R-core inrush protection

[sthcoaster]

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

 

[phase]

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.

 

[Vovk Z]

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.

 

[analog_sa]

Does the same apply for R-core transformers?

No doubt. But at 300VA i think you don't need any protection.

 

[Vovk Z]

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.

 

[Theodosis O.]

Sthcoaster, Put a NTC resistor, e.g. CL 60, in series with transformer primary.
That’s all.