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IBM 5150 PC speaker watts?

M

musicforlife

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Jan 10, 2018
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I noticed that in my IBM the pc speaker on the backside lable says 0.2w while in later machines like my 286 it's always 0.5w instead. Does this mean that you can't replace 0.2w speaker with 0.5w? Anyone knows why there's such difference?
 
Mostly a matter of what's available--there's little practical difference at the 5150's power level. There might be a slight difference in volume, if one, say were 8 ohm impedance vs. 16 ohms.
 
Have a look at the schematic and see what's driving it, likely a logic gate through a transistor to the 5v rail and an in-line resistor. So we just need the inline resistor value to determine the peak-to-peak voltage at the speaker, find the RMS value from that, divide by the speaker nominal impedance and there is your power output.
 
So it looks to be a 33R resistor inline.

Allowing for maybe 0.7V across the transistor, we get total 41R and 4.3V assuming 8R speaker. So peak-to-peak at the speaker terminals will be 8 * 4.3 / 41 = 0.84V, about 0.6V RMS. Then from Ohms law average current is 0.6/8 = 0.075A and as P=IV so the driver dissipates about 0.075 * 0.6 = 0.05W.
 
We've been here before within the last couple of months about why an XT speaker is louder than an AT one. It boils down to that series resistor--the AT uses a 100R, while the XT uses a 33R. Or maybe I'm remembering the post wrong.

Bottom line is that it doesn't really matter. As far as power goes, note that the speaker's being driven with a square wave that goes from somewhere around Vcc to 0V. So average DC dissipation is about half that calculated.

In other words, no big deal.
 
It does have a low pass filter that will soften the square, I’m not sure sure how effective it is.
 
I think you're right, it's only a 0.01uF cap according to the schematic so effective at about 100kHz or so. Probably there only for the purpose of EMI suppression.
 
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