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Offline Hood

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Re: Power Supply Unit
« Reply #130 on: January 27, 2011, 02:27:43 AM »
Sorry about the £30 bit, thas what I can get one for in this country, it would cost a foryune to send it across to you I think. I was just surprised that you could not get one in Canada about a similar kind of price.

Yes I noticed the 4.5Amp bit but think its a typo, if you download the data sheet you will see its 4.5Amps per secondary and there are 2 secondaries so that would be 9 amps total. Also if you work it out 9 amp x 25v = 225VA
BTW you would be connecting the secondary in parallel rather than series as series would give you 50VAC
Hood
Re: Power Supply Unit
« Reply #131 on: January 30, 2011, 06:59:33 PM »
I went to purchase the capacitors from my local store but I couldn't find anyone with 50VDC or more neither with 20000uF or more. The closest I found were:

15500uF, 25VDC
10000uF, 25VDC
8200uF, 25VDC
5500uF, 25VDC
34000uF, 15VDC
25000uF, 15VDC
10000uF, 10VDC

Can someone tell me if a combination of the above is suitable?

If not, then I will have to purchase them from the Web

Thanks


I haven't read all this thread yet (only up to page 7), so this may have already been answered...

If you can't find the correct size capacitor, you could always buy more of the same size lower voltage ones and wire them in series to get over your operating voltage. Series means wiring + to - in a line... So 3 x 15500uF @ 25V in series would give you in effect 1 capacitor of 15500uF @ 75V.

If you bought 6 of these 15500uF 25V capacitors, wired 2 sets of 3 in series and then took those 2 sets of three and wired them in parallel, you'd end up with the equivalent of 1 capacitor of 31000uF @ 75V.

I know it sounds weird to do it that way, but I usually work with what I can get my hands on at a reasonable price too.

Graham

Offline kolias

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Re: Power Supply Unit
« Reply #132 on: January 30, 2011, 07:08:30 PM »
Thanks for the tip Graham nice to know, I think we discussed connecting them in series or parallel, I forget too, but Hood send me the capacitors I need and I'm all set for now.
Nicolas
Re: Power Supply Unit
« Reply #133 on: January 30, 2011, 07:11:37 PM »
That's really not a good practice with electrolytics, unless you provide a LARGE voltage safety factor, which kinda defeats the purpose.  The tolerance on the capacitance of large electrolytics is typically very broad (like +/-30%), which means some will charge MUCH faster than others, which means some can have considerably more voltage across them than others when connected in series.

Regards,
Ray L.
Regards,
Ray L.
Re: Power Supply Unit
« Reply #134 on: January 30, 2011, 07:49:45 PM »
That's really not a good practice with electrolytics, unless you provide a LARGE voltage safety factor, which kinda defeats the purpose.  The tolerance on the capacitance of large electrolytics is typically very broad (like +/-30%), which means some will charge MUCH faster than others, which means some can have considerably more voltage across them than others when connected in series.

Regards,
Ray L.

That's why I suggested 3 x 25V types in series to give a 75V capability, they should easily handle the 40 or so volts being used in this circuit with almost a 40% margin for safety. It's always worth a try if you can't get the correct size. They don't exactly go off like a hand grenade, just monitor the voltage across them individually and make sure they don't get too warm :)

...and I thought the purpose was to get up and running as quickly as possible :P
Re: Power Supply Unit
« Reply #135 on: January 30, 2011, 07:53:22 PM »


If you bought 6 of these 15500uF 25V capacitors, wired 2 sets of 3 in series and then took those 2 sets of three and wired them in parallel, you'd end up with the equivalent of 1 capacitor of 31000uF @ 75V.



If I'm not mistaken, the total capacitance in a circuit with capacitors in series is reduced.  If I'm figuring right, the total capacitance for three 15500uF caps in series would be about 5200uF.  To maintain a total capacitance of 15500uF you would have to have three 46500uF caps.  That said, I'm not sure total capacitance even matters that much in a power supply.

In the above example, the 2 sets of three would equal about 5200uF each then putting those sets in parallel would raise the total capacitance to about 10400uF.
Re: Power Supply Unit
« Reply #136 on: January 30, 2011, 08:31:07 PM »


If you bought 6 of these 15500uF 25V capacitors, wired 2 sets of 3 in series and then took those 2 sets of three and wired them in parallel, you'd end up with the equivalent of 1 capacitor of 31000uF @ 75V.



If I'm not mistaken, the total capacitance in a circuit with capacitors in series is reduced.  If I'm figuring right, the total capacitance for three 15500uF caps in series would be about 5200uF.  To maintain a total capacitance of 15500uF you would have to have three 46500uF caps.  That said, I'm not sure total capacitance even matters that much in a power supply.

In the above example, the 2 sets of three would equal about 5200uF each then putting those sets in parallel would raise the total capacitance to about 10400uF.

Oops... I forgot that bit,  I got the voltage bit right though, it's been a while since I've done any electronics.. Though I seem to remember being able to draw off odd voltages at low currents into a regulator between the capacitors to use when no other source was available. Though I may have been dreaming :P

http://www.electronics2000.co.uk/calc/series-parallel-capacitor-calculator.php
« Last Edit: January 30, 2011, 08:41:44 PM by GrahamIT »
Re: Power Supply Unit
« Reply #137 on: January 30, 2011, 08:37:20 PM »
Yes, you're absolutely correct.  Capacitors in series combine the same way resistors do in parallel.  Similarly, capacitors in parallel combine the same way resistors do in series.  So, two 10,000 uf capacitors in series will give 5000 uF,in parallel will give 20,000 uF.

"That said, I'm not sure total capacitance even matters that much in a power supply." - It matters VERY much, if you don't want your output voltage to sag during the low phases of the AC input.  Too little capacitance will result in output voltage ripple.  The higher the current, or the lower the capacitance, the larger ripple.  On a stepper motor, that will lose torque, and top speed, and could, in extreme cases, cause the stepper driver to misbehave.

Regards,
Ray L.
Regards,
Ray L.

Offline kolias

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Re: Power Supply Unit
« Reply #138 on: January 30, 2011, 08:52:57 PM »
Hey Ray, I dont understand what you are saying but I try.... I only wish I knew a fraction of what you know......

Anyway its nice to have you here to help people like me
Nicolas

Offline alenz

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Re: Power Supply Unit
« Reply #139 on: January 30, 2011, 10:56:49 PM »

Nicolas,
I know better than to butt in but re capacitors………………
I think you are seeing viewpoints from two opposite sides of the spectrum. Perhaps (just guessing) one from a professional who designs commercial products to meet a certain spec and a hobbyist that just wants to build something that will do what he wants with the parts at hand (even tho it's not optimal). I say don't worry about the second decimal when dealing with components that have a 20% tolerance to begin with. Try to stay in the ballpark and it should turn out just fine. Our steppers may not be operating at 100% (how would we know?) but if they are doing what we want then as hobbyist we are happy. Ignorance is bliss.
Al
P.S. Fire away, I'm flameproof<grin>
P.S.S. I deeply respect the wealth of knowledge of both of the above unnamed posters.
AL