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Old 19th November 2013, 09:46 PM   #111
AJT is offline AJT  Philippines
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i have done a lot of power amps without this soft start circuit and never regretted not having one....
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Old 19th November 2013, 11:01 PM   #112
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I thought it was preferable to power a soft start using a separate low VA transformer... Although there are plenty of designs where no transformer is used, these are "less safe" and seem to be ill-advised for DIY use where someone without the proper knowledge of safety ratings for mains caps (for instance) or other components can put himself in danger of electrocution.

I need to implement a soft start for an upcoming project having a delay of 5-10 seconds. I am planning on using a separate 10VA transformer (cost is only $10 for a nice one), an inexpensive bridge rectifier and a single smoothing cap. This will be regulated to 12V by a cheap LM7812, and then a 555 timer is used to switch the resistor/thermistor bypass relay. This is very easy to design, safe, and adjusting the delay time anywhere between tens of milliseconds and tens of seconds is very simple. It's also simple to adapt the circuit to different relay coil voltages by changing the regulator, and the pull-in characteristics of the relay are unimportant.

Perhaps the added expense and complexity of a transformer and regulator are not desirable to this audience?
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Old 20th November 2013, 12:56 AM   #113
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Quote:
Originally Posted by CharlieLaub View Post
I thought it was preferable to power a soft start using a separate low VA transformer...
yes, in my case i use a small "housekeeper" traffo...

Quote:
Originally Posted by CharlieLaub View Post
Perhaps the added expense and complexity of a transformer and regulator are not desirable to this audience?
the soft start circuit was designed with simplicity in mind, regulators? not absolutely required....
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Old 24th January 2014, 10:01 PM   #114
pthep is offline pthep  Canada
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I am not a quick builder, hence still working on the V2 edition.
From the published BOM, it seems T1 to T5 2SC945 are hard to find.
I have some BC546 on hand, the pin outs are different so I gather
twisted leads will result. Just a backup opinion required. Thanks.
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Old 24th January 2014, 10:02 PM   #115
pthep is offline pthep  Canada
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Sorry wrong forum, should be in Speaker protect.
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Old 24th January 2014, 10:59 PM   #116
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Quote:
Originally Posted by pthep View Post
I am not a quick builder, hence still working on the V2 edition.
From the published BOM, it seems T1 to T5 2SC945 are hard to find.
I have some BC546 on hand, the pin outs are different so I gather
twisted leads will result. Just a backup opinion required. Thanks.
the 2sc945 can be harvested out of discarded atx psu's generics will have them...
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Old 25th January 2014, 02:45 PM   #117
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Quote:
Originally Posted by pthep View Post
I am not a quick builder, hence still working on the V2 edition.
From the published BOM, it seems T1 to T5 2SC945 are hard to find.
I have some BC546 on hand, the pin outs are different so I gather
twisted leads will result. Just a backup opinion required. Thanks.
BC546 will work, just please make sure the leads are properly oriented. 2SC828 is another generic candidate.
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Old 25th January 2014, 09:23 PM   #118
pthep is offline pthep  Canada
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Thanks for the replies, used the BC546, had to leave the leads a little longer
to allow for the twist and some insulated sleeving. Soldered in place, tested out
and works just as the build guide described. Thanks again.
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Old 3rd February 2014, 12:10 PM   #119
SRMcGee is offline SRMcGee  United States
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Folks:

I thought you might find this interesting. This SoftStart board came out of an F5T V2 stereo amp. I'm not sure what the cause of the failure was but one lead on each of the four power resistors was blown out of its hole and one of the four power resistors is fried (the others appear fine). The transformer is a 1kw toroid with 24V secondaries and the power supply has 160kuF in capacitance. I was using 75R power resistors as recommended by AndrewT in this thread (as opposed to the 150 to 180R values in the BOM) and a 4A slow blow fuse.

Do you suggest I increase the resistance on R15-R18, lower the value of the fuse or try something else?

Regards,
Scott
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Old 3rd February 2014, 11:27 PM   #120
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Quote:
Originally Posted by SRMcGee View Post
Folks:

I thought you might find this interesting. This SoftStart board came out of an F5T V2 stereo amp. I'm not sure what the cause of the failure was but one lead on each of the four power resistors was blown out of its hole and one of the four power resistors is fried (the others appear fine). The transformer is a 1kw toroid with 24V secondaries and the power supply has 160kuF in capacitance. I was using 75R power resistors as recommended by AndrewT in this thread (as opposed to the 150 to 180R values in the BOM) and a 4A slow blow fuse.

Do you suggest I increase the resistance on R15-R18, lower the value of the fuse or try something else?

Regards,
Scott


Have you tested the board without the transformer connected? This way you can be sure that the relay works and shorts out the power resistors even without a transformer connected.
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