Prev: Samsung C3010 Price List and Features
Next: CF Lightbulb Dimmer
From: Tim Wescott on 24 Mar 2010 19:52
lerameur wrote:
> On Mar 24, 6:43 pm, Tim Wescott <t...(a)seemywebsite.now> wrote:
>> lerameur wrote:
>>> Hi,
>>> I believe it is possible but I would just like a confirmation.
>>> I would like to know if I can use an AC capacitor and make it behave
>>> like a DC cap, I would like to do a rail gun. Are DC capacitor better
>>> suited, do they perform better for a quick discharge.??
>> Um -- what the heck are you talking about?
>>
>> There are capacitors that are made primarily for service with zero DC
>> bias, there are capacitors (polarized electrolytics) that are made for
>> service _with_ a DC bias, and there are capacitors made for service
>> attached to the electrical grid.
>>
>> Open a Digi-Key catalog and eliminate the half-dozen most popular
>> dielectrics used to build capacitors with and you'll still have entries
>> to spare -- and that's not including oddball dielectrics, nor is it
>> taking into account the wide variation in ways that capacitors are
>> constructed _using_ these dielectrics.
>>
>> I've seen a lot of different categories under which capacitors are sold,
>> I don't recall ever seeing capacitors sold as "AC capacitors" or "DC
>> capacitor".
>>
>
> What I meant is that DC capacitors are polarized and AC are bipolar.
> I would like to charge a, AC Capacitor with direct current
> (electrolytic capacitor ) , there just need the positive to zero.
> From what I can read from you, I will be able to do what I need to
> do.
> I am just saying AC capacitor because you can see VAC on these
> capacitors.
> http://www.bridgat.com/files/CBB65_175uf_air_conditioner_capacitor.jpg
> cheers

That looks like an engine start/run capacitor; while it is rated in VAC,
that doesn't make "AC capacitor" a meaningful term.

Here in the US, DC is polarized and the rest of us are tending to get
bipolar over it, but that has nothing to do with capacitors.

I'll reiterate -- ask what you really mean to ask. If you're looking
for good caps for a rail gun, then ask that. If you know what the rail
gun is going to ask from it's caps (bipolar voltage? Always unipolar?
Quick discharge?).

I suspect that a rail gun needs to discharge its caps very quickly --
this is an issue with aluminum electrolytic caps; you can't just use any
old 'lytic and expect it to deliver tons-o-current all at once. So
normally storing lots of energy as DC would be a task for 'lytics, but
if you _also_ need to discharge that energy lightning-fast, you may need
to either carefully select your 'lytics, or use some other, far bulkier,
cap.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: George Herold on 24 Mar 2010 21:38
On Mar 24, 7:25 pm, lerameur <leram...(a)yahoo.com> wrote:
> On Mar 24, 6:43 pm, Tim Wescott <t...(a)seemywebsite.now> wrote:
>
>
>
>
>
> > lerameur wrote:
> > > Hi,
>
> > > I believe it is possible but I would just like a confirmation.
> > > I would like to know if I can use an AC capacitor and make it behave
> > > like a DC cap, I would like to do a rail gun. Are DC capacitor better
> > > suited, do they perform better for a quick discharge.??
>
> > Um -- what the heck are you talking about?
>
> > There are capacitors that are made primarily for service with zero DC
> > bias, there are capacitors (polarized electrolytics) that are made for
> > service _with_ a DC bias, and there are capacitors made for service
> > attached to the electrical grid.
>
> > Open a Digi-Key catalog and eliminate the half-dozen most popular
> > dielectrics used to build capacitors with and you'll still have entries
> > to spare -- and that's not including oddball dielectrics, nor is it
> > taking into account the wide variation in ways that capacitors are
> > constructed _using_ these dielectrics.
>
> > I've seen a lot of different categories under which capacitors are sold,
> > I don't recall ever seeing capacitors sold as "AC capacitors" or "DC
> > capacitor".
>
> What I meant is that DC capacitors are polarized and AC are bipolar.
> I would like to charge a, AC Capacitor  with  direct current
> (electrolytic capacitor ) , there just need the positive to zero.
> From what I can read from you, I will be able to do  what I need to
> do.
> I am just saying AC capacitor because you can see VAC on these
> capacitors.http://www.bridgat.com/files/CBB65_175uf_air_conditioner_capacitor.jpg
> cheers- Hide quoted text -
>
> - Show quoted text -

Sure, your 'bipolar' caps are great at DC. Sometimes lots better than
your 'DC' caps.
DC or poalrized caps are great because they pack a lot of charge into
a small volume. But you pay for this density with all sorts of odd
behavior.

