On Sun, 21 Sep 2008 21:24:09 +0100, "ian field"
put finger to keyboard and composed:


"Franc Zabkar" wrote in message
.. .
On Sun, 21 Sep 2008 15:54:26 +0100, "ian field"
put finger to keyboard and composed:

Finding a way to reduce the duty cycle might be a means to get away with a
more commonly available lower impedance speaker.

AFAICS, at 25VDC a 40 ohm speaker would draw 600mA when the transistor
switches on, regardless of duty cycle, assuming the transistor has a
high enough gain at whatever base drive the IC provides. An MPS-A20
transistor is only rated for 100mA.

As the speech coil is an inductor it will exhibit a linear rising current
waveform from the initial application of voltage, obviously that will be
shorter for a lower impedance/inductance speech coil so the pulse width must
be made shorter to ensure it terminates before the inductor saturates.

I used Bob Parker's ESR meter to measure various 8 ohm speakers. AIUI,
the meter delivers 8us pulses to the device being tested.

FWIW, here are my results:

10W/8R - 35 ohms
2W/8R - 22 ohms
0.5W/8R - 15 ohms
20W/8R/20kHz tweeter - 18 ohms

- Franc Zabkar
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