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| need a speaker a resistor? |
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| Zero999:
--- Quote from: janoc on May 01, 2018, 10:00:06 pm --- --- Quote from: Hero999 on May 01, 2018, 08:17:21 pm ---There seems to be a fundamental lack of understanding of how the circuit works here. Please read the Wikipedia article: it's very good! https://en.wikipedia.org/wiki/Multivibrator#Operation --- End quote --- Duh, you are right, I didn't realize the capacitors get their positively charged plate (since when does a cap have an "anode" & "cathode"?) connected to the ground when the transistor turns on, so they are effectively presenting a negative voltage to the base of the other one. --- End quote --- Well a polarised capacitor certainly does have an anode and cathode, but I agree it was probably not the correct terminology. --- Quote ---On the other hand, I have never seen this circuit implemented with the protection diodes - and it is a classic, possibly one of the first things all kids starting with electronics used to do (these days it would be probably an Arduino something but I am old school). This is the first time I see someone claim that the diodes are necessary. How much is the breakdown a problem, really? When the breakdown occurs the BE junction behaves like a Zener diode (i.e. the breakdown is reversible) and the capacitor very rapidly discharges. The current spike could be large but for a very short time, so the thermal effects on the semiconductor (what could actually damage it) are going to be limited. Here a guy actually measured a BC546 in the breakdown regime: https://cr4.globalspec.com/thread/72501/Emitter-Base-Breakdown-Voltage-of-NPN-Used-as-Zener He used up to 100mA of current and doesn't report any adverse effects. There is another commenter there saying that most devices he tested couldn't handle more than about 3mA of reverse current - but that was also long time ago, with old components. Not sure how much does that apply to (semi-)modern transistors. And he also doesn't say for how long the current has been flowing. I would say that as long as the device isn't getting hot due to excessive currents, this isn't likely to cause problems in this application. --- End quote --- If the circuit is operated at a low voltage, then the transistors will never go into breakdown. There are other effects than thermal at work here: operating transistors in the zener breakdown region can permanently damage them and reduce their Hfe. I suppose this circuit will work with low gain transistors and the reverse bias pulses are short, so the damage doesn't affect the circut operation. I don't have much personal experience of this, because it's something I avoid. |
| janoc:
--- Quote ---If the circuit is operated at a low voltage, then the transistors will never go into breakdown. There are other effects than thermal at work here: operating transistors in the zener breakdown region can permanently damage them and reduce their Hfe. I suppose this circuit will work with low gain transistors and the reverse bias pulses are short, so the damage doesn't affect the circut operation. I don't have much personal experience of this, because it's something I avoid. --- End quote --- The article I have linked mentions substrate passivation which reduces the Hfe, but that requires quite a bit of current. Also the post mentions that the it is more likely to happen with the old, metal can transistors. Anyhow, it is easy to drop the power supply voltage to 5V or so or to choose transistors with higher Vbe reverse voltage if this should be a concern. |
| tpowell1830:
--- 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.^^ 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 --- Irukandji, as stated the transistor is rated at 100 mA, so 0.100 A=9V/R?. Using ohm's law we can rearrange this as 9/0.100=R?=90 ohms minimum, so a 100 ohm resistor or higher can be used. If you have a 325 ohm potentiometer, you can place in series with the 100 ohm resistor and change the volume. The 100 ohm will limit the maximum current when you have the potentiometer cranked to 0 ohms. Hope this helps... |
| Zero999:
--- Quote from: tpowell1830 on May 02, 2018, 04:31:58 pm --- --- 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.^^ 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 --- Irukandji, as stated the transistor is rated at 100 mA, so 0.100 A=9V/R?. Using ohm's law we can rearrange this as 9/0.100=R?=90 ohms minimum, so a 100 ohm resistor or higher can be used. If you have a 325 ohm potentiometer, you can place in series with the 100 ohm resistor and change the volume. The 100 ohm will limit the maximum current when you have the potentiometer cranked to 0 ohms. Hope this helps... --- End quote --- A 0.5W 8Ω speaker, a 9V supply and an astable multivibrator, is a bad combination. Either use a higher impedance speaker, such as 64Ω or a much lower supply voltage, say 3V. The transistors should also have a higher current rating: I'd recommend the BC337 or similar. |
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