AndrewT said:
Current issue of Elektor magazine had same theory(maybe it was submitted by a reader, cant remember)
http://www.jmwa.demon.co.uk/EMCLF0010.htm
http://www.isce.org.uk/engnotes/ENote5X.pdf
Hi everybody,
i have 2 big,big,big transfo 70VCT, 2500VA in 120v i need too a soft start because if i connect my transfo without soft start the circuit breaker open. what you to advise me? thank you!!!
i have 2 big,big,big transfo 70VCT, 2500VA in 120v i need too a soft start because if i connect my transfo without soft start the circuit breaker open. what you to advise me? thank you!!!
Hi Maxpou,
have you tried starting it with a 150W light bulb in series with it yet?
If it starts this way and then you short out the light bulb it indicates that a soft start resistor and/or thermistor will do the job.
For 120Vac I would try 50W of 22R as the dropper resistor and relay it out after about 1 second.
have you tried starting it with a 150W light bulb in series with it yet?
If it starts this way and then you short out the light bulb it indicates that a soft start resistor and/or thermistor will do the job.
For 120Vac I would try 50W of 22R as the dropper resistor and relay it out after about 1 second.
AndrewT said:
Andrew, Thanks.
My understanding from what you post is that the soft-start citcuit (adding more coomplexity) is not really needed in my case (480VA on 220V) ? Please confirm as I always scare of dealing or problems with 220V. Thanks!
Hi,
I cannot say exactly for your case but let me expand by relating one of my experiences.
I have a pair of 625VA toroids from the same manufacturer. The first transformer starts on a T6.3A fuse that rarely blows (once in about ten years). The second bought a few years later will never start on a T6.3A fuse and usually starts on a T8A. It was as a result of this that I investigated a relay operated dropper resistor (stolen from a redundant slot car controller) to allow a smaller fuse to be used. (at that time I never knew that soft start even existed). Both stereo amps now run reliably on T3.1A fuses and both have +-20mF/channel.
Have you consulted the current time tables for high value mains fuses? The breaking current for a T6.3A and T8A are frightening.
That's part of the reason for the tens of kAmp rating of switch gear in the distribution board.
I cannot say exactly for your case but let me expand by relating one of my experiences.
I have a pair of 625VA toroids from the same manufacturer. The first transformer starts on a T6.3A fuse that rarely blows (once in about ten years). The second bought a few years later will never start on a T6.3A fuse and usually starts on a T8A. It was as a result of this that I investigated a relay operated dropper resistor (stolen from a redundant slot car controller) to allow a smaller fuse to be used. (at that time I never knew that soft start even existed). Both stereo amps now run reliably on T3.1A fuses and both have +-20mF/channel.
Have you consulted the current time tables for high value mains fuses? The breaking current for a T6.3A and T8A are frightening.
That's part of the reason for the tens of kAmp rating of switch gear in the distribution board.
The simplest option is a current-limiter NTC. When cold, its resistance will limit the inrush current; in operation, it heats up from I^2*R and its resistance drops. They're spec'd by nominal operational current.
I'd recommend one at 300VA even if your transformer won't trip the breaker regularly.
BTW, you will find one of these in the input of most current PC SMPUs. Lots of other nice components can be salvaged from dead PC supplies, too, and plenty of these are around for free these days, often with blown output caps as the only problem ;-)
I'd recommend one at 300VA even if your transformer won't trip the breaker regularly.
BTW, you will find one of these in the input of most current PC SMPUs. Lots of other nice components can be salvaged from dead PC supplies, too, and plenty of these are around for free these days, often with blown output caps as the only problem ;-)
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