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Yet Another Soft Start Circuit
Yet Another Soft Start Circuit
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Old 7th September 2019, 06:06 AM   #51
tauro0221 is offline tauro0221  United States
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Hi,
Quote:
Any subsequent rectifier and large filter capacitor will also take a few mains cycles to charge up, as the transformer secondary is not an ideal low impedance path for high charging currents during the initial cycles
That it is the way I do when do the ramping. The triac it is fired 6 times in sequence at the same phase angle to allow the capacitors to slowly charge up. Then the next face angle it is increase and repeat the cycle until it reach the AC voltage peak. The voltage will be slowly raise up until it reach the voltage peak. At that moment when it reach the peak the triac it is turn on full time. Also at that moment will wait for delay of few milliseconds then the triac is bypass with a relay contact because it no longer need it. The reason of bypassing the traic it will keep firing at the zero crossing and may caused electrical noise in the amplifier. It did the job of bringing the voltage up and longer need it. Also by doing the ramping you will minimizes the inrush current that occur when power on a device with a component like a transformer and a bank of discharged capacitors. This method as been working in my amplifier flawless for more than 4 years using an Arduino uno with zero failures. I advice anyone to built one and report back the finding. It does not take too much effort to built one. You will not regret building it.

Last edited by tauro0221; 7th September 2019 at 06:09 AM.
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Old 9th September 2019, 05:14 AM   #52
johnhenryharris is offline johnhenryharris  United States
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to TRobbins: Quote(Why an amp would want what appears to be a humongous power transformer (although the amp audio output rating is not disclosed), or a humongous level of filter capacitance, with no valid reason, is another matter. I'd be happy if there was a valid reason, with some test distortion results to show up the need, but this thread is up to 50 posts with no sign of a technical base.)


The large toroid and capacitor bank has an audible effect, more effortless dynamics, I wish I had a distortion analyzer to show the reduction in higher order harmonics that more bias current brings, but there are articles on that out there. BTW my amps are 160 watts Class A into 8 ohms, has a 2.4KVA toroid, and .760f in a CLC config.


Also see:
Power Supplies | Pass DIY


But the thread is about soft starting the amp, the big power supply is just the reason why.
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Old 9th September 2019, 07:00 AM   #53
spookydd is offline spookydd  France
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After looking at the various methods and how far to go with the soft starting, I think I'll do it this way:

Since I'll be using a micro-controller to handle all the housekeeping and sequential power ups for everything, I might as well go all the way and do s few more simple steps.

- I'll insert a resistor after the bridges before the cap banks, to put a damper on the charging, which should make the transformer's inrush profile far more dominant at first

- so the initial inrush current will be much more transfo based and can be dealt with by powering it up with a short delay programmed in the micro-controller after it detects the zero crossing

- a resistor in the transfo's primary would greatly limit the inrush current, but the timing of the power up would also do so

- zero crossing is easy to detect and provide to a micro-controller, which info can then be used by the program to add a proper delay, and I'm thinking about also having at the same time a threshold level set by a divider and given to the micro-controller as well, such as is done by bryston on their amps. they don't use any limiting resistors, they just power up with a triac using zero crossing and a threshold

- after the initial power up with the delay, an other short delay added and short out the primary's limiting resistor, which will speed up the cap banks charging, still limited by the resistor there as well

- and then, after perhaps some 3 seconds or so, the cap banks limiting series resistor can also be shorted (by relay) to finish the charging

- a detection of the output rails levels on the cap banks can then be used to send a signal back to the micro-controller that it's all done, which allows moving on to powering up the next amp psu

The toroids won't all be the same size, but at the moment I think the largest will be something like 800VA or so, and probably 2 of the amps will have similar sized psu/cap banks.
This is a 4 way multi-amp, with a xover that would be powered up even before the amps, and all the amps would have to power up muted, and nothing would be unmuted until the whole chain would be fully powered up, with no failures. All fully automatic, no human intervention, triggered by the presence of a signal, and an absence of signal for several minutes would cause a complete shutdown of everything.
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Old 17th September 2019, 03:59 AM   #54
WhiteDragon is offline WhiteDragon  United States
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Simple is nice.

this thread is reminding me that my last concern before my next build is a soft start.

Though new to the art of soft start. A thought that passed my mind too. If a relay is going to be involved. then might as well have a circuit that would place very low current on the main on off switch. So its a interesting approach and i enjoyed the Original Posts design

Then again im lazy and just want simple. dirt dirt simple. But seems relay is more professional. just leaving a NTC thermistor in circuit would be horrible.
then im wondering if dealing with say 250 to 330 VA transformer. would it really be that incredibly horrible. I think 150 to 220 VA might be the limit on just leaving thermistors in circuit.