George H.
From: George Herold on 24 Mar 2010 21:44
On Mar 24, 7:52 pm, Tim Wescott <t...(a)seemywebsite.now> wrote:
> lerameur wrote:
> > On Mar 24, 6:43 pm, Tim Wescott <t...(a)seemywebsite.now> wrote:
> >> lerameur wrote:
> >>> Hi,
> >>> I believe it is possible but I would just like a confirmation.
> >>> I would like to know if I can use an AC capacitor and make it behave
> >>> like a DC cap, I would like to do a rail gun. Are DC capacitor better
> >>> suited, do they perform better for a quick discharge.??
> >> Um -- what the heck are you talking about?
>
> >> There are capacitors that are made primarily for service with zero DC
> >> bias, there are capacitors (polarized electrolytics) that are made for
> >> service _with_ a DC bias, and there are capacitors made for service
> >> attached to the electrical grid.
>
> >> Open a Digi-Key catalog and eliminate the half-dozen most popular
> >> dielectrics used to build capacitors with and you'll still have entries
> >> to spare -- and that's not including oddball dielectrics, nor is it
> >> taking into account the wide variation in ways that capacitors are
> >> constructed _using_ these dielectrics.
>
> >> I've seen a lot of different categories under which capacitors are sold,
> >> I don't recall ever seeing capacitors sold as "AC capacitors" or "DC
> >> capacitor".
>
> > What I meant is that DC capacitors are polarized and AC are bipolar.
> > I would like to charge a, AC Capacitor  with  direct current
> > (electrolytic capacitor ) , there just need the positive to zero.
> > From what I can read from you, I will be able to do  what I need to
> > do.
> > I am just saying AC capacitor because you can see VAC on these
> > capacitors.
> >http://www.bridgat.com/files/CBB65_175uf_air_conditioner_capacitor.jpg
> > cheers
>
> That looks like an engine start/run capacitor; while it is rated in VAC,
> that doesn't make "AC capacitor" a meaningful term.
>
> Here in the US, DC is polarized and the rest of us are tending to get
> bipolar over it, but that has nothing to do with capacitors.
>
> I'll reiterate -- ask what you really mean to ask.  If you're looking
> for good caps for a rail gun, then ask that.  If you know what the rail
> gun is going to ask from it's caps (bipolar voltage?  Always unipolar?
> Quick discharge?).
>
> I suspect that a rail gun needs to discharge its caps very quickly --
> this is an issue with aluminum electrolytic caps; you can't just use any
> old 'lytic and expect it to deliver tons-o-current all at once.  So
> normally storing lots of energy as DC would be a task for 'lytics, but
> if you _also_ need to discharge that energy lightning-fast, you may need
> to either carefully select your 'lytics, or use some other, far bulkier,
> cap.
>
> --
> Tim Wescott
> Control system and signal processing consultingwww.wescottdesign.com- Hide quoted text -
>
> - Show quoted text -

Hi Tim, Just wondering if you'd want the rail gun cap to discharge
like a transmission line??
You've got to hold some energy as the 'slug' travels down the rail.
You might even want to try and shape it.

George H.
From: Sjouke Burry on 24 Mar 2010 21:46
lerameur wrote:
> Hi,
>
> I believe it is possible but I would just like a confirmation.
> I would like to know if I can use an AC capacitor and make it behave
> like a DC cap, I would like to do a rail gun. Are DC capacitor better
> suited, do they perform better for a quick discharge.??
>
>
> K
An ac cap is just a cap, with the added advantage of
being able to survive ac current and voltage.
It pays for that with a much lower capacity_per_volume
compared with the dc cap, but the voltage range is
much higher.
For a railgun, the problem is, that it forms a resonance
circuit with your cap, and your design needs to address
the problems that causes, like a blocking diode to
stop the current from reversing, and swithes and diodes
being able to stand very high currents and voltages.

