soft start calculation

[rtate]

heaveno, I would like to build the softstart circuit to use on my Leach superamps.
Each amp has 80,000 uf capacitance and 10 amp toroid trannies.
Mains is 110 VAC.[/u]

What value should I use for R1?
Should I just be concernd about the wattage, or is the resistance also an issue?
Thanks

[peranders]

How many VA is the toroid? The size is important, not the load. Chosen resistor can handle 600 VA without any problem.

[rtate]

Thanks you for the quick reply  PA!

I belive the transformers are 1 KVA.
Do I need to increase the wattage as well as the resistance??

[peranders]

It might very well work. If the temperature rise is below 40 degrees, (60-70 deg resistor temperature) I wouldn't worry. Suggested value is maybe a bit lower, 47 ohms, possibly 10 W => 3 A peak

[rtate]

I read this on another site with a similar design(esp project 39) regarding the use of higher value resistors.Should I be concerned using the" SST-01" and the "Leach superamp ". :-D

C/P:
In reality, the worst case peak current will never be reached, since there is the transformer winding resistance and mains impedance to be taken into account. On this basis, a reasonable compromise limiting resistor (and the values that I use) will be in the order of 50 Ohms for 240V (3 x 150 ohm/ 5W), or 11 Ohms (3 x 33 ohm/ 5W) for 120V operation. Resistors are wired in parallel. You may decide to use these values rather than calculate the value from the equations above, and it will be found that this will work very well in nearly all cases, and will still allow the fuse to blow in case of a fault.

This is in contrast to the use of higher values, where the fuse will (in all probability) not blow until the relay closes. Although the time period is short, the resistors will get very hot, very quickly.

Another good reason to use a lower value is that some amplifiers have a turn-on behaviour that may cause a relatively heavy current to be drawn for a brief period. These amplifiers may not reach a stable operating point with a high value resistance in series, and may therefore cause a heavy speaker current to flow until full voltage is applied. This is a potentially disastrous situation, and must be avoided at all costs. If your amplifier exhibits this behaviour, then the lower value limiting resistors must be used

[peranders]

My purpose of the softstart is to avoid blowing the fuses in the wall so therefore I need a very short delay, 300 ms is enough and under this time I want the transformer to get magnetized.

[rtate]

I do plan on using the C4 and C5 @ 1 uf 63 vdc combination to increase the time delay a little.

Do you think R1 of  50 ohms 20 watt(using 2 -100 or 2-47 ohm resistors in parallel) would give me a time delay of aprox.600-800 ms??

[peranders]

I'll suggest that you take two 100 ohms, 5 W, in parallel.

[rtate]

I'LL try that!!
Thanks for your patience....