simple capacitor test circuit

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?

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.

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).

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.

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.

timing circuit with a led light indicator but I think I can figure something out with those methods.

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

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

Hey, it's even worse than you were thinking :).

Well. Ummm. Right.

Compute the nominal capacitance using the plate capacitor formula and a dielectric constant of 80.

Are you doing high-voltage pulse forming? There is a big literature.

Joe Gwinn

It may be one of those free energy things.

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I remember seeing that calculation somewhere.. Thanks for reminding me abou t it. I am not sure what you mean by pulse forming and I am not quite sure what I am intending on. All I know is I will be using a pulsed (I have to f igure out an ideal frequency to step charge the capacitor) DC power supply at about 3KV or higher with a bifilar wound coil and one of these as a capa citor and maybe a choke coil in there.. The goal is to continuously resonat e the tank circuit.

So like, Tesla coil stuff?

You may find the capacitor is lossier than the coil, and consuming all the power you're trying to push into it means it won't resonate very well. Water has a noticable dielectric loss tangent (think of it this way: it's a *really bad* capacitor all the way up at microwave frequencies, in fact it's an excellent resistor!).

Tim

What made you choose water?

perhaps build ac bridge and compare them with a known capacitor.

for some examples of that approach google "555 capacitance meter"

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).

tap water dielectric won't withstand voltages over 1.48V so the 555 approach isn't going to work, perhaps you can put them in series with a smaller capacitor that's good to 10V or so and measure the pair.

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.

He lives in the South Pole...

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.

--- If you put together a capacitive voltage divider like this:

(View with a fixed-pitch font)

AC>---+---E1 | [C1] | +---E2 | [C2] | AC>---+---GND

Then, if C1 is the known good capacitor and C2 is your home-made capacitor,

E2 Xc1 Xc2 = --------- (1) E1 - E2

Where X is the reactance of the capacitor, in ohms, and is equal to:

with f being equal to the frequency of the AC source, in Hz, and C being equal to the capacitance of the capacitor in farads.

For example, say:

E1 = 12VRMS, E2 = 6VRMS, C1 = 100nF, and f = 60Hz.

Then 1 1 Xc1 = --------- = ---------------------- = 26539 ohms. 2pi f C 6.28 * 60Hz * 1e-7F

Plugging that reactance into (1) gives us:

Then rearranging (2) to solve for C2, we have:

-- JF

Just using the 528 mm^2, 2 mm separation, and 80 for a dielectric constant, it calculates out to about 187 pF.

Not very large in capacitance and, with tap water for a shunt resistance, you probably won't even be able to measure the capacitance.

He said 12 by 44 cm, not mm. So that should be about 19nF. With tap water, it would still be more of a resistor than a capacitor.

To the OP: What are you trying to do with all this?

Jeroen Belleman