Capacitor Question

S

Steve

Guest
Hi,

I've got 4 capacitors with the following specs:

CAPACITOR, 4700UF 400V;
Capacitor dielectric type: Aluminium Electrolytic;
Temperature, operating (a) max: 85°C;
Temperature, operating (a) min: -40°C;
Time, operating life: 3000h;
Current, ripple AC @ 100Hz: 14.22A;
Current, ripple AC @ 10kHz: 19.91A;
Frequency, ESR: 100Hz;
Resistance, ESR: 38mR;
Temperature, ESR: 20°C;
Temperature, operating life: 85°C;
Temperature, ripple current: 85°C;
Tolerance, +: 20%;

I plan to combine the 4 to make a large capacitor of 800V @ 9400uF (2 pairs
in parallel connected together in series). I was then hoping to use this
large capacitor in a coilgun (e.g. charge them up and discharge them through
a coil). My question is, are these capacitors suitable for such an
application? In particular, should these be discharged through a coil (they
won't blow up will they)? Im aware of the electrical dangers (touch them
and die basically) but I'm not sure about the suitability for my
application.

If you have any info or advice then I'll be glad to here it.

Thanks in advance.
 
Steve wrote:
Hi,

I've got 4 capacitors with the following specs:

CAPACITOR, 4700UF 400V;
Capacitor dielectric type: Aluminium Electrolytic;
Temperature, operating (a) max: 85°C;
Temperature, operating (a) min: -40°C;
Time, operating life: 3000h;
Current, ripple AC @ 100Hz: 14.22A;
Current, ripple AC @ 10kHz: 19.91A;
Frequency, ESR: 100Hz;
Resistance, ESR: 38mR;
Temperature, ESR: 20°C;
Temperature, operating life: 85°C;
Temperature, ripple current: 85°C;
Tolerance, +: 20%;

I plan to combine the 4 to make a large capacitor of 800V @ 9400uF (2 pairs
in parallel connected together in series). I was then hoping to use this
large capacitor in a coilgun (e.g. charge them up and discharge them through
a coil). My question is, are these capacitors suitable for such an
application? In particular, should these be discharged through a coil (they
won't blow up will they)? Im aware of the electrical dangers (touch them
and die basically) but I'm not sure about the suitability for my
application.

If you have any info or advice then I'll be glad to here it.

Thanks in advance.
Click to expand...
These specs are tailored for a very different use, so there are some
open questions about how they will fair in your application. The
problem centers around the 38 milliohm internal resistance and the
heating produced in this resistance during your pulses, and the fact
that this resistance is not well distributed throughout the thermal
mass of the capacitor. So, even though you can check the case
temperature and find out if you are exceeding the maximum, you cannot
well predict the internal hot spot temperature. As long as your peak
current were not more than the rated ripple current for the
capacitors, his would not be a worry, but I suspect that you are going
to pass peaks much larger than 20 amperes through these.

--
John Popelish
 
"Steve" <steve@steve.com> wrote in message
news:ce3c2c$rc4$1@news8.svr.pol.co.uk...
Hi,

I've got 4 capacitors with the following specs:

CAPACITOR, 4700UF 400V;
Capacitor dielectric type: Aluminium Electrolytic;
Temperature, operating (a) max: 85°C;
Temperature, operating (a) min: -40°C;
Time, operating life: 3000h;
Current, ripple AC @ 100Hz: 14.22A;
Current, ripple AC @ 10kHz: 19.91A;
Frequency, ESR: 100Hz;
Resistance, ESR: 38mR;
Temperature, ESR: 20°C;
Temperature, operating life: 85°C;
Temperature, ripple current: 85°C;
Tolerance, +: 20%;

I plan to combine the 4 to make a large capacitor of 800V @ 9400uF (2
pairs
in parallel connected together in series). I was then hoping to use this
large capacitor in a coilgun (e.g. charge them up and discharge them
through
a coil). My question is, are these capacitors suitable for such an
application? In particular, should these be discharged through a coil
(they
won't blow up will they)? Im aware of the electrical dangers (touch them
and die basically) but I'm not sure about the suitability for my
application.

If you have any info or advice then I'll be glad to here it.

Thanks in advance.
Click to expand...

