Electronics > Beginners
need a speaker a resistor?
<< < (5/6) > >>
hamster_nz:

--- 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 ---

2nd rule of hobby electronics - if it smells you are most likely doing it wrong. :D
james_s:

--- Quote from: hamster_nz on May 01, 2018, 08:01:00 pm ---
2nd rule of hobby electronics - if it smells you are most likely doing it wrong. :D

--- End quote ---

Turn the voltage up until it burns out, then back it off 10% :)
Zero999:

--- Quote from: janoc on May 01, 2018, 03:58:24 pm ---
--- 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.

--- End quote ---
That's not true.The bases see a negative voltage, irrespective of whether the load is in the emitter or collector. A reverse voltage spike is generated when the opposite transistor turns on and connects the anode of the capacitor to 0V. The cathode of the capacitor will peak at the supply voltage, minus a diode drop. In the simulation shown below, it's two diode drops, because of the protection diode in series with the bases.



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
janoc:

--- 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.

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.

james_s:
I've built a lot of multivibrators and don't recall ever adding the diodes. I would classify that as something worth taking note of, but not really worrying about it at this point. It's not often that one will be building a multivibrator out of a pair of transistors beyond basic experimentation and they certainly do work without diodes.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod