Question about Slow charging capacitors for amplifier power supply

Thread Starter

Nutball

Joined Jan 29, 2016
23
I think a fixed time delay would be as easy as a main capacitor voltage dependent switch that, using the big cap's voltage, can't turn on full power until the voltage is high enough.
 

tcmtech

Joined Nov 4, 2013
2,867
Generally, you don't need the slow turn-on. I have essentially 40,000 uF at 50 V in my amp and I NEED the slow turn-on,
I have about 400,000 uF @ 70 - 80 volts in my welder and I don't think twice about turning it on.
Same with my plasma cutter that has around 20,000 - 30,000 uf @ 340 volts.

I also have a battery charger that will top 400 amps at 12 volts into dead battery if the supply line can support it and I don't think twice about turning that on either.
What is the capacitive value of a pair of near dead 12 volt 120 aH group 31 battery any ways? A few million Farads? :oops:

I also have a 15 HP single phase air compressor that has a ~2400 uf start capacitor bank that draws upwards of 120 amps at 240 VAC and that starts direct online every time and can take 4 - 8 seconds to get up to speed depending on my shop temperature. What's the capacitive value of that things start up load, 2 - 3 Farads at 340 VDC? o_O

My point is for some who deal with small scale electronics circuits several tens of thousands do uf at several tens of volts seems like a huge amount but when compared to other systems its insignificantly small.
I mean if that value of capacitance is a concern how can you possibly do a direct line connection with the transformers that power those circuits anyways?
Ever do a theoretical calculation on what the energizing currents are to get those big iron cores up to speed is? If you did you would be wanting to soft start those then soft start load your capacitor banks behind those and so on. :rolleyes:
 
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tcmtech

Joined Nov 4, 2013
2,867
The theory doesn't take into account all that is possible due to ignorance. In my case I expect a surge, but I want to be on the safe side to avoid blowing fuses because I don't know the actual draw and duration.
Until a problem actually presents itself it not actually a problem.

Your over analyzing and putting too much unjustified 'what if worst case' theory behind a common action that is done an has been done billions of times now in billions of other circuits and devices that have way higher power capacities than anything you are working with.

Also there's a reason someone invented the 'slow blow' fuse which is very common in electronics power supplies.

http://aemcomponents.com/applications/faq/
 
My circuit does other things such as turn-on/turn-off thump suppression and if one rail fails, the amp turns off. I did have the single rail failure.
I accidentally switched the NPN and PNP output transistors and suffered no damage but he expected resistor.
 

Thread Starter

Nutball

Joined Jan 29, 2016
23
tcmtech, I assume those tools could have their own soft starts built in, if not they at least had the brains behind them knowing how to make it work. I don't, so I thought I'd play it safe with a simple soft start circuit. I've always thought those billions of other circuits out there had soft start features because of all of the extra components involved in the power supply area. I do have experience with 1500-8000uF causing problems with circuits not designed by me. One very unusual experience was connecting an 8v lithium battery to an ESC with 1500uf and I think no motor was attached. I didn't use the soft start resistor and got a huge pop and in less than a second the 10AWG battery wires were hot. That's where I'm coming from.

Anyway I think I may have found the problem, so I'll rearrange my components and might try without any soft start too.
 

tcmtech

Joined Nov 4, 2013
2,867
tcmtech, I assume those tools could have their own soft starts built in, if not they at least had the brains behind them knowing how to make it work. I don't, so I thought I'd play it safe with a simple soft start circuit. I've always thought those billions of other circuits out there had soft start features because of all of the extra components involved in the power supply area. I do have experience with 1500-8000uF causing problems with circuits not designed by me. One very unusual experience was connecting an 8v lithium battery to an ESC with 1500uf and I think no motor was attached. I didn't use the soft start resistor and got a huge pop and in less than a second the 10AWG battery wires were hot. That's where I'm coming from.

Anyway I think I may have found the problem, so I'll rearrange my components and might try without any soft start too.

It depends on who makes something. I have devices and equipment with huge capacitor banks that have no soft start or start up current limiting in them and then I have small devices that have it despite having small fractions of the equivalent capacitance.
The one thing I have found is many of the devices that have the soft start built in have that as their primary point of failure. Either The current limiting resistors burn out or the bypass relay/contactors burn out. The direct online fire up and go system either work or they don't and what failed when they don't work anymore is usually pretty obvious.
 
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