Hi Steve,
I agree with John about predictability, but let me respectfully add this.
Your question implies that you have limited experience with high energy
devices that can kill you. Please get sound advice before "messing" with 400
Volt capacitors. Coil guns make an interesting and attractive project, they
may also be classified as firearms in your location.
I won't say that these concerns ever stopped me, but I did ask a lot of
questions when I was over my head, and I still do.
Best Regards,
Tom
 
"Steve" <steve@steve.com> wrote in message
news:ce3c2c$rc4$1@news8.svr.pol.co.uk...
Hi,

I've got 4 capacitors with the following specs:

CAPACITOR, 4700UF 400V;
Capacitor dielectric type: Aluminium Electrolytic;
Temperature, operating (a) max: 85°C;
Temperature, operating (a) min: -40°C;
Time, operating life: 3000h;
Current, ripple AC @ 100Hz: 14.22A;
Current, ripple AC @ 10kHz: 19.91A;
Frequency, ESR: 100Hz;
Resistance, ESR: 38mR;
Temperature, ESR: 20°C;
Temperature, operating life: 85°C;
Temperature, ripple current: 85°C;
Tolerance, +: 20%;

I plan to combine the 4 to make a large capacitor of 800V @ 9400uF (2
pairs
in parallel connected together in series). I was then hoping to use this
large capacitor in a coilgun (e.g. charge them up and discharge them
through
a coil). My question is, are these capacitors suitable for such an
application? In particular, should these be discharged through a coil
(they
won't blow up will they)? Im aware of the electrical dangers (touch them
and die basically) but I'm not sure about the suitability for my
application.

If you have any info or advice then I'll be glad to here it.

Thanks in advance.
Click to expand...
Thanks for the replies.

I thought the 38 milliohm internal resistance was a plus point if these caps
are used for a device such as a coilgun. What would you suggest as a 'good'
internal resistance? Would the internal heat cause problems? The caps
would be charged up very slowly and then discharged (fired).

Thanks again.
 
"Steve" <steve@steve.com> wrote in message
news:ce3c2c$rc4$1@news8.svr.pol.co.uk...
Hi,

I've got 4 capacitors with the following specs:

CAPACITOR, 4700UF 400V;
Capacitor dielectric type: Aluminium Electrolytic;
Temperature, operating (a) max: 85°C;
Temperature, operating (a) min: -40°C;
Time, operating life: 3000h;
Current, ripple AC @ 100Hz: 14.22A;
Current, ripple AC @ 10kHz: 19.91A;
Frequency, ESR: 100Hz;
Resistance, ESR: 38mR;
Temperature, ESR: 20°C;
Temperature, operating life: 85°C;
Temperature, ripple current: 85°C;
Tolerance, +: 20%;

I plan to combine the 4 to make a large capacitor of 800V @ 9400uF
(2 pairs
in parallel connected together in series). I was then hoping to use
this
Click to expand...

You have the math wrong. two parallel caps gives you twice the
capacitance, but if you then series the result, you'll get 1/2 the
capacitance of the result, yielding 4700uF at 800V max. You would need
4 more of these caps to get 9400uF.

Also, note that the caps may not take on charge at the same rate,
meaning that if you build a series of caps like this, you need to
ensure that they are actually balanced by putting some resistors in
parallel with each of them. Otherwise, you'll find that one of the
series pair will have more voltage across it, possibly causing
catastrophic failure.

Regards,
Bob Monsen
 
On Mon, 26 Jul 2004 17:38:32 +0100, "Steve" <steve@steve.com> wrote:

Hi,

I've got 4 capacitors with the following specs:

CAPACITOR, 4700UF 400V;
Capacitor dielectric type: Aluminium Electrolytic;
Temperature, operating (a) max: 85°C;
Temperature, operating (a) min: -40°C;
Time, operating life: 3000h;
Current, ripple AC @ 100Hz: 14.22A;
Current, ripple AC @ 10kHz: 19.91A;
Frequency, ESR: 100Hz;
Resistance, ESR: 38mR;
Temperature, ESR: 20°C;
Temperature, operating life: 85°C;
Temperature, ripple current: 85°C;
Tolerance, +: 20%;

I plan to combine the 4 to make a large capacitor of 800V @ 9400uF (2 pairs
in parallel connected together in series). I was then hoping to use this
large capacitor in a coilgun (e.g. charge them up and discharge them through
a coil). My question is, are these capacitors suitable for such an
application? In particular, should these be discharged through a coil (they
won't blow up will they)? Im aware of the electrical dangers (touch them
and die basically) but I'm not sure about the suitability for my
application.

If you have any info or advice then I'll be glad to here it.

Thanks in advance.
Click to expand...
When I was a kid, I used to cobble together bunches of old
electrolytic caps, charge them to 350 volts or so, and discharge them
hard into few-turn coils to magnetize things, or fire surplus runway
flashtubes. Never had a cap fail or blow up or anything as a result of
high-current discharge. But to be prudent, keep the caps in some sort
of sturdy box, just in case one decides to pop.

