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# How to measure capacitance?

What is the most inexpensive way that I can measure capacitance? I have some photoflash capacitors that aren't marked with their capacitance. I went to radio shack, and the cheapest multimeter that measured it was around 60 dollars. Is there a cheap way that I can find out the rating of my capacitors?

You're obviously quite smart in this field, so I pose a question to you.

How do I find out how much capacitance I need? Is there an equation? I need 5v discharged over 2 seconds from a capacitor for my diy project, but no one can shoot me a straight answer to the value of the capacitance that I need. Any suggestions?

Jacek

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Jacek, check out this page:

http://www.learningaboutelectronics.com/Articles/Capacitor-discharge-calculator.php

and use the formula provided. There you enter the 5 volts as the initial voltage and then put time equals 2 seconds. Then you can solve for the RC value.

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I don't know how to measure the capacitance of a capacitor accurately but I can estimate the capacitance of a capacitor near to the actual value with cheap parts.

All you need to do is to connect a resistor with known value( in Mega ohm), a digital multimeter and the capacitor which is going to be measured in parallel.

Before you connect the capacitor with the resistor and the multimeter in parallel,

charge the capacitor with a known constant voltage source(battery work best). After the capacitor is fully charged, prepare a stopwatch and let the stopwatch start counting as soon as you connected the capacitor in parallel.

Set a voltage reference in such a way that you stop the stopwatch counting after you observed the value of the voltage reference displayed by the multimeter , ie 100mV. Record the time taken. Record the data as follows:

Initial Voltage, Vo = voltage of power source, in Volt

Resistance, R = Resistance of the resistor used, in ohm

Final voltage, V = voltage reference, in Volt

Time taken, t = The time when the stopwatch stop counting, in seconds

Then use the formula V = Vo x e^(-t/CR)

ln(V) = ln(Vo)-t/CR

ln(V/Vo) = -t/CR

ln(Vo/V) = t/CR

Finally,

C = t/[ln(Vo/V)R], in Farad

Since the capacitor and multimeter have internal resistance, the measured value will be slightly differ from the actual value. I tried it before and it does estimate the capacitance of an unknown capacitor.

Hope this can help.

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What if during exam, i can`t remember the formula????

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some good info

method one on that page is with an RC time circuit. he points out that you would never be able to actually measure this time because it’s so small, but that’s not true if you have a microcontroller like a basic stamp 2.

in an RC time circuit, the resistance in ohms multiplied by the capacitance in Farads equals time in seconds. therefore, capacitance equals seconds divided by resistance. C = T/R

here’s how the circuit would look:

—————————> to input pin of basic stamp

| |

unknown C known R

| |

—————————> to ground

so what you do is, you set the pin high for a few milliseconds to charge the cap. then you change the pin to an input, start a counter, and tell the basic stamp to watch for a logic low.

of course, you would need to “calibrate” your basic stamp first, by measuring how long it takes to go through the counting area of code. for example, tell it to go through a counting loop like ten thousand times, then time how long it takes with a stopwatch. a higher number will give you a good average after doing the calibration a few times. the basic stamp operates in milliseconds, so ten thousand loops might be close to ten seconds, but it depends on the complexity of the loop and a bunch of chaos theory variables.

so anyway, now that you know how long each count takes in your counter, you can insert the actual time in seconds into the equation to find a very good approximation of capacitance.

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I would go to www.dealextreme.com and order a cheap multimeter there. You can get one with capacitance function for 12$ or less with free shipping.

The circuit orksecurity describes is a lot of fun to make, and costs less than a dollar (schmitt trigger hex inverters are great, 555 timers will work too), although in my experience you need an oscilloscope (more expensive than a multimeter), or a multimeter with a frequency count function to use it with acceptable convenience and accuracy. You can keep switching in and out a bunch of known capacitances until you can't hear a signal too... but that method does not appeal to me much!

A cheaper and more convenient method might be to MAKE SURE they are discharged first, then charge them with a 9v or 12v battery, and connect them in series to a resistor and an LED. You will have to know how to do math and use a stopwatch.

From the LED datasheet you can see the voltage drop across the LED (or use a multimeter with a diode test function). When the applied voltage drops below this threshold it will turn off. The voltage output of a capacitor across a resistor is a decay function as described here: http://en.wikipedia.org/wiki/Capacitor#DC_circuits

You are basically using the LED as a primitive voltage indicator. I suggest red due to low theshold voltage. The time the LED stays on will let you calculate the capacitance, as you now know the time it took to decay from a known initial voltage to a known final voltage across a known resistance. A bigger resistor will give more accurate results (due to the internal resistance of the capacitor and longer times the LED will be on) but the LED will be dimmer; find a good balance. Try a few hundred ohms first.

Of course if you have a multimeter with a voltage measurement function, just connect it in parallel across the capacitor instead (at a 9v or 12v charge this is no problem), this will be much more sensitive than an LED. You will see the voltage drop as it decays across the resistor and you can time it to some arbitrary point.

In my (limited) experience and (very limited) memory, camera flash capacitors are rated somewhere near 330v and vary in capacitance from approximately 100 to 300 microfarads. Knowing the capacitance does not tell you the safe voltage rating, be VERY AWARE of this. Connecting them in series and naively assuming this increases the voltage tolerance can also lead to disastrous failures...

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Build an oscillator whose frequency depends on the capacitance. Calibrate using known capacitors. Hook up the one you want to test and compare. If it's too fast or too slow for you to hear, replace with larger or smaller resistor and try again.

If you understand the math behind the circuit, you can probably go fairly directly from frequency produced by the oscillator to capacitance.

Caveat: Photoflash caps may be able to store unpleasantly large amounts of energy. Never assume a large capacitor is safe until after you've shorted the leads.

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