Determining Capacitance
Larry Brown
Chemistry/Physics course. Some of the capacitors have been damaged. I can
replace all components except for the capacitors. I see no recognizable
numbers on the disk capacitors which I have bought at RS. I also have not
found a simple circuit for determining the capacitance of them. Is there
something like an RC circuit, or must I set up an oscillator and use an
oscilloscope to tune the inductor to determine resonance and calculate the
capacitance? Obviously, I am not an electronics expert.
These kits are excellent, fitting with our curriculum which has included
simple electrical circuits, both 6 volt and 120 volt house-wiring, both on
home make kits. The problem is with damaged components.
Larry Brown
Donald E. Simanek
> We use Radio Shack 160-in-One electronics kits as part of our Freshman
> Chemistry/Physics course. Some of the capacitors have been damaged. I can
> replace all components except for the capacitors. I see no recognizable
> numbers on the disk capacitors which I have bought at RS.
Does this mean the numbers are unreadable, or you need to know the code?
Or, perhaps they have dots or stripes? Radio Shack used to sell a
handbook of data, tables and info which included component code schemes.
Typically there's a string of numbers, the first two giving two
significant figures, the third giving the multiplier, the result being in
picofarads. Sometimes they just write out the value in pf.
You'll be swamped with methods for measuring capacitance.
(1) Find the resistance of a multimeter on its volts scale. [Older analog
meters like the Simpson had about 2000 ohm, FET electronic multimeters may
be megohms. See instruction manual]. Connect capacitor to multimeter and
apply a voltage. across both (all in parallel). Disconnect the voltage,
and time the discharge. It's an RC circuit, with the meter supplying the
R.
(2) Find some known capacitors of the same type (disk--don't mix types)
and set up a capacitance comparison bridge (Wheststone, two arms known
resistors, two arms capacitors. Use AC generator at about 1000 Hz, low
voltage (2 to 5 volts is enough). Use sensitive AC meter as the null
detector, or use an oscilloscope for that.
(3) Put capacitor, square wave generator, and oscilloscope in parallel.
Set square wave generator so its period is several times the expected time
constant of C with the R of the oscilloscope. Observe the exponential
charge/discharge curves directly on the screen. Check against known
capacitor if you are unsure of the R of the oscilloscope with probe) It
should be megohms.
Usually the capacitors required in these circuits (in kits) are
predictable from their function. Coupling capacitors between amplifier
stages will be .1 or .01 microfarads. Filter capacitors in power supplies
(electrolytic capacitors) will be maybe 20 or 30 microfarads. You probaly
won't find picofarad capacitors except possibly in a radio receiver or
transmitter circuit, and they will be physically small, maybe 1 cm across.
A .1 microfarad disk capacitor will likely be smaller than a quarter.
Also, the values aren't generally critical to the performance of a circuit
except in those cases where the capacitor *is* part of an RC or LC
oscillator circuit.
> These kits are excellent, fitting with our curriculum which has included
> simple electrical circuits, both 6 volt and 120 volt house-wiring, both on
> home make kits. The problem is with damaged components.
Usually resistors get fried. *How* in the world are your students
damaging disk capacitors? Electrolytic capacitors can be destroyed by
applying reverse polarity--but *disk* capacitors? Never underestimate the
ability of a student to destroy something.
-- Donald
Scott Rippetoe
measure capacitances ranging from 20 microfarad down to picofarads. If
you do not have access to similar multimeters, take the kits back to
Radio Shack and ask them to measure the capacitance using one of their
digital multimeters.
Scott Rippetoe
Academy of Science and Technology