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Yet Another Soft Start Circuit
Yet Another Soft Start Circuit
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Old 24th July 2020, 02:15 PM   #151
rhythmsandy is offline rhythmsandy
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As what OP has recommended the actual inrush current shouldnt go beyond 12Amps for 230V AC. It should be a hard limit. How to put a hard limit is the actual question. Run it for few secs and bypass with relay would be wonderful. This way no matter what kind of load it just hard limits the current to 12A. I thought of using Mosfets but how to limit current since its AC so no idea. Secondly using mosfets in mains is it safe?
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Old 24th July 2020, 04:33 PM   #152
Mark Johnson is offline Mark Johnson  United States
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Yet Another Soft Start Circuit
The soft start board by jhofland / XRK uses MOSFETs to switch the mains. The ones they have chosen are kinda spendy but designing your own PCB and sending it off to fab isn't exactly cheap either.
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Old 24th July 2020, 06:33 PM   #153
johnhenryharris is offline johnhenryharris  United States
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<Which values of NTCs can be used in that case? >
If you look at the BOM you will see large NTCs that can hold the in-rush to 12 amps which is safe for home wiring.


The NTC is rated for continuous use. If you look at the Power supply for the F5T it has a NTC in-line ALL the time. I like to bypass the NTC after the initial in-rush as I do not like adding heat to the inside of the amp, just my preference.


For large power amps I like relays with a contact rating of 30 amps, just for a little added safety and durability.


But each to his/her own, that is why we has a forum for discussion of ideas. Even though I have my own biases I like seeing what others think and have done.
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Old 24th July 2020, 06:37 PM   #154
ximikas is offline ximikas
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Hard current limit 12 ? I think only three ways are possible there:

1. Measure current with current transformer or small resistor and use some kind of feedback to to control SCR gate angle ,or two mosfets in series ,whatever you prefer and easier to control .

2. Use some NTC in series ,if you want 12Amp max , 230V / 12A = about 20 ohms ,in hot state , in cold their will have even higher resistance and less inrush current .
3. Use fixed resistor about 20 ohms high power , like 25 - 50 W ,it will not heat up so quickly in second or two ,but can withstand overload .



Soft start should be bypassed with relay or SSR, SCR, whatever prefer .
Can be fixed time delay or amplifiers capacitors voltage detector , simple divider and TL431 driving optocoupler from some aux voltage ,or from amplifiers capacitors .In case of fault voltage will be too low and 431 will not turn on optocoupler and soft start will not be bypassed .To prevent resistor overload soft start must have limited operating time , lets say 3 secs .But it may sound not simple ...
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Old 24th July 2020, 07:06 PM   #155
johnhenryharris is offline johnhenryharris  United States
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NTC resistance is given in cold state, 10 ohms each, 2 in series for 20 ohms for 230V and two 5 ohms for 120v. (the spec also gives hot resistances in the sub ohm)


some of what you mention is cover in the first post of this thread.
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Old 24th July 2020, 07:30 PM   #156
ximikas is offline ximikas
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So for (230V) 995-SL32-10015-B resistance changes from 10 ohms when cold to 0,05 ohms when hot ,at 15Amps . I see there a drawback - thermal inertia , if mains voltage gets lost for a second or two , capacitors may discharge , but limiter 32mm (big one) may not cool down and not limit inrush when mains voltage appears again .
Simple solution ,but need to carefully select limiter model according to output capacitors and their charge time ,not to get NTC resistance too low too early ,when capacitors not fully charged yet .I would prefer fixed resistor ,selected for few amperes .In my previous amplifier was 180w rated transformer ,loaded with 10000uf per rail , and when turning on power switch , even lights dimmed for short moment ,no soft start or current limiting was there .
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Old 25th July 2020, 05:33 AM   #157
johnhenryharris is offline johnhenryharris  United States
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what you are calling thermal inertia is not an issue. Start up the amp, circuit bypasses the NTC in 3 to 10 seconds, time is selectable, then measure temp of NTC and resistance, didn't get hot enough to change the resistance much. Do repeated power ups and down and check the NTCs, still not an issue.


In the event of a bypass relay failure the NTC will go to low enough resistance to not affect the current flow, a fixed resistor wouldn't do as well in this case.


An amp with a 180VA rated transformer would not need a soft start circuit, IMO.
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Old 25th July 2020, 09:08 AM   #158
ximikas is offline ximikas
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About thermal inertia - i mean simple solution,used widely, with only NTC resistor , when no bypassing relay present .
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