What are you going to use for a switch? I just made sort of a knife
switch out of #12 house wiring; it welded itself closed every shot and
I had to pry it open.

For max bang, keep the wiring resistance and inductance down.

Fun experiment: get a flashtube that can handle a few hundred joules.
Aim it at a newspaper on the floor. Dark adapt in a very dark room,
stare at the paper, and flash. The sharp, bright afterimage is
astounding.

I bet somebody out there has paralleled hundreds of caps from
disposable cameras. A PC board to do this (modular, bolt-together)
would be cool. A kilojoule or so would make a satisfying bang.

John
 
"Robert C Monsen" <rcsurname@comcast.net> a écrit dans le message de
news:6%cNc.162022$a24.82074@attbi_s03...
"Steve" <steve@steve.com> wrote in message
news:ce3c2c$rc4$1@news8.svr.pol.co.uk...
Hi,

I've got 4 capacitors with the following specs:

CAPACITOR, 4700UF 400V;
Capacitor dielectric type: Aluminium Electrolytic;
Temperature, operating (a) max: 85°C;
Temperature, operating (a) min: -40°C;
Time, operating life: 3000h;
Current, ripple AC @ 100Hz: 14.22A;
Current, ripple AC @ 10kHz: 19.91A;
Frequency, ESR: 100Hz;
Resistance, ESR: 38mR;
Temperature, ESR: 20°C;
Temperature, operating life: 85°C;
Temperature, ripple current: 85°C;
Tolerance, +: 20%;

I plan to combine the 4 to make a large capacitor of 800V @ 9400uF
(2 pairs
in parallel connected together in series). I was then hoping to use
this

You have the math wrong. two parallel caps gives you twice the
capacitance, but if you then series the result, you'll get 1/2 the
capacitance of the result, yielding 4700uF at 800V max. You would need
4 more of these caps to get 9400uF.

Also, note that the caps may not take on charge at the same rate,
meaning that if you build a series of caps like this, you need to
ensure that they are actually balanced by putting some resistors in
parallel with each of them. Otherwise, you'll find that one of the
series pair will have more voltage across it, possibly causing
catastrophic failure.

Regards,
Bob Monsen
Click to expand...
Plus add a *huge* diode antiparallel with each caps pair, as you are not
guaranteed they'll have the same capacitance. When shorting the bank, you'll
have one pair that will see a negative voltage across it. If they differ by
say 10%, then you'll have a reverse polarity 40V across a pair. Not
something you want to happen.

BTW, I believe that internal mechanical strains will be much more a problem
than the low duty cycle heating of the caps.


--
Thanks,
Fred.
 
"Steve" <steve@steve.com> wrote in message
news:ce3c2c$rc4$1@news8.svr.pol.co.uk...
Hi,

I've got 4 capacitors with the following specs:

CAPACITOR, 4700UF 400V;
Capacitor dielectric type: Aluminium Electrolytic;
Temperature, operating (a) max: 85°C;
Temperature, operating (a) min: -40°C;
Time, operating life: 3000h;
Current, ripple AC @ 100Hz: 14.22A;
Current, ripple AC @ 10kHz: 19.91A;
Frequency, ESR: 100Hz;
Resistance, ESR: 38mR;
Temperature, ESR: 20°C;
Temperature, operating life: 85°C;
Temperature, ripple current: 85°C;
Tolerance, +: 20%;

I plan to combine the 4 to make a large capacitor of 800V @ 9400uF (2
pairs
in parallel connected together in series). I was then hoping to use this
large capacitor in a coilgun (e.g. charge them up and discharge them
through
a coil). My question is, are these capacitors suitable for such an
application? In particular, should these be discharged through a coil
(they
won't blow up will they)? Im aware of the electrical dangers (touch them
and die basically) but I'm not sure about the suitability for my
application.

If you have any info or advice then I'll be glad to here it.

Thanks in advance.
Click to expand...
Yes you're right, I do have the maths wrong, my mistake.

For a switch I'm planning on using SCR's. Everything is going to be
controlled and monitored via a microcontroller. I work as a C programmer,
so my high voltage electronics knowledge is very limited (as you've probally
already guessed). I want to make a coilgun just out of interest and for
fun.

I can understand the need for resistors and diodes. I'll make sure
everything is safe and correct before applying any power.

