Re: Toroidal transformer ratings?? What after being rectified?

WONDERFUL!!! I think I will frame this and hang it on my wall.
Beautiful details, good explanation, cited sources, and all
positive. I can understand what is being said, even if I am not an
electronics expert. This is why I read every single post to this
board, hoping I will run across information like this. Thanks again,
and if we ever cross paths, the Big Mac is on me!
Randy


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "cnc_4_me" <cnc4me@g...>
wrote:

>
> The following is a example of a stepper motor power supply design.
> For servo motors you would need a 50% larger margin of safety on
> current rating...
>
> Well...In my opinion your power supply is not to big, as i can
> demonstrate...
>
> First you should design for worst case, 3 motors moving.
>
> This is how i would design supply...
>
> 3 motors 7amps each = 21 amps DC
>
> 67% current required for gecko = 14amps dc (.66667x21A)
>
> The required AC current into a bridge rectifier capacitor combo is
> 1.8 x DC current (i think you missed this post i made). So 1.8 x

14A

> DC = 25.2 amps AC from transformer......Hard to belive isn't it!!!
>
>
> With that said, before i go on, i pulled a fast one. I read ahead
> and looked at the link Les posted. Here is a page from it on the
> amount of current through the transformer, bridge, and capacitor...
>
> http://sound.westhost.com/power-supplies.htm#xfmr_series_r
>
> Giving it a quick once over i see i was to conservative on 2

things.

> To calculate AC current in transformer they used a number closer to
> 1.9 than 1.8....And in previous post i said ripple current in cap

is

> = to DC current...According to this article the ripple current is

1.6

> x the DC current...I stand corrected....Sort of...SIGH...So many
> conflicting rules...I just looked up the cap ripple current

derating

> in a power supply data sheet i have been using for years...It

states

> ripple current in cap is 2 to 3 times higher than output DC

current,

> not 1.6 as the web article says.....
>
>
> OK so now we have to pick a capacitor value...Notice in the artical
> the guy kind of side steps the issue and says 2000uF per amp...This
> is because it depends on to many factors that are design
> specific...Thats why we go with the gecko capacitor selection
> formula...We are designing this supply to work with geckos, and the
> mfg told us what cap size his product needs....
>
> So we have a 34VDC supply (aprox) at 14ADC...
> C= (80,000 x I) / E
> (80,000 x 5) / 34 = 32,941 or about 33KuF MINIMUM cap size
>
> We also need to calc our ripple current in the capacitor...I am

going

> to use the web article spec of 1.6 for derating...
>
> 1.6 x DC current.
> 1.6 x 14ADC = 22.4ADC of ripple current
>
> Now Go to mouser and grab your ripple current charts i posted in
> previous post and find a 33KuF cap at 22.4ADC ripple...MIN specs..
>
> Here we are at mouser, looking at Vishay and Cornell Dubilier

caps...

> First lets narrow the search down and find a voltage range to look
> at...Power supply rated at 34VDC can drift at least 15% or 39vdc or
> so...
>
> We see there are standard voltage caps at 50 and 75VDC...
>
> In my mind i am thinking with 39VDC and regenitive braking to worry
> about, that 50vdc cap is on the edge....I suspect with all the low
> cost power supply designs around people are putting out, many would
> use this voltage...But i would not...Even though there is 2X price
> penelty to pay for security....My opinon, use the 75VDC

cap...Notice

> i am ignoring the surge rating here, a 50VDC cap has a surge rating
> of 65VDC...I have not taken this in account because i have never

seen

> a spec on it...Do not know how long of a surge it can take or how

it

> affects life....
>
> Next i will look at the 2 different capacitor series offered and

look

> for bang for the buck at 75VDC...I won't go into a lot of boring
> detail of looking at different caps. But try to find a cap or
> combination caps of at least 33Kuf and capable of handiling at

least

> 22.4ADC ripple....
>
> While selecting caps couple of things i do...I do not mix and
> match...I will select a value of cap and use multiples of it. Like

4

> pcs of a 10Kuf, not a 10Kuf and a 20KuF...I think the current

shares

> better this way...
>
> And 1 problem you will run into...The values offered by mouser do

not

> always match the sheets from the mfg...So on some values you will

not

> have a ripple current spec...Only thing you can do is use the

ripple

> current value from the next highest value cap. If you are brave

you

> can try to interpolate the value...But be very conservative...
>
> And finally only parallel caps, do not put them in series, unless

you

> really know what you are doing. Putting caps in parallel the uF

adds

> up and voltage rating stays the same...Series halfs the uF value

and

> doubles the voltage rating, and you have to play games with voltage
> divider resistors on the caps...
>
> OK, let me look at the mouser catolog now...Something in the 33Kuf,
> 22.4ADC range.
>
> 2 choices.
>
> 75-36DY183F075BC2A 18kUF 13.7A $18.00 2PCS 36KuF 27.4A $36.00
> 539CGS75V17000 17Kuf 9.8A $33.00 2pcs 34Kuf 19.4A $66.00
>
> Of these 2 choices notice number 2 does not meet current spec...
> So we use this one...
>
> 75-36DY183F075BC2A 18kUF 13.7A $18.00 2PCS 36KuF 27.4A $36.00
>
>
> OK...We getting close...Still need a bridge rectifer though...So

lets

> go to the recton bridge rectifier pages...
>
> We need a bridge rated at at least 14ADC and i see 15Amp
> bridges...But once again this is on the edge...And it would

probably

> work fine with heatsink and knowing the fact that all 3 axis are

not

> usally used at same time...But i would not use it...As i said in a
> earlier post, i usually select a current value 50 to 100%
> bigger...Also i usually select a 400piv diode...Why, do i do this,
> for many reasons, by going 2x on current rating i do not have to
> worry about the surge current to the caps burning out the bridge.
> with the high voltage rating i do not have to worry about inductive
> kickback, or any kind of spike...I picked 400volt rating because

it

> is usually only 25 to 50 cents more. So lets make a chart of specs
> and prices.
>
> 15amp 400piv 300Amp Surge $2.79
> 20amp 400piv 300Amp Surge $2.35
> 25amp 400piv 300Amp Surge $2.84
> 35amp 400piv 400Amp Surge $3.26
>
> Notice the small price differance and 20 amp is less than 15 amp
> (cause of ul rating)...
>
> I would go with the 35 amp bridge, 50 cents more gets you 2x the
> current you need and 8x the voltage rating...And look at the surge
> current rating, 400 amps, this is also a magic number...Remember

Jon

> Elson's post, small transformers can put out 400 amps peak...Surge
> problem solved...
>
> Bonus information.
>
> Wire sizes i would use for different current ratings.
>
> 10ga 30A
> 12ga 20A
> 14ga 14A
> 16ga 9A
> 14ga 6A
> 18ga 4A
> 20ga 3A
> 22ga signal wire only
> 24ga and smaller not recomended (fragile hard to work with)
>
> More bonus information
>
> Do not forget what the gecko power supply design manual says about
> drives that are more than 18 inches from power supply. Or if you

put

> a fuse in the drive power line. You then need a small cap

installed

> directly in the drive power in terminals. A 470uF 75 Volt cap for
> this example.
>
>
> Wally