What is a "Digital Capacitor"

[ashardD]

I see these "Digital Capacitors" being advertised for car audio and they just seem too good to be true. They claim to be 24V capacitors with values from 1 to 5 Farads! They only cost between $30 to $150 dollars. When I look at anything that comes close from digikey it costs around $3000 dollars. Could someone explain what this discrepancy is all about.

I am looking for a low cost capacitor bank for a spot welder I am designing. Heck, if these things are for real they would provide a voltmeter for free. Something tells me this isn't going to work though.

Thanks for reading,
Ashard

 

[ScottyUK]

Google 'ultracapacitor' for some of the background. They are real, but think of them akin to a cell or battery rather than a normal cap.



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If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

I can only recommend that you test them. I was also very surprised to see that such capacitors can deliver kA+ for hundreds of milliseconds. Meaning that powerful spot welders can operate off a 10 A outlet without blowing the fuse. All you need to do is to limit spot frequency so the fuse doesn't overheat and trip.

You may need to discharge a set sequentially so you don't get a very high initial discharge and next to nothing the following few hundred milliseconds. It depends. If you are working with heavy plates (all is relative) like car body work, then you need several hundred milliseconds. But if you are welding thin material, a single discharge will work quite well.

If you tested these capacitors about ten - fifteen years ago, you would have been very disapointed. They had a rather high internal resistance and could typically be used as a memory back-up supply only. That has changed drastically.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ashardD]

OK, I'll give it a shot. First I'll see how single discharge cycles go. I am particularly worried that the caps won't like the near "short circuit" current draw. I haven't seen one that quotes it's maximum current rating or most other specs of interest. I fear it will blow up or something. I will start out with a 10W 0.04 Ohm resistor in the loop to measure the current draw, but ideally the cap will hold its own under short circuit conditions. The present single-discharge mode prototype is not using a current limiter. It has only the small resistance of an SCR and the variable contact resistance of the electrodes.

Definitely using ear plugs and safety glasses for this test. I'll let you know how it goes.

Thanks for your thoughts,
Ashard

 

[itsmoked]

Yep safety first!

The ghetto blaster usage can be pretty close to short circuiting also.

I think it will be fine.

I've seen builds using them for spot welding.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

Ashard (as hard can be? )

Inductance in the loop will probably play a greater role than your .04 ohms. So, try different lead configurations. As close as possible is usually best for highest discharge rate and minimum ringing.

And, for measuring the current pulse, use a fast Rogowski coil. The ones from Fluke have limited band-width and do not work. Also, their current range is too low. A coaxial shunt is usually best for these measurements.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...