F5 Turbo Builders Thread

[AndrewT]

Originally Posted by AudioSan

well. that only helps the main fuse and the transformer. but it does nothing with the charging current to the caps. and that is hard for the transformer.
AndrewT may have a recipe for this one.

I have posted many times that soft start and slow charge are very different strategies.
Soft start is to allow the transformer to start up with blowing the close rated fuse in the primary circuit. This is a short term requirement of about 100 to 300ms.
Slow charge is the current limiting AFTER the transformer has started.
Power Thermistors in the secondary circuit do this slow charge duty very well. This is a much longer term requirement usually longer than 5s.

Both types of current limiter, whetcher using resistor or thermistors increase the supply source impedance. Both types of limiter need to be shorted out to bring back the advantages of low source impedance.

And now the contentious part:
Don't try to achieve the dual duty of soft start and slow charge by inserting a long time duration current limiter in the primary circuit.

[AndrewT]

I have posted many times that soft start and slow charge are very different strategies.
Soft start is to allow the transformer to start up without blowing the close rated fuse in the primary circuit. This is a short term requirement of about 100 to 300ms.

Slow charge is the current limiting in the smoothing capacitor circuit AFTER the transformer has started.
Power Thermistors in the secondary circuit do this slow charge duty very well. This is a much longer term requirement, usually longer than 5s.

Both types of current limiter, whether using resistor or thermistors, increase the supply source impedance. Both types of limiter need to be shorted out to bring back the advantages of low source impedance.

And now the contentious part:
Don't try to achieve the dual duty of soft start and slow charge by inserting a long time duration current limiter in the primary circuit.

They won't let me edit out the errors in my earlier post so I have rewritten it here.

[a.wayne]

Schematic example Andrew ...?

[AndrewT]

schematic for a series resistor and a relay bypass?

[nrg2009]

I appreciate the explanation. The point you made was lost on me also, so i appreciate your insights. Knowing this now, i am looking for slow charge.

[dazed2]

Andrew,
Could you please explain why you shouldn't have soft start AND slow charge in the same module prior to the transformer? If I don't have to, I'm not keen on adding a circuit after the transformer. and before the cap bank.

[AndrewT]

Because a primary side Rs does not give an effective current limiter in the secondary circuit.
Because a primary side Rs that is not bypassed quickly generates an enormous amount of heat.

[Zen Mod]

softest start solution is "common" soft start circ on primary side, filled with NTC chain , instead of fixed resistors

[oiphy]

My english is not very tec.... please someone draw for me. maybe Zen????
this is interesting for me since I'm going for 4x47000 for 1 channel. What trafos size VA and Output Volt?
what rail volt is max when cascoded?
will be water cooled, so heat no problem, thinking no insulator between output and sink....

[BigE]

Gentlemen,

Is there a formula for the resistance required to slow start a set of capacitors that relates resistance to capacitance?

Thank you in advance!

Found it Tau = RxC. R in Ohms, C in Farads 5xTau = steady state.