If I buy some new capacitors, then what properties should I look out for
(other than voltage and capacitance)? Are the ESR resistance and ripple
current the most important things? If so, what sort of values should they
have? If anyone can recommend any particular capacitors that would be
great, I don't mind spending upto Ł150 on caps. I tend to buy most of my
stuff from RS and Farnell (I'm based in England).

Thank you for your time.
 
Steve wrote:
"Steve" <steve@steve.com> wrote in message
news:ce3c2c$rc4$1@news8.svr.pol.co.uk...
Hi,

I've got 4 capacitors with the following specs:

CAPACITOR, 4700UF 400V;
Capacitor dielectric type: Aluminium Electrolytic;
Temperature, operating (a) max: 85°C;
Temperature, operating (a) min: -40°C;
Time, operating life: 3000h;
Current, ripple AC @ 100Hz: 14.22A;
Current, ripple AC @ 10kHz: 19.91A;
Frequency, ESR: 100Hz;
Resistance, ESR: 38mR;
Temperature, ESR: 20°C;
Temperature, operating life: 85°C;
Temperature, ripple current: 85°C;
Tolerance, +: 20%;

I plan to combine the 4 to make a large capacitor of 800V @ 9400uF (2
pairs
in parallel connected together in series). I was then hoping to use this
large capacitor in a coilgun (e.g. charge them up and discharge them
through
a coil). My question is, are these capacitors suitable for such an
application? In particular, should these be discharged through a coil
(they
won't blow up will they)? Im aware of the electrical dangers (touch them
and die basically) but I'm not sure about the suitability for my
application.

If you have any info or advice then I'll be glad to here it.

Thanks in advance.



Thanks for the replies.

I thought the 38 milliohm internal resistance was a plus point if these caps
are used for a device such as a coilgun. What would you suggest as a 'good'
internal resistance? Would the internal heat cause problems? The caps
would be charged up very slowly and then discharged (fired).

Thanks again.
Click to expand...
The 38 milliohms is not a problem, per se. The problem may be that
much of that resistance occurs in the welds between bits of aluminum
that connects the terminals to the wound foil. You may simply fuse
these tabs off the capacitor the first time you dump the charge.

--
John Popelish
 
Steve wrote:
"Steve" <steve@steve.com> wrote in message
news:ce3c2c$rc4$1@news8.svr.pol.co.uk...
Hi,

I've got 4 capacitors with the following specs:

CAPACITOR, 4700UF 400V;
Capacitor dielectric type: Aluminium Electrolytic;
Temperature, operating (a) max: 85°C;
Temperature, operating (a) min: -40°C;
Time, operating life: 3000h;
Current, ripple AC @ 100Hz: 14.22A;
Current, ripple AC @ 10kHz: 19.91A;
Frequency, ESR: 100Hz;
Resistance, ESR: 38mR;
Temperature, ESR: 20°C;
Temperature, operating life: 85°C;
Temperature, ripple current: 85°C;
Tolerance, +: 20%;

I plan to combine the 4 to make a large capacitor of 800V @ 9400uF (2
pairs
in parallel connected together in series). I was then hoping to use this
large capacitor in a coilgun (e.g. charge them up and discharge them
through
a coil). My question is, are these capacitors suitable for such an
application? In particular, should these be discharged through a coil
(they
won't blow up will they)? Im aware of the electrical dangers (touch them
and die basically) but I'm not sure about the suitability for my
application.

If you have any info or advice then I'll be glad to here it.

Thanks in advance.



Yes you're right, I do have the maths wrong, my mistake.

For a switch I'm planning on using SCR's. Everything is going to be
controlled and monitored via a microcontroller. I work as a C programmer,
so my high voltage electronics knowledge is very limited (as you've probally
already guessed). I want to make a coilgun just out of interest and for
fun.

I can understand the need for resistors and diodes. I'll make sure
everything is safe and correct before applying any power.

If I buy some new capacitors, then what properties should I look out for
(other than voltage and capacitance)? Are the ESR resistance and ripple
current the most important things? If so, what sort of values should they
have? If anyone can recommend any particular capacitors that would be
great, I don't mind spending upto Ł150 on caps. I tend to buy most of my
stuff from RS and Farnell (I'm based in England).

Thank you for your time.
Click to expand...
All other things being equal, I would go for high RMS ripple current
rating worry about ESR secondly. If you can find capacitors rated for
peak current, you probably have something designed for pulse
applications.

I would also try to figure out how to use just parallel combinations,
and eliminate the series connections. It raises the effective ESR and
has both charging and discharge problems that others have spoken
about. You should be able to change the winding of the coils and
achieve the same energy peak with lower voltage caps, with only the
switch current being a problem to solve.

--
John Popelish
 
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