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Inrush current limiting necessary for EI transformers
Inrush current limiting necessary for EI transformers
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Old 26th October 2006, 01:49 AM   #1
Bama Slamma is offline Bama Slamma  United States
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Default Inrush current limiting necessary for EI transformers

I've seen a lot of sites that recommend some type of soft start circuit for toroid transformers. Are these necessary for mid to high power EI laminate transformers?
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Old 26th October 2006, 01:50 AM   #2
anatech is offline anatech  Canada
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Inrush current limiting necessary for EI transformers
Hi Bama Slamma,
Yes. Highly recommended.

-Chris
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Old 5th November 2006, 02:59 AM   #3
Bama Slamma is offline Bama Slamma  United States
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The transformer is a 320VA, 120 VAC primary (standard US power) 40-0-40 VAC secondary. It will be used to power two aussie amps NX150 power amp modules. It will give me the +-56V rails recommended for these amps.
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Old 5th November 2006, 03:02 AM   #4
anatech is offline anatech  Canada
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Inrush current limiting necessary for EI transformers
Hi Bama Slamma,
Let's just say that your power switch will last longer with an inrush current limiter. It depends somewhat on how much capacitance you have in there too.

You don't really "need" a limiter, but they are nice to have. You can always just use a large, high current power switch.

-Chris
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Old 5th November 2006, 11:51 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
an advantage of soft start is the ability to use a close rated mains fuse giving better protection in event of a fault.

As an example your 320VA on 120Vac supply may run reliably with a T2.5A or T3A fuse when soft started.

Without soft start it may require a T6.1A or T8 just to get it started, or maybe worse.

BTW. are you planning to run 150W +150W into 8ohm speakers from a 320VA transformer?

Seems a bit low, particularly if the speakers are really 6ohm or 4 to 8ohm as has become popular of late.

Don't even think about using 4ohm speakers.

You can recover some of the amplifier ability by using larger than normal smoothing caps (or more in parallel) but this may aggravate the on-line starting.
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Old 5th November 2006, 04:13 PM   #6
anatech is offline anatech  Canada
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Inrush current limiting necessary for EI transformers
Hi Andrew,
Quote:
You can recover some of the amplifier ability by using larger than normal smoothing caps (or more in parallel) but this may aggravate the on-line starting.
Yes, absolutely!

Your other points are well made and have merit.

-Chris
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Old 5th November 2006, 04:46 PM   #7
AndrewT is offline AndrewT  Scotland
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It's nice when great minds think alike (well I'm allowed to dream).
Or is that too ridiculous to imagine?
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Old 5th November 2006, 04:56 PM   #8
anatech is offline anatech  Canada
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Inrush current limiting necessary for EI transformers
Hi Andrew,
You've put more thought into what Bama Slamma is doing than I have. I must say that is is wonderful when great minds think alike, but you though ahead too!

Excellent advice Andrew.

-Chris
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Old 6th November 2006, 03:56 PM   #9
Bama Slamma is offline Bama Slamma  United States
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I checked with the designer and he said a 320 VA transformer will run two amps for stereo. All my speakers are 8 ohm. Since the amps are class AB, don't they only draw max current at max output? What I plan to use my creation for is home listening at low to moderate volumes and maybe small PA system use. With +-56 volt rails, the amp is rated 230 watts rms at 4 ohms. A 320 VA transformer does seem undersized for running at max power int 4 ohms.
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Old 6th November 2006, 03:59 PM   #10
Bama Slamma is offline Bama Slamma  United States
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Recommended capacitance is 20,000uF per rail for stereo.
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