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Soft Start Circuit Help
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Old 20th June 2021, 04:50 AM   #1
kptseng is offline kptseng  Singapore
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Default Soft Start Circuit Help

Hello Everyone,

I'm planning to put in a soft start circuit for the class A/B amp I am building now, which has 2 x 350va toroidal transformers and total 80,000uF of smoothing capacitance (20,000uF for each power rail).

I want to go on the safe side and the one by Ron Elliott seems to be a decent and simple enough one to build. (see figure 5 on Soft-Start Circuits)

Ron's notes suggest that the soft start time is 100ms, but I am thinking of increasing that to 500ms to 1sec, but I am not sure which component / value to change to enable a longer soft start time.

Can someone help me with some advice?

Many thanks and have a safe weekend.

Last edited by kptseng; 20th June 2021 at 04:55 AM.
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Old 20th June 2021, 06:33 AM   #2
bansuri is offline bansuri  Italy
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You have to increase the power rating of the resistors by factor 3-5 for such a long time,

There is also no advantage, using more than 200ms, the caps are already sufficiently charged.
Actually all calculations are on that page
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Old 20th June 2021, 12:39 PM   #3
kptseng is offline kptseng  Singapore
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Hello Bansuri,
I've done some reading and yes you are right.
I'll still to what's stated in the document. Many thanks.
Do you know how should I connect the ground of the soft start circuit?
Those encapsulated auxiliary transformer has just L and N, with no ground nor 0v. Should the ground of the circuit be connected to the chassis ground?
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Old 20th June 2021, 01:03 PM   #4
Mooly is offline Mooly  United Kingdom
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Soft Start Circuit Help
It should connect to mains ground. If your chassis is mains grounded then it can connect there provided the connection is suitably rated to withstand a fault current and blow the mains fuse.

It is all safety related in the event of fault scenarios.
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Old 20th June 2021, 01:50 PM   #5
kptseng is offline kptseng  Singapore
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Thanks Mooly,
The sift start circuit will be sitting closed to the IEC socket at the back of the chassis, so I guess the connecting to the ground of the socket would be the best.
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Old 20th June 2021, 07:01 PM   #6
Mooly is offline Mooly  United Kingdom
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Soft Start Circuit Help
Yes, that would be OK.
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Old 23rd June 2021, 03:33 AM   #7
Andersonix is offline Andersonix  Sweden
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Soft Start Circuit Help
80,000 uF split in four rails is not enough surge to bother with any soft start. Move on!
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Old 23rd June 2021, 05:55 AM   #8
dotneck335 is offline dotneck335  United States
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That's ROD Elliott, not Ron!! Don't you be dissin' one of the true diy gurus/heroes!!
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Old 23rd June 2021, 06:23 AM   #9
bansuri is offline bansuri  Italy
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The same circuit can be used for a speaker protection relay at the output, then your 1+sec makes more sense.
Just parallel from R1 to the relay and use bigger C and R as described.


You do not need to ground the circuit, the transformer and the relay coil are isolated, so the thing can float and there is less danger to make a short by touching an element with a probe ground.

Last edited by bansuri; 23rd June 2021 at 06:31 AM.
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Old 25th June 2021, 05:35 AM   #10
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by kptseng View Post
Ron's notes suggest that the soft start time is 100ms, but I am thinking of increasing that to 500ms to 1sec, but I am not sure which component / value to change to enable a longer soft start time.

Can someone help me with some advice?
The soft start delay depends on which soft start device (resistor or NTC/inrush limiter) you use. I have a pretty comprehensive analysis available here: The Ultimate Guide to Soft Start Design – Neurochrome

The best way to optimize the soft start is by measuring the current into the transformer primary during startup. You can do that with a simple current probe that you can build for less than $10: The $10 Current Probe - How to Measure Current with an Oscilloscope – Neurochrome

Tom
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