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.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

"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.

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.

If my inductance measurements make any sense, then the time constant
of the speaker would be L/R = 0.5mH/40R = 12us, or 80uH/40R = 2us.
That's a very short pulse.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

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
--
Please remove one 'i' from my address when replying by email.