Charging up a cap

L

Larry B

Guest
I have a 10,000mfd/16v electrolytic cap that I need to charge up. I am
using a 120vac/12vdc 240ma transformer to do this. About how long
should it take to complete the charge?

Thanks, Lar
 
"Civil Engineers are Queer" is the memonic that I remember from
4o years ago, meaning that C*V=q=It (actually the archaic form C*E=q)

solve for t:

t=C*V/I

First, figure how long it will take to charge to 12V from
a supply that has 12V effective output.

t = 10,000e-6 * 12 / 0.25 = 0.48 s

HOWEVER, a transformer rated at 0.24A will deliver much more than 0.24A
into a dead short, meaning that it will charge the cap to 12V in much
less
than 1/2 sec. The calculation above assumes full-wave rectification.

If the rectification is half wave, then current will
flow into the capacitor only half the time, meaning double the time.

ALSO, an unloaded capacitor will eventually charge to 1.4(14) * 12V,
but as the capacitor charges beyond 12V, the current will flow for
less and less of the full/half rectified cycle, approaching the
steady state asympotically, so you could say it will take forever.

You also need to consider things like the impedance of the transformer,
the forward drop in the diode(s), the effective series resistance of the
capacitor, the inductance of the wires, etc, etc.

You gotta give us some more info...

MikeM




Larry B wrote:
I have a 10,000mfd/16v electrolytic cap that I need to charge up. I am
using a 120vac/12vdc 240ma transformer to do this. About how long
should it take to complete the charge?

Thanks, Lar
Click to expand...
 
Larry B wrote:
I have a 10,000mfd/16v electrolytic cap that I need to charge up. I am
using a 120vac/12vdc 240ma transformer to do this. About how long
should it take to complete the charge?
Click to expand...
Hooking it up to the power supply and measuring the voltage would have
been way quicker than typing up your message.
That said, t=R*C , where R is the impeadance of your charging circuit. A
rule of thumb says it should be full in approx. 5*t.

Michael
 
"MikeM" <nowhere@nohow.com> wrote in message
news:3FB87A53.397781E6@nohow.com...
"Civil Engineers are Queer" is the memonic that I remember
Click to expand...
I think Civil Engineers might prefer Queen = Victoria Cross and then a bit
of maths.
 
"Michael Hofmann" <westbound@gmx.net> wrote in message
news:bp9udd$1kmg4u$1@ID-25863.news.uni-berlin.de...
Larry B wrote:
I have a 10,000mfd/16v electrolytic cap that I need to charge up.
I am
using a 120vac/12vdc 240ma transformer to do this. About how long
should it take to complete the charge?

Hooking it up to the power supply and measuring the voltage would
have
been way quicker than typing up your message.
That said, t=R*C , where R is the impeadance of your charging
circuit. A
rule of thumb says it should be full in approx. 5*t.

Michael
Click to expand...
Sorry my response took so long. Wife had surgery. AOK now.

I know enough to be dangerous and do not need an exact length of time
on the charging issue. I don't have most of the requested variables
(impedence of the transformer, wire inductance, forward drop of the
diode).

If it is true that overcharging (assuming a 14vdc charge into a 16vdc
cap) will only reach a "stasis" point and not damage the cap, what
would be a safe amount of charging time needed to get close to a full
charge (making typical assumptions for the unknowns).

Thanks, Lar
 
Larry B wrote:
"Michael Hofmann" <westbound@gmx.net> wrote in message
news:bp9udd$1kmg4u$1@ID-25863.news.uni-berlin.de...

Larry B wrote:

I have a 10,000mfd/16v electrolytic cap that I need to charge up.
using a 120vac/12vdc 240ma transformer to do this. About how long
should it take to complete the charge?

circuit. A
rule of thumb says it should be full in approx. 5*t.

what
would be a safe amount of charging time needed to get close to a full
charge (making typical assumptions for the unknowns).
Click to expand...
First approximation of t in your case is 1 second. So in about 5 secs it
should be fully charged.