You might consider an agn lead battery instead, which
can supply a shortcircuit current of about 4000 amps.

Switching that will be very difficult, maybe a heavy
mercury wetted swith might work.

To experiment, you can of course even close the wire-
cicuit any way you like, but expect your wiring too
burn and melt.
Something has to melt of course because the battery
can stand the short for only a few tenth of a sec.

You might switch the current by moving the railgun
slidebar from an insulated part of the rail to the
bare part of the rail.
Again, be carefull, use thick cotton or wool protective
clothing and welding handgloves.
And tell us how things went.

From: Tim Wescott on 24 Mar 2010 22:20
George Herold wrote:
> On Mar 24, 7:52 pm, Tim Wescott <t...(a)seemywebsite.now> wrote:
>> lerameur wrote:
>>> On Mar 24, 6:43 pm, Tim Wescott <t...(a)seemywebsite.now> wrote:
>>>> lerameur wrote:
>>>>> Hi,
>>>>> I believe it is possible but I would just like a confirmation.
>>>>> I would like to know if I can use an AC capacitor and make it behave
>>>>> like a DC cap, I would like to do a rail gun. Are DC capacitor better
>>>>> suited, do they perform better for a quick discharge.??
>>>> Um -- what the heck are you talking about?
>>>> There are capacitors that are made primarily for service with zero DC
>>>> bias, there are capacitors (polarized electrolytics) that are made for
>>>> service _with_ a DC bias, and there are capacitors made for service
>>>> attached to the electrical grid.
>>>> Open a Digi-Key catalog and eliminate the half-dozen most popular
>>>> dielectrics used to build capacitors with and you'll still have entries
>>>> to spare -- and that's not including oddball dielectrics, nor is it
>>>> taking into account the wide variation in ways that capacitors are
>>>> constructed _using_ these dielectrics.
>>>> I've seen a lot of different categories under which capacitors are sold,
>>>> I don't recall ever seeing capacitors sold as "AC capacitors" or "DC
>>>> capacitor".
>>> What I meant is that DC capacitors are polarized and AC are bipolar.
>>> I would like to charge a, AC Capacitor with direct current
>>> (electrolytic capacitor ) , there just need the positive to zero.
>>> From what I can read from you, I will be able to do what I need to
>>> do.
>>> I am just saying AC capacitor because you can see VAC on these
>>> capacitors.
>>> http://www.bridgat.com/files/CBB65_175uf_air_conditioner_capacitor.jpg
>>> cheers
>> That looks like an engine start/run capacitor; while it is rated in VAC,
>> that doesn't make "AC capacitor" a meaningful term.
>>
>> Here in the US, DC is polarized and the rest of us are tending to get
>> bipolar over it, but that has nothing to do with capacitors.
>>
>> I'll reiterate -- ask what you really mean to ask. If you're looking
>> for good caps for a rail gun, then ask that. If you know what the rail
>> gun is going to ask from it's caps (bipolar voltage? Always unipolar?
>> Quick discharge?).
>>
>> I suspect that a rail gun needs to discharge its caps very quickly --
>> this is an issue with aluminum electrolytic caps; you can't just use any
>> old 'lytic and expect it to deliver tons-o-current all at once. So
>> normally storing lots of energy as DC would be a task for 'lytics, but
>> if you _also_ need to discharge that energy lightning-fast, you may need
>> to either carefully select your 'lytics, or use some other, far bulkier,
>> cap.
>>
>> --
>> Tim Wescott
>> Control system and signal processing consultingwww.wescottdesign.com- Hide quoted text -
>>
>> - Show quoted text -
>
> Hi Tim, Just wondering if you'd want the rail gun cap to discharge
> like a transmission line??
> You've got to hold some energy as the 'slug' travels down the rail.
> You might even want to try and shape it.

I have no clue -- I've never messed with rail guns.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
First  |  Prev  |  Next  |  Last
Pages: 1 2 3
Prev: Samsung C3010 Price List and Features
Next: CF Lightbulb Dimmer