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Old 14th January 2011, 03:40 PM   #571
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I'm busy designing a power supply for my amp. Now I'm having 2 250VA (25V 10A) toroid transformers which needs to be powered. Because it's more then 300VA there's the need of some inrush current limiting.

The primary side of the transformers are parallel and the secondary side is put in series for +- 35V.

I found this CL-60 thermistor as the easiest way for an inrush current limiter. It will be bypassed after a second or what with a relay. The only problem is that I can't find it in a shop with normal shipping cost (not Ä20 for 2 pieces) or a shop where I don't have to order for a minimum of Ä50.

Now I found these alternatives:
- SG26
- 10D15

Is it safe to use one of these instead of the CL-60. It will be used with 2 250VA transformers parallel. If not, can you tell me what else to use? I'd like to power the transformers with one NTC, because else I need another relay to power it.
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Old 14th January 2011, 04:33 PM   #572
AndrewT is offline AndrewT  Scotland
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Hi,
have you read elsewhere that transformers <=300VA do not need inrush current limiting?
Do you plan to use a bypass across the resistive element after it's done it's inrush attenuation job?
Have you considered using a resistor (or stack of series resistors) to replace the CL60?
Have you considered replacing the fuse with a bigger one until it allows successful start up and does not suffer nuisance blowing? If the distribution board MCB suffers next, maybe you need to consider increasing it's trigger value as well.

Now I'll give you my advice.
Close rate the fuse feeding each transformer, i.e. 250VA/115Vac=T2A fuses
Insert a resistor bank for each transformer than allows the transformer to start up without blowing that T2A fuse after repeated start ups over an operational period exceeding just a few months.
Add a bypass dpst relay (one pole to bypass each resistor bank) with a time delay of ~ 100ms to 300ms after switch on.

I'll estimate that a bank of 5off 10r 5W wirewound resistors (equivalent to 50r 25W) will start up a 250VA transformer on a 115Vac supply.
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Old 23rd January 2011, 10:36 PM   #573
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Default Trafo. for LM4780.

Hi Guys,
I need close to max power for my LM4780, Looking at the Anteks, You think i will be cutting it close with the AS-4428,
rated at 400VA,28 vac, ie 39.2vdc ?
The next lower is 25vac.
The 4780's will be driving only woofers.

Last edited by keyboard123; 23rd January 2011 at 10:44 PM.
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Old 23rd January 2011, 11:49 PM   #574
AndrewT is offline AndrewT  Scotland
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Hi,
what's the rated input voltage for that rated output voltage?
What is your maximum supply voltage?
What is the regulation of the proposed transformer.

I think you will find there are some operating conditions where the PSU will supply more than+-42Vdc.
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Old 24th January 2011, 12:48 AM   #575
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Hi AT,
Yeah, I assumed the input voltage being constant. But in reality swings higher.110- 140vac. that will throw it close to its limits.
Regulation worst case would be less than 1%.
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Old 24th January 2011, 11:30 AM   #576
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by keyboard123 View Post
Regulation worst case would be less than 1%.
transformer regulation is much worse than that.
1kVA ~3 to 4%.
300VA ~5 to 6%
10VA ~15% to 30%
and EI are usually worse than toroids.
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Old 26th January 2011, 09:26 PM   #577
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Hi, guys I'm currently building my first power amp and have a few newbie questions. It's going to be two p2p mono block lm3886s encased in a wooden blocks.
I have just been following been following the schematics in brianGT's chipamp manual. I done a bit of looking round and seen a few variations on carlosFM's PS and was wondering which is the currently recommended one, the one in the chipamp manual or another one? If it makes any difference I have 21v 80VA trafos and will be powering a 4ohm load to no more than 20w.

Any help will be greatly appreciated.
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Old 27th January 2011, 12:40 AM   #578
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80 va is a bit low, will work, thats if you have one lying around. power up your amp under load and check the voltage at the trafo secondary to see if there is any substantial drop.
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Old 27th January 2011, 09:52 AM   #579
AndrewT is offline AndrewT  Scotland
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Hi,
tell us more about the 80VA transformer.
Rated input voltage?
Rated output voltage/s?
Open circuit output voltage when fed with rated input voltage?
Number of separate windings?
Number of tappings on each winding?
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Old 27th January 2011, 10:11 PM   #580
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Hi, they've got dual 120v primaries so I wire them in series for the 240v I have. They've got a 52v center-tap which is what it measures open circuit. Shall I test it loaded? If so what load?

The center tap is easily accessible so I was planning to convert it to dual secondaries although with my searching I never found any hard evidence that dual rectifiers is better, what do you think?

Thanks for the help I appreciate it.
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