Anyhoo. If a triac was used with a Diac. Im curious if this automatically gives ideal or non ideal 0 crossing
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Old 18th September 2019, 09:57 PM   #55
john20851 is offline john20851  United Kingdom
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Hi,
Can someone please explain to me why with these soft start circuits they all use low ohm resistors? I modified all my amateur radio power supplies with the soft start resistor as 10k 50w, and all have not had the cracking thump at switch on (13.8v 25A). A relay bypasses this resistor when the voltage goes above 10v, and these have worked for twenty years with no problems. The question is, have I been doing this wrong?
regards john
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Old 18th September 2019, 11:28 PM   #56
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by john20851 View Post
Hi,
Can someone please explain to me why with these soft start circuits they all use low ohm resistors? I modified all my amateur radio power supplies with the soft start resistor as 10k 50w, and all have not had the cracking thump at switch on (13.8v 25A). A relay bypasses this resistor when the voltage goes above 10v, and these have worked for twenty years with no problems. The question is, have I been doing this wrong?
regards john
A 10k resistor will have zero effect on mains.
However on the secondary of a valve supply will have an effect.
I suspect your getting mixed up between mains side soft starts and secondary side soft starts.
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Old 19th September 2019, 08:12 PM   #57
john20851 is offline john20851  United Kingdom
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Hi, and thanks for answering. I actually have these 25amp amateur radio power supplies that are in use daily. To prevent problems with switch-on arcing and replacing mains switches, I put a 10k 50watt aluminium clad resistor in series with the mains transformer with a relay that bypasses this when the secondary volts goes over 12v. These have worked like a charm for best part of twenty years. I had thought of using a picaxe chip to switch relays, and as a voltage adc with timing, but so far I haven't needed to. I worked this out roughly to 24mA at 240v, and even at this current the transformer still gives a barely audible hum at startup. Without anything the power supply would almost move at switch on. They are Diamond gsv3000 linear power supplies, and are quite heavy. With this resistor the relay kicks in after about 2 seconds. I suppose for the sake of safety I should fit a thermal fuse in contact with these resistors, but they are never left unattended, and I did check on one recently and the resistor has not discoloured in any way. They are bolted onto the metal back plate of the power supply as a heatsink. Even if the relay failed to trip, the wattage amounts to just under 6 watts - well within limits.
The thing that interests me though, is why both systems appear to work ok, low value resistors, and the 10k I've been using.
Thanks again
john
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Old 19th September 2019, 08:22 PM   #58
nigelwright7557 is offline nigelwright7557  United Kingdom
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10k doesn't make any sense.
10R is a more usual value.
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Old 20th September 2019, 07:40 PM   #59
johnhenryharris is offline johnhenryharris  United States
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Hi John20851,
The reason for using low ohm resistors is simply ohm's law. You want to limit the current to some maximum value. For 240v mains a 20 ohm resistance will limit the current in-rush to 12 amps, this is the value I choose, you can use different if you like.
The initial current in-rush will drop off exponentially as the capacitor bank is charged so you are able to bypass the resistance after a few seconds. A large resistance would limit the current but too much, charging the capacitor bank too slowly putting a strain on the toroid.
Thanks,
John

Last edited by johnhenryharris; 20th September 2019 at 07:44 PM.
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Old 20th September 2019, 10:19 PM   #60
john20851 is offline john20851  United Kingdom
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Hi,
I agree there is quite a difference between 10 and 10r, thing is, it works. Four of them have been in almost daily use since around 1986 when I got my amateur radio licence. I would call 12amps at 240v quite a nasty surge, and not going to do switch contacts any good over a short time, the only other thing , these power supplies don't use a toroidal transformer. I did some experiments, seeing as I have quite a few 50w clad resistors of 1k to 15k in value. They all seemed to work, with obviously the 1k switching the relay the fastest. But it also made the transformer thump more when switched on. I measured the ac current with a 10k in series with the psu mains, and it was 25mA for about 2.5S then the bypass rely kicks in as the output voltage goes over 12v. I haven't had chance to test these on some 650watt audio amps I have with toroidal transformers, funny thing is they don't "crack" when switched on. Maybe they already have some system. I still don't like the idea of 12A surge, that's over 2.8kW albeit a short time.
regards
john
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