Michael
 
"Michael Hofmann" <westbound@gmx.net> wrote in message
news:bpkgf4$1nu5fu$1@ID-25863.news.uni-berlin.de...
Larry B wrote:
"Michael Hofmann" <westbound@gmx.net> wrote in message
news:bp9udd$1kmg4u$1@ID-25863.news.uni-berlin.de...

Larry B wrote:

I have a 10,000mfd/16v electrolytic cap that I need to charge up.
using a 120vac/12vdc 240ma transformer to do this. About how long
should it take to complete the charge?

circuit. A
rule of thumb says it should be full in approx. 5*t.

what
would be a safe amount of charging time needed to get close to a
full
charge (making typical assumptions for the unknowns).

First approximation of t in your case is 1 second. So in about 5
secs it
should be fully charged.

Michael

I must have a bad cap. Once charged, if I short it to discharge it, I
Click to expand...
get no spark or snap.

Larry
 
You can test the cap using an analog volt meter, test for impedance, then reverse the
leads, the needle should bounce and then the cap would discharge. Usually works for me,
that and a brief visual inspection as most caps that I've seen have visibly failed
(overheated, cover has shrunk, etc.)


--
Myron Samila
Toronto, ON Canada
Samila Racing
http://204.101.251.229/myronx19
"Larry B" <hatespam@hatespam.gov> wrote in message
news:Vvqvb.8162$sb4.5618@newsread2.news.pas.earthlink.net...
"Michael Hofmann" <westbound@gmx.net> wrote in message
news:bpkgf4$1nu5fu$1@ID-25863.news.uni-berlin.de...
Larry B wrote:
"Michael Hofmann" <westbound@gmx.net> wrote in message
news:bp9udd$1kmg4u$1@ID-25863.news.uni-berlin.de...

Larry B wrote:

I have a 10,000mfd/16v electrolytic cap that I need to charge up.
using a 120vac/12vdc 240ma transformer to do this. About how long
should it take to complete the charge?

circuit. A
rule of thumb says it should be full in approx. 5*t.

what
would be a safe amount of charging time needed to get close to a
full
charge (making typical assumptions for the unknowns).

First approximation of t in your case is 1 second. So in about 5
secs it
should be fully charged.

Michael

I must have a bad cap. Once charged, if I short it to discharge it, I
get no spark or snap.

Larry
Click to expand...
 
the cap (which is brand new) reads open in one direction (+ to +).

"Myron Samila" <myronx19@removenospamhotmail.com> wrote in message
news:46uwb.4468$dt2.468352@news20.bellglobal.com...
You can test the cap using an analog volt meter, test for impedance,
then reverse the
leads, the needle should bounce and then the cap would discharge.
Usually works for me,
that and a brief visual inspection as most caps that I've seen have
visibly failed
(overheated, cover has shrunk, etc.)


--
Myron Samila
Toronto, ON Canada
Samila Racing
http://204.101.251.229/myronx19
"Larry B" <hatespam@hatespam.gov> wrote in message
news:Vvqvb.8162$sb4.5618@newsread2.news.pas.earthlink.net...

"Michael Hofmann" <westbound@gmx.net> wrote in message
news:bpkgf4$1nu5fu$1@ID-25863.news.uni-berlin.de...
Larry B wrote:
"Michael Hofmann" <westbound@gmx.net> wrote in message
news:bp9udd$1kmg4u$1@ID-25863.news.uni-berlin.de...

Larry B wrote:

I have a 10,000mfd/16v electrolytic cap that I need to charge
up.
using a 120vac/12vdc 240ma transformer to do this. About how
long
should it take to complete the charge?

circuit. A
rule of thumb says it should be full in approx. 5*t.

what
would be a safe amount of charging time needed to get close to
a
full
charge (making typical assumptions for the unknowns).

First approximation of t in your case is 1 second. So in about 5
secs it
should be fully charged.

Michael

I must have a bad cap. Once charged, if I short it to discharge
it, I
get no spark or snap.

Larry
Click to expand...
 
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