Large Capacitors

[Jay C]

Guys, I have all the parts for my power supply but the large capacitor. I have located a likely candidate at www.mouser.com but they don't have any brackets to go with it. I'd love to shop in one place (to save on shipping). Is there any other place I should be looking for a minimum 35K uF 35VDC computer grade capacitor? Here is the one I'm looking at
http://www.mouser.com/search/product...6DY403F040BC2A

I have tried 3 pages of Google links without much luck locating these two, and eBay pricing/shipping is outrageous.

Suggestions? Worst case I'll use a pipe clamp I guess

Jay

[Megahertz]

I noticed the capacitor was 2" in diameter. What about a 1 1/2" plumbing PVC end cap? (PVC pipe is measured by the inner diameter. Therefore, the outer diameter would be close to 2". I wonder if they make 1 3/4 PVC?)

[Jay C]

Well I went ahead and ordered it. Yeah, I've been contemplating my DIY hacks to keeps this bad boy from moving once it's installed. It shouldn't be too hard, and I'll try to make it look non-ghetto I'll post a photo of my solution once I get the part in hand while I stalk the isles at Lowes

Jay

[BobWarfield]

Lots of big capacitors for sale on eBay. I got a box of 5 for less than a single one would have cost me on Mouser.

FWIW, it sometimes helps to search for "Tesla capacitor". I guess people are using them to make Tesla coils.

Best,

BW

[Jay C]

Bob, why would you want 5 35,000uF capacitors when 1 will do Also, the Telsa capacitor search brings up HV caps, not high capacitance. In either case, shipping is more than the cap I bought.

Thanks,
Jay

[BobWarfield]

Originally Posted by Jay C Bob, why would you want 5 35,000uF capacitors when 1 will do Also, the Telsa capacitor search brings up HV caps, not high capacitance. In either case, shipping is more than the cap I bought. Thanks, Jay

So far, I've got a lathe and a mill underway, and I have no doubt a router table or plasma won't be far behind. I can hook 2 in parallel for more capacity if I want 4 axes and big servos on the mill, and they're sitting there on the shelf next time I want one. At $40 delivered for 5, vs Mouser's $16 + shipping, it was well worth it to me.

BTW, the ratings on your cap are a lot different than the recommendations I saw on the Gecko board. I'm sure they're right for your application, but I am sort of curious about it.

The cap you list is 40V, whereas I'm running 68V out of the transformer for my supply since the Geckos will run up to 80V and I didn't want to leave anything on the table unless I had to. The figures I've seen for stepper power supply caps would indicate you want your max stepper voltage / 0.8 as a voltage rating, and you'd want to run (80000 * desired amperage) / target voltage to get the capacitance in microfarads. In my case, I wanted 80V and 4.5 amps for the lathe's power supply.

Working backwards from your cap's 40V capacity, that would imply 40V * 80% = 32V target voltage, which seems a lot lower than what I am used to for steppers. Mariss recommends allowing 3x overvoltage with his drivers for your steppers but more is better as the drive will automatically reduce the current to keep the overall power from burning up the motor. Basically, you'll get better acceleration performance if you allow the over voltage.

The current capacity with 30,000 microfarads would be 12 amps @ 32V, which is again, an interesting value. I guess at the lower voltages (possibly dictated by the transformer you scrounged?), you'll need more current.

In any event, I'm glad you found what you were looking for.

Best,

BW

[CountZero]

A capacitor is not just a capacitor, you would be amazed as how much inductance a electrlytic cap can display at higher frequencies. With big caps this high frequency can be as low as 100's of Hz. To keep the performance you expect you can parallell several caps, this adds the capacitance but reduces the inductance (and series resistance).

[Jay C]

Originally Posted by BobWarfield BTW, the ratings on your cap are a lot different than the recommendations I saw on the Gecko board. I'm sure they're right for your application, but I am sort of curious about it.

I have the HobbyCNC board I bought a year ago that I'm using. Also I'm following Dave Rigotti's PS plan and he calls for a minimum 35K 35V cap. The 40K 40V cap was cheaper

The cap you list is 40V, whereas I'm running 68V out of the transformer for my supply since the Geckos will run up to 80V and I didn't want to leave anything on the table unless I had to.

Yeah, my drivers take a max of 42V so I went with a 24V transformer. That'll give me ~33VDC. Plenty for the 200 oz-in steppers with 5TPI ballscrews for my X2.

The figures I've seen for stepper power supply caps would indicate you want your max stepper voltage / 0.8 as a voltage rating, and you'd want to run (80000 * desired amperage) / target voltage to get the capacitance in microfarads. In my case, I wanted 80V and 4.5 amps for the lathe's power supply. Working backwards from your cap's 40V capacity, that would imply 40V * 80% = 32V target voltage, which seems a lot lower than what I am used to for steppers. Mariss recommends allowing 3x overvoltage with his drivers for your steppers but more is better as the drive will automatically reduce the current to keep the overall power from burning up the motor. Basically, you'll get better acceleration performance if you allow the over voltage. The current capacity with 30,000 microfarads would be 12 amps @ 32V, which is again, an interesting value. I guess at the lower voltages (possibly dictated by the transformer you scrounged?), you'll need more current. In any event, I'm glad you found what you were looking for. Best, BW

Bob, now you have my attention. These values are exactly the ones I'm working with. Edit: I figured out where I was solving for the wrong "unknown". I was using too small of a current. I changed my values to add a 10% current cushion and get 32KuF for 4 steppers.

Very good post, and I appreciate your taking the time to lay it all out. Would make a nice addition to your blog too.

Jay

[Jay C]

Originally Posted by CountZero A capacitor is not just a capacitor, you would be amazed as how much inductance a electrlytic cap can display at higher frequencies. With big caps this high frequency can be as low as 100's of Hz. To keep the performance you expect you can parallell several caps, this adds the capacitance but reduces the inductance (and series resistance).

Well, I have 5 6800uF 50VDC caps I can use that don't take up nearly the amount of space as the big one. The only reason I didn't was becaue the plan said do not use less than 35K uF, and I didn't want to try to mix in some even smaller caps to make up the difference. What kind of ill effects are we concerned with? This is just a brute force unregulated power supply.

Look forward to your input,
Jay

[BobWarfield]

Originally Posted by Jay C Bob, now you have my attention. These values are exactly the ones I'm working with. Edit: I figured out where I was solving for the wrong "unknown". I was using too small of a current. I changed my values to add a 10% current cushion and get 32KuF for 4 steppers. Very good post, and I appreciate your taking the time to lay it all out. Would make a nice addition to your blog too. Jay

They're just the same equations I gave above--I just worked them backwards in the spreadsheet I use. I'm pretty sure they came off the Yahoo Gecko group and indirectly (or directly) via Mariss. He has quite a lot to say about running up to 20x rated voltages and at least 3x on step motors to get better performance with his drives. I chose not to add it to the blog because its basically his information and I don't know how focused it is around his drive designs.

Best,

BW

[Jay C]

I would say one would need to modify the max to coincide with their drivers max voltage.

[BobWarfield]

Originally Posted by Jay C I would say one would need to modify the max to coincide with their drivers max voltage.

Of course! That's why I said, "since the Geckos will run up to 80V" in my note above.

Best,

BW