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R-core inrush protection
R-core inrush protection
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Old 1st December 2020, 04:27 AM   #1
sthcoaster is offline sthcoaster  United Kingdom
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R-core inrush protection
Default R-core inrush protection

I have a pair of Electronic R-core 300VA 230V primary 2x 21v secondary that I plan use in Peter Daniels LM3875 monoblocks.

I understand for larger toroids it is a good idea to use inrush/soft start protection. Does the same apply for R-core transformers?

I have Sjostrom SST03 soft start modules that I could use but if not necessary would hold them for another project.

Thanks
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Old 1st December 2020, 04:48 AM   #2
phase is online now phase  United States
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My understanding is that such a circuit would be needed when a large bank of capacitors were to put a strain on the rectifier array upon startup. A transformer won’t produce enough heat in the short burst at power on to be a problem in most cases.
I have large amps with no means of soft starting, and are working fine after some 40 years.

A speaker protection circuit with a relay to eliminate the possibility of a blast of DC at startup is a great thing.
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Old 1st December 2020, 07:29 PM   #3
Vovk Z is offline Vovk Z  Ukraine
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Quote:
Originally Posted by phase View Post
A transformer won’t produce enough heat in the short burst at power on to be a problem in most cases.
A transformer won't produce enough heat during energizing but will definitely produce enough inrush current.
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Old 1st December 2020, 07:34 PM   #4
analog_sa is offline analog_sa  Europe
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R-core inrush protection
Quote:
Originally Posted by sthcoaster View Post
Does the same apply for R-core transformers?

No doubt. But at 300VA i think you don't need any protection.
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Old 1st December 2020, 07:42 PM   #5
Vovk Z is offline Vovk Z  Ukraine
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Quote:
Originally Posted by sthcoaster View Post
I understand for larger toroids it is a good idea to use inrush/soft start protection. Does the same apply for R-core transformers?
Rigid inseparable R-core transformers have a similar core design as toroids but a bit different form, so they may possibly have a large inrush current too (but toroids are the worst). If R-core has a separable core (glued?) then of cause inrush current will be not so large.
I think it is always a good idea to use a soft-start (at least small value NTC) with any transformer. You may test the exact inrush current value with an oscilloscope (or any fast AC ammeter). Or, if you have a lot of fuses of different ratings then you may test it with a go through a few different fuses (from a small value to large or vice versa).
My simple estimated rule is this: if a fuse rated as the twice as rated load doesn't burn then it is ok. If you have to use a much higher fuse value then use some soft-start.
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Old 5th December 2020, 08:19 PM   #6
Theodosis O. is online now Theodosis O.  Greece
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Sthcoaster, Put a NTC resistor, e.g. CL 60, in series with transformer primary.
That’s all.
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