At my job I work with alarm panels.
One of them has an output for an 8 ohm speaker and the recommended wattage is 15.
Months ago I took apart a very old computer I use to own. I have an 8 ohm 0.5 watt speaker I haven’t found a use for, until now. I think if I add enough resistance I could connect it to the speaker output and use it (without seeing any smoke :eek:).
How much resistance would I need?
You could connect it up, but personally I’d just buy a bigger speaker. It would save you dumping most of the energy straight into heat.
Moved MPSIMS --> IMHO.
Manhours to source and install (plus potential cost of) sufficient resistance will be greater than the cost of a new speaker.
15 watts into an 8 ohm speaker means the output voltage is about 11 volts. Your 0.5 watt speaker has a max current of 0.25 amps. To get that amount of current from an 11 volt source you need a total resistance of about 44 ohms, which includes your speaker in series. Subtract the 8 ohms for your speaker and your series resistor needs to be about 36 ohms. It needs to be able to dissipate 2.25 watts of heat.
That’s a lot less resistance than I thought. Thanks.
I already have use of a 15 watt, 8 Ohm speaker if I want, but for a few reasons I’d really rather use my 0.5 watt speaker.
8 ohms in this instance is impedance, not resistance, and the 8 ohms in just the nominal value. If you were to chart the actual impedance of the speaker by frequency across the audible range it would vary significantly from the nominal value.
The speaker output from the panel is unlikely to be anywhere near 15 watts. I’ll guess around 5. The recommended spec will have ample headroom built in.
Simply adding resistance is going to dramatically decrease the volume coming out of the speaker - by 15dB if the math done in above posts is correct.
Most of the energy in an electrical audio signal is in the low frequencies. You’d probably do as much or more to protect the speaker with a high-pass filter at 500Hz or so, which I believe you could accomplish with an appropriate capacitor - at the cost of extremely tinny sound.
Doesn’t really cost much to experiment, though, so have at it. 
One of the reasons I want to use my tiny speaker is because of the low volume.
I could buy some kind of volume control for the larger speaker, but I have access to free resistors at work plus I’ll have a use for my tiny speaker instead of having to throw it out.
Power is drawn, not provided. If the impedance matches, the lower power should be okay; they’ll just draw less power.
According to Ohm’s law, to get voltage it’s the square root of Watts * Ohms. So my little 15Watt, 8Ohm speaker can handle…wow, 10V. That’s a lot more than I thought.
Oh yeah, engineer_comp_geek, I forgot that you calculated 11V. Sorry about that.
Oh wait, I’m a moron. My speaker is .5Watt, 8Ohm, so it can handle 2V. Yeah, I think I’ll to use a resistor.
8 * .5 = 4
Sqr(4) = 2