coil resistors.

How can i calculate load current and load time with specific resistor?

Please help beginner.
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Old 25th November 2004, 02:04 PM   #2
boholm is offline boholm  Denmark
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(40 volts *1.4 ) - 0.6 or 1.2 volts = about 55 volts.

At startup the capacitor is uncharged and therefore acts as a shortcircuit: 55 volts / 0 ohm = infinite ampere.

Due to internal impedance in the transformer and the diode bridge, the max. current will be more modest, but it will still be large.

Why not build this:

http://sound.westhost.com/project15.htm
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Old 25th November 2004, 04:07 PM   #3
Electrons are yellow and more is better!
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Absolutely max inrush current is huge. Measure the DC resistance of the primary winding. Take 230 x 1.4 = 322 volt and divide this with 3-5 ohms = 100 A!

Best way to start up this huge power supply of yours is to have 100 ohms 7-10 watts wirewound i series with your primary winding and then let a relay bypasss it after 1-3 seconds. If you have this device on the primary side you don't have to deal with so high currents as if you had it on the secondary side. The rectifier bridge will probably handle this well with any extra parts.

With a current probe (using hall elements) connected to an oscilloscope I measured 77 A peak for my 600 VA/ 4 x 22000 uF PS (+- 60 volts) This extremely high peak lasts only one or a couple of mains periods. Since you have a huge capacitor battery maybe the current peak will be streched a bit in time.
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Old 25th November 2004, 04:36 PM   #4
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Default Re: load current

Quote:
Originally posted by k1jroth
I`m trying to design soft start circuit for 2x40V 1000VA toroid and
filter capasitors (63V 144000uF).
Wouldn't have to if you used smaller caps.

Tim
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Old 25th November 2004, 04:51 PM   #5
Electrons are yellow and more is better!
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Default Re: Re: load current

Quote:
Originally posted by Sch3mat1c


Wouldn't have to if you used smaller caps.

Tim
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Old 25th November 2004, 05:00 PM   #6
djQUAN is offline djQUAN  Philippines
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I have only a 500VA toroid with eight 10000uF caps. dims the house lights. I definitely need a soft start installed soon.
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