** A good cap will charge - hold that charge, then let it loose with a
bang.
Good caps will NOT let loose any charge with a bang. If the cap lets
the charge loose with a bang the cap is stuffed and probably has an
internal short. A good cap should hold a charge for hours.
Try charging the cap with a 600 volt 1 amp diode and 100 ohm 5 watt
resistor in series off the AC - be careful and have a safety switch on the
circuit !!!!
Extremely dangerous thing to recommend Phil. If the cap is shorted the
diode and resistor will smoke. The cap will potentially have 340V DC
(1.4 x 240VAC) on it, which is pretty lethal. A safety switch (I assume
you mean an RCD device) will not protect you against potential
electrocution if you touch both terminals of the cap.
Leave it for a few minutes then short the terminals with a clip lead -
should give a nice loud bang and spark.
This could destory the capacitor, and cause molten metal to fly from the
arc you create. The current in shorting a large capacitor charged to 340V
DC is only limited by its internal resistance and that of the clip lead,
and could be 100 amps or more. This could also melt the internal
connection of the cap if it was not designed for high current discharges.
If you really want to try this, PLEASE WEAR SAFETY GLASSES to prevent bits
of molten metal and exploding capacitor getting in your eyes.
A much safer way of doing this, if you have access to multimeter, would be
to charge the capacitor from a 9 Volt or 12 Volt battery, with a series
current limiting resistor (or from a current limited power supply if you
have one), and let the cap charge to the battery voltage. Check the
leakage by measuring the current into the cap. It should be less than a mA
or so after the cap has charged.
You could then get an estimate of the capacitance by discharging the cap
into a know resistance. The cap will discharge to approx 37% of its inital
voltage in one time constant, which is the product of R and C. If you
discharged it with a 100K resistor, the voltage should be 37% of the
starting voltage after approx 6.7 seconds. You may need to take into
account the resistance of the voltmeter. By measuring how long it takes to
discharge you can work out its capacitance.
David