Forums

simple capacitor test circuit

Started by Unknown March 17, 2013
Hello, I am new to discussion groups. I have a home built capacitor (it sho=
uld have a fairly large capacitance) and I would like to find out what its =
value is as accurately as possible with a standard multimeter. I wonder if =
it is possible to build a simple circuit with a led light indicator or some=
thing that with a little math and good capacitors can tell me ruffly how la=
rge it is. Please help. I only need to get a couple of measurements for my =
project.
On Sun, 17 Mar 2013 17:06:18 -0700 (PDT), diablosdemon7272@gmail.com wrote:

>Hello, I am new to discussion groups. I have a home built capacitor (it should have a fairly large capacitance) and I would like to find out what its value is as accurately as possible with a standard multimeter. I wonder if it is possible to build a simple circuit with a led light indicator or something that with a little math and good capacitors can tell me ruffly how large it is. Please help. I only need to get a couple of measurements for my project.
What's the ballpark capacitance that you expect? Is it polarized? -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On 3/17/2013 7:06 PM, diablosdemon7272@gmail.com wrote:
> Hello, I am new to discussion groups. I have a home built capacitor (it should have a fairly large capacitance)
and I would like to find out what its value is as accurately as possible with a standard multimeter. I wonder if it is possible to build a simple circuit with a led light indicator or something that with a little math and good capacitors can tell me ruffly how large it is. Please help. I only need to get a couple of measurements for my project.
>
Charge it to a voltage, say 30 volts, then put a resistor across it's terminals and see how long it takes to discharge to 10 volts. Compare that to known capacitors, add capacitance until the times are equal. Or, do a search on RC time constant and you can calculate, using the resistor (R) you pick and the time. Discharge to 7.5 volts is 3 time constants. These pages discus RC time constants.
> http://www.electronics-tutorials.ws/rc/rc_1.html
> http://www.tpub.com/neets/book2/3d.htm
> http://www.kean.edu/~asetoode/home/tech1504/acdc/rctc.htm
The discharge time will depend on the capacitance of your capacitor and the resistor you choose. If you think your capacitor is not very leaky, I would make the time for discharge 20 or 30 seconds to improve your measurement accuracy. What is the input impedance of your meter, or what kind of meter do you have. How did you build your capacitor? Mikek
diablosdemon7272@gmail.com wrote:

> Hello, I am new to discussion groups. I have a home built capacitor (it should have a fairly large capacitance) and I would like to find out what its value is as accurately as possible with a standard multimeter. I wonder if it is possible to build a simple circuit with a led light indicator or something that with a little math and good capacitors can tell me ruffly how large it is. Please help. I only need to get a couple of measurements for my project.
Don't have any idea how large is large on your end? But, you can use a resistor to charge the cap and time it for large types. Time_TO_Charge_To_63.2% = R * C; So, if you connect lets say a 1 volt supply through a 1k ohm R to the cap and have the other end of the cap back to the opposite terminal of the supply and then, measure the time it takes to get to 0.632 volts. So Time_Calculated / R = C . To start the test, you just short the cap and as soon as you release the short, start the timer. I am just assuming you have a supper cap!... Just a thought.... Jamie
On Sun, 17 Mar 2013 17:30:21 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>On Sun, 17 Mar 2013 17:06:18 -0700 (PDT), diablosdemon7272@gmail.com wrote: > >>Hello, I am new to discussion groups. I have a home built capacitor (it should have a fairly large capacitance) and I would like to find out what its value is as accurately as possible with a standard multimeter. I wonder if it is possible to build a simple circuit with a led light indicator or something that with a little math and good capacitors can tell me ruffly how large it is. Please help. I only need to get a couple of measurements for my project. > >What's the ballpark capacitance that you expect? Is it polarized?
--- A polarized home built capacitor? Get real! -- JF
Thanks for all the feedback! I was actually thinking of some kind of simple=
 timing circuit with a led light indicator but I think I can figure somethi=
ng out with those methods.

