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Old 12th May 2008, 08:29 AM   #1
95Honda is offline 95Honda  Germany
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I am building a large voltage step-down transformer set up for my subwoofer amplifiers. I moved to Germany where the voltage is 230 and my pair of A250s came from the states. The amps are about 1kw or so each as I am currently running them.

Before I left the US I bought a bunch of transformers, one of them is a 3Kw dual primary (115/230V) and dual secondary (115/230V) potted transformer I will use for the sub amps.

My concern is inrush (obviously) across the 230V mains when I turn it on. I have a timer and relay for a step start where the first 1 second or so after turn on will have the primary winding in series with a power resistor. I am going to use one of the aluminum housed Dale power resistors, probably a 50 watt or 75. I guess I am trying to figure out a good value. I was thinking around 5 ohms would be good enough to limit the inrush current and allow the tranny to saturate within the 1 second or so delay. After about 1 second, the resistor is bypassed and the primary is connected straight to the line.

I am only going to use a step-start with this, no Thermistors or solid state devices.

Any thoughts on the resistor value?

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Old 12th May 2008, 09:15 AM   #2
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Du plan to use 3kVA on 230 VAC in a regular outlet?

Is it a toroid?

100 ohms, 20 Watts, delay 0,5-1 second if this for magnetizing the toroid only. Sufficient time is only maybe 10 cycles.
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Old 12th May 2008, 11:09 AM   #3
eduard is offline eduard  Netherlands
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One time i did make a 15 ohm 25 watt aluminium resistor explode by using it as a mean to reduce inrush current in an Hiraga 30 watt power amp. It didn't have such a big transformer 500va EI or 800 va toriod but it did have lots of capacity. 25 years ago it did happen , if i remember correctly i did change it into a few hundred ohms. This way it could be connected to the power all the time and just use a few watts to save energy greetings, eduard
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Old 12th May 2008, 11:50 AM   #4
95Honda is offline 95Honda  Germany
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Yes, directly on 230V line, regular socket (16 amp).

And yes, to magnatize transformer only (it is potted in epoxy, I have no idea the type, but I highly doubt torroid), there will be no load initially on the secondary, that will come later as the amplifiers are turned on.

100 ohms will charge in a second or so? OK, I will try that. I kind of figured it would be more around 5-10 ohms.

I don't have the equipment to see what the primary impedance measures, I guess if I did I could calculate this, or at least calculate enough resistance to keep the current flow down around 10 amps or so initially...

Thanks for the replies!
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Old 14th May 2008, 11:57 AM   #5
AndrewT is online now AndrewT  Scotland
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to magnetise the toroid core takes just a few cycles.
Try a range of ceramic cased wirewounds. (I use 5off 10r 10W in series for between 300VA and 1000VA and they never get warm).
Use somewhere between 20r and 50r at start up and relay bypass after between 100mS and 300mS.
Aim to use the lowest value of T rated fuse you can get away with without incurring nuisance blowing.
You may find that T8A or T10A works just fine. Rather than the T40A that would normally be used for a transformer without soft start up and the rule of 3times VA/Vac.

If you want/need to slow down the charging of the smoothing capacitance then use a separate NTC or NTC+resistor in the secondary circuit, again with relay bypass but this time the delay should be between 1S and 5S.
regards Andrew T.
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Old 14th May 2008, 12:37 PM   #6
95Honda is offline 95Honda  Germany
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Great, thanks!

Nice thing is, I don't have to worry about any capacitance on the other side, I am using this as an isolation transformer, and the load won't be connected to the secondary for at least a few seconds after initial turn on. So no caps to charge. All I want to do is make sure I don't trip my house mains breakers (16A) every time I turn this thing on. I will have it off all the time I am not using the stereo.
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