Metronome

[zachtheterrible]

Hi, I'm using this here metronome to test capacitors. you just plug the capacitor into the socket, and depending on the value, the speaker will produce a series of clicks, or whine.

When I plug a .01 uf capacitor into the circuit, it will buzz, which is normal. Then when I plug a .047 uf cap into it, it melted the bloody speaker, and the transistors got very hot. Anyway, I take it that this means the capacitor is bad, but why does it do this?

Is using a metronome a good way to test capacitors?

P.S. sorry for the cruddy drawing. does anyone know how to publish a picture using the simulation program Crocodile Tech?

 

[Phasor]

You haven't shown the polarities of your transistors, however I assume the left-hand one in NPN, and the right-hand one is PNP, is this correct?

If the capacitor is removed, current feeds directly into the base of the NPN, and the combined gain of the transistors causes a large DC current to flow in the PNP transistor and speaker. Your transistor may not be rated for this current (possibly 1-2A). Also speakers are not fond of DC. Remember that a speaker, as far as DC current is concerned, is just a low value resistor. It will heat up just like a resistor, the power being dissipated in it, equal to I^2*R. As the speaker is composed of very fine wire, the wire quickly melts when heated.

I would consider this to be a very poor circuit to use for capacitor testing.

 

[grrr_arrghh]

in crock tech, use the print screen button, then paste into a graphics program, then save as a .gif

err, croc is a pretty poor simulation software - i'd advise you look at something else...