| Electronics > Beginners |
| need a speaker a resistor? |
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| hamster_nz:
--- Quote from: Hero999 on May 01, 2018, 10:17:05 am ---It will work as originally drawn, but it doesn't mean it's a good design. The collector resistor on the speaker side, should be a lower value, to increase the volume and more importantly, the transistors need protection diodes, as their reverse base-emitter voltage ratings are likely being exceeded, which is bad for reliability. --- End quote --- Sheesh! you guys are a tough audience! Does it really have to be a great design? So what if it works for 10 minutes or 10 seconds. Let the guy/gal burn out a few transistors and learn from their mistakes. It isn't as though they are designing for SpaceX (not yet, but maybe one day with encouragement..). Give them credit for sketching out drawing a schematic that a random somebody else could follow to make something that did what they thought it should first time. That is a lot more than a lot of beginners seem to be able to do. |
| Zero999:
--- Quote from: hamster_nz on May 01, 2018, 10:29:48 am --- --- Quote from: Hero999 on May 01, 2018, 10:17:05 am ---It will work as originally drawn, but it doesn't mean it's a good design. The collector resistor on the speaker side, should be a lower value, to increase the volume and more importantly, the transistors need protection diodes, as their reverse base-emitter voltage ratings are likely being exceeded, which is bad for reliability. --- End quote --- Sheesh! you guys are a tough audience! Does it really have to be a great design? So what if it works for 10 minutes or 10 seconds. Let the guy/gal burn out a few transistors and learn from their mistakes. It isn't as though they are designing for SpaceX (not yet, but maybe one day with encouragement..). Give them credit for sketching out drawing a schematic that a random somebody else could follow to make something that did what they thought it should first time. That is a lot more than a lot of beginners seem to be able to do. --- End quote --- That's all good and no one's detracting from any of it, but if no one told him how the design can be improved, then he'd never lean anything. |
| janoc:
--- Quote from: Hero999 on May 01, 2018, 10:17:05 am ---It will work as originally drawn, but it doesn't mean it's a good design. The collector resistor on the speaker side, should be a lower value, to increase the volume and more importantly, the transistors need protection diodes, as their reverse base-emitter voltage ratings are likely being exceeded, which is bad for reliability. --- End quote --- Don't over-complicate things ... The normal way how to build a multivibrator like this would be to put the loads in the collectors of the transistors, not emitters. Then you don't need any protection diodes because the BE junctions have no chance to ever be reverse polarized (bases can never get lower voltage than emitters, at best equal). Even the original circuit will work just fine - these transistors have Vbe_max of about 5V, the LED in the emitter will not cause a 5V voltage drop. OTOH, if this is to drive a speaker, then an extra amplification stage would be desirable. |
| Irukandji:
I tried the circuit without a resistor an then with a resistor and a potentiometer. Without a resistor i cant control the noise. It goes from slow to higher frequency and the speaker stunk after a short time.^^ with 325 Ohm resistor this speaker makes a quiet noise. With the 325 Ohm potentiometer the noise is much louder but when i turn it to much up the transistor or the potentiometer goes hot.^^ |
| janoc:
--- Quote from: Irukandji on May 01, 2018, 06:31:18 pm ---I tried the circuit without a resistor an then with a resistor and a potentiometer. Without a resistor i cant control the noise. It goes from slow to higher frequency and the speaker stunk after a short time.^^ --- End quote --- 9V / 8 \$\Omega\$ = 1.125A (disregarding the voltage drop across the transistor, that is negligible). Are you surprised that the speaker "stunk"? You have most likely damaged it too. --- Quote from: Irukandji on May 01, 2018, 06:31:18 pm ---with 325 Ohm resistor this speaker makes a quiet noise. With the 325 Ohm potentiometer the noise is much louder but when i turn it to much up the transistor or the potentiometer goes hot.^^ --- End quote --- See above. Neither the transistor nor the potentiometer are designed to handle such current - the BC547 is rated for 100mA maximum, you are trying to push 10x as much current thought it. If you let it go like that, the transistor will destroy itself. The same for a potentiometer - normal cheap carbon pots are good only for small currents, not an amp. Even wirewound potentiometer would be getting really hot in such case. If you want the speaker to be louder, you need either a higher impedance speaker (e.g. a piezo element) or an amplifier. |
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