AC capacitor question

L

lerameur

Guest
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
 
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.??
Click to expand...
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".

Perhaps if you said something like "I have very little practical
knowledge of capacitors and I want to select the type that's best for a
rail gun" then folks would be able to help you out.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
 
lerameur wrote:
On Mar 24, 6:43 pm, Tim Wescott <t...@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
Click to expand...

It will cost you quite a bit more, for lower performance. DC
electrolytics pack a lot more storage into the same space and cost a lot
less. AC electrolytics aren't intended for DC applications.


--
Lead free solder is Belgium's version of 'Hold my beer and watch this!'
 
"lerameur"

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


** That is an oil filled film ( likely polypropylene) capacitor with two
sections, 5 uF and 17 uF.



..... Phil
 
lerameur wrote:
On Mar 24, 6:43 pm, Tim Wescott <t...@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
Click to expand...
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
 
On Mar 24, 6:43 pm, Tim Wescott <t...@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".
Click to expand...
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
 
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
Click to expand...
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.
 
George Herold wrote:
On Mar 24, 7:52 pm, Tim Wescott <t...@seemywebsite.now> wrote:
lerameur wrote:
On Mar 24, 6:43 pm, Tim Wescott <t...@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.
Click to expand...
I have no clue -- I've never messed with rail guns.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
 
On Mar 24, 7:25 pm, lerameur <leram...@yahoo.com> wrote:
On Mar 24, 6:43 pm, Tim Wescott <t...@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 -
Click to expand...
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.
 
On Mar 24, 7:52 pm, Tim Wescott <t...@seemywebsite.now> wrote:
lerameur wrote:
On Mar 24, 6:43 pm, Tim Wescott <t...@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 -
Click to expand...
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.
 
Basically I need about 6kv with 30uF, with a fast discharge.
I am trying to build a DC tesla coil.
as shown here: http://www.richieburnett.co.uk/dcresist.html

I found on ebay this capacitor
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=200450854027&ssPageName=STRK:MEWAX:IT

Do you think it will work for me?
or do I need an electrolytic capacitor?

k
 
In <f5621e41-4cdd-47a9-af5b-d688a7a18f95@g19g2000yqe.googlegroups.com>,
lerameur wrote:

Basically I need about 6kv with 30uF, with a fast discharge.
I am trying to build a DC tesla coil.
as shown here: http://www.richieburnett.co.uk/dcresist.html

I found on ebay this capacitor
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=
200450854027&ssPageName=STRK:MEWAX:IT

Do you think it will work for me?
or do I need an electrolytic capacitor?
Click to expand...
Non-electrolytic capacitors are OK for Tesla coils, whether AC or
DC is used.

Meanwhile, 30 or 60 uF sounds high to me for making a Tesla coil that
is not extremely huge. That sounds high to me by about 3 orders of
magnitude for this voltage range for a table-top size Tesla coil. And
these huge capacitors can probably electrocute an elephant.

- Don Klipstein (don@misty.com)
 
On Wed, 24 Mar 2010 15:28:38 -0700 (PDT), lerameur
<lerameur@yahoo.com> 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
You are getting into something where the internal construction of the
Click to expand...
cap may matter more than type.

For a rail gun, you want all the energy out of the cap in one fast
discharge - so the cap has to have low "external series resistance"
and a low inductance type of construction - interleaved sheets of
dielectric/plate work better than a cap with the dielectric and sheets
wound up.

Large heavy braided copper connections - or copper pipe with large
surface area, and short connections to minimize losses.

Caps designed for "commutation" use are a good bet - non polar and
designed for relatively high voltage, high discharge rates.

RF caps another good choice - very low ESR/inductance, very high
voltages. AND very expensive as a rule

Check out what Tesla coilers use - they have the same requirements,
high voltage high discharge etc.. Do it yourself glass plate caps are
fun - maybe too low in capacity for your aps, but they are
inexpensive, big bulky, and can be made nearly indestructible. (a
paper cutter will cut thin sheets of aluminum flashing material - to
make some dynamite stacked foil caps)

Photoflash caps are a good choice - DC electrolytic designed for fast
discharge at moderately high voltages.

Lastly, an ordinary electrolytic or HV cap will sometimes fail open
when the lead connecting to the internal foil vaporizes at the
connection (usually a spot-weld between copper and aluminum)
commutating caps, photoflash, RF caps are usually designed with that
in mind.

Rail gun or coil gun? Same type of cap requirements though.

These days AC caps can be made quite small - so a lot of the "motor
run" caps are now non polar, non electrolytic, with good "dissipation
factor."
--
 
Don Klipstein wrote:
And these huge capacitors can probably electrocute an elephant.
Click to expand...

Once again proving that Elephants have no business playing with high
voltage! ;-)


--
Lead free solder is Belgium's version of 'Hold my beer and watch this!'
 
"lerameur" <lerameur@yahoo.com> wrote in message
news:a115bc51-cf30-408f-b3e6-1a339c006d3d@15g2000yqi.googlegroups.com...
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
Click to expand...
Check out http://4hv.org
 
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