I actually have two capacitors to measure. Each are made with 2 sheets of s=
tainless steel foil with 2mm thick corrugated plastic as a spacer on the en=
ds of the foil (the center is not supported by the spacers). They are wound=
 around each other on a PVC pipe in a double coil so they have two plates b=
ut they have about 3/4 more plate surface area. I hope that makes sense. Th=
e dielectric will be tap water so I plan on them being very leaky.. The sma=
ller coil plates (foil) are 12cm by 22cm (264 square cm). the larger one ar=
e double the length at 12cm by 44cm (528 square cm).
On Sun, 17 Mar 2013 19:58:09 -0500, John Fields <jfields@austininstruments.com>
wrote:

>On Sun, 17 Mar 2013 17:30:21 -0700, John Larkin ><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >>On Sun, 17 Mar 2013 17:06:18 -0700 (PDT), diablosdemon7272@gmail.com wrote: >> >>>Hello, I am new to discussion groups. I have a home built capacitor (it should have a fairly large capacitance) and I would like to find out what its value is as accurately as possible with a standard multimeter. I wonder if it is possible to build a simple circuit with a led light indicator or something that with a little math and good capacitors can tell me ruffly how large it is. Please help. I only need to get a couple of measurements for my project. >> >>What's the ballpark capacitance that you expect? Is it polarized? > >--- >A polarized home built capacitor? > >Get real!
Why not? People used to build homemade electrolytic rectifiers. Think about using anodized aluminum, or aluminum foil, to make your own electrolytic cap. He did say that he expects a "large" capacitance, whatever that means. No harm in asking. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On 3/17/2013 8:07 PM, diablosdemon7272@gmail.com wrote:
> Thanks for all the feedback! I was actually thinking of some kind of simple timing circuit with a led light indicator but I think I can figure something out with those methods. > > I actually have two capacitors to measure. Each are made with 2 sheets of stainless steel foil with 2mm thick corrugated plastic as a spacer on the ends of the foil (the center is not supported by the spacers). They are wound around each other on a PVC pipe in a double coil so they have two plates but they have about 3/4 more plate surface area. I hope that makes sense. The dielectric will be tap water so I plan on them being very leaky.. The smaller coil plates (foil) are 12cm by 22cm (264 square cm). the larger one are double the length at 12cm by 44cm (528 square cm). >
Hmm, I'm not sure that will be as big as I was envisioning. Let us know. Mikek
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message 
news:8hqck85rqcn5uuuinlmfnm987ag3aqgupm@4ax.com...
> On Sun, 17 Mar 2013 19:58:09 -0500, John Fields > <jfields@austininstruments.com> > wrote: > >>On Sun, 17 Mar 2013 17:30:21 -0700, John Larkin >><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: >> >>>On Sun, 17 Mar 2013 17:06:18 -0700 (PDT), diablosdemon7272@gmail.com >>>wrote: >>> >>>>Hello, I am new to discussion groups. I have a home built capacitor (it >>>>should have a fairly large capacitance) and I would like to find out >>>>what its value is as accurately as possible with a standard multimeter. >>>>I wonder if it is possible to build a simple circuit with a led light >>>>indicator or something that with a little math and good capacitors can >>>>tell me ruffly how large it is. Please help. I only need to get a couple >>>>of measurements for my project. >>> >>>What's the ballpark capacitance that you expect? Is it polarized? >> >>--- >>A polarized home built capacitor? >> >>Get real! > > Why not? People used to build homemade electrolytic rectifiers. Think > about > using anodized aluminum, or aluminum foil, to make your own electrolytic > cap. > > He did say that he expects a "large" capacitance, whatever that means. No > harm > in asking. > > > > > --
Hey, it's even worse than you were thinking :).
On Sun, 17 Mar 2013 21:49:15 -0400, "tm" <No_one_home@white-house.gov> wrote:

> >"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message >news:8hqck85rqcn5uuuinlmfnm987ag3aqgupm@4ax.com... >> On Sun, 17 Mar 2013 19:58:09 -0500, John Fields >> <jfields@austininstruments.com> >> wrote: >> >>>On Sun, 17 Mar 2013 17:30:21 -0700, John Larkin >>><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: >>> >>>>On Sun, 17 Mar 2013 17:06:18 -0700 (PDT), diablosdemon7272@gmail.com >>>>wrote: >>>> >>>>>Hello, I am new to discussion groups. I have a home built capacitor (it >>>>>should have a fairly large capacitance) and I would like to find out >>>>>what its value is as accurately as possible with a standard multimeter. >>>>>I wonder if it is possible to build a simple circuit with a led light >>>>>indicator or something that with a little math and good capacitors can >>>>>tell me ruffly how large it is. Please help. I only need to get a couple >>>>>of measurements for my project. >>>> >>>>What's the ballpark capacitance that you expect? Is it polarized? >>> >>>--- >>>A polarized home built capacitor? >>> >>>Get real! >> >> Why not? People used to build homemade electrolytic rectifiers. Think >> about >> using anodized aluminum, or aluminum foil, to make your own electrolytic >> cap. >> >> He did say that he expects a "large" capacitance, whatever that means. No >> harm >> in asking. >> >> >> >> >> > >Hey, it's even worse than you were thinking :). >
Well. Ummm. Right. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators