need a speaker a resistor?

[Irukandji]

Hi ,
i have a speaker with 0,5W and 8 Ohm. Did i need a resistor when i put this mini speaker on a 9V batterie?

[james_s]

Why would you put a speaker on a 9V battery? Do you also have some electronics to make it do something?

[Irukandji]

I will try to make a noise with 2 transistors and 2 capacitor. But i don't know, if i need a resistor for the speaker, when i try this with a 9V battery.

[tpowell1830]

I will try to make a noise with 2 transistors and 2 capacitor. But i don't know, if i need a resistor for the speaker, when i try this with a 9V battery.

Hi Irukandji, welcome to the forum. If you want better responses you will need to provide better information about what you are trying to do and some schematic of your work.

[james_s]

Probably not specifically for the speaker, but post the schematic you are trying to use and that will help.

[Jwillis]

2 transistors .Sounds like either a Class A with a darlinton configuration or a class B.If either are low power then a resistor probably isn't necessary.If you amp has a bit of power to it then a signal transformer between you output  and speaker would be you best bet . If your just making a simple fixed oscillator then you could put a potentiometer in series to your speaker to control the volume. 

[buck converter]

I think he may be referring to the classic multivibrator http://4.bp.blogspot.com/-hEkdkfaf268/TxrQU1fJc5I/AAAAAAAAAgw/_fE4Y_DAnuU/s1600/Astable+multivibrator-2+LED+flashing+circuit-2N3904.gif

please give more infromation!

[Jwillis]

He said he wanted to make noise .I don't know .More information would be most helpful.

[Brumby]

Indeed ... Welcome to the forum.

If you want better responses you will need to provide better information about what you are trying to do and some schematic of your work.

This. ^ ^ ^

There are many ways to "make a noise" and whether you need a resistor or not (and if you do - what value) will depend very much on the circuit you propose to use.  Trying to answer your question with the information given so far is like trying to drive a car blindfolded.

Help us to help you.

[hamster_nz]

Can't anybody be nice. After all it is a beginners section? Like...

Hi!

You should put a 100 ohm resistor in series with that speaker (to limit power), along with a large value capacitor 100uF or preferably larger (to stop any DC component - speakers hate DC).

Full explanation.

With a 9V battery, the biggest AC signal you will see is +/-4.5V AC. In series with the 100 Ohm resistor this gives a power draw of (4.5*4.5)/(100+8) = ~0.2W, of which 10% (8/(100+8)) will be used by the speaker.

This will make it impossible to burn up either the speaker of the resistor due to excessive power. The resistor might get a bit hot though.

As 90% of the power is lost as heat in the 100 ohm resistor this is not a particularly efficient way to listen to signals - you will be better to add a small 9V amplifier module to drive the speaker.

Is is really that hard?

[Irukandji]

Hi i tryed to calculate and draw (with paint) the circuit. Did i need a resistor on the place with the question symbol?

[hamster_nz]

I am pretty sure that will work. Make the unknown resistor 325 ohm too and you should be fine. It might be quiet as only 8/333ths of the power will be used in the speaker (maybe 4mW, if I can do the numbers in my head) .

If you need a bit of help, I could most likely build one here from parts I have to hand. The values for the capacitors will also need to be known, as the also control the frequency.

PS.the wattage shown on the speaker has one too many zeros

[Zero999]

Yes, a resistor is required, for the circuit to work properly.

The transistor's bases or emitters should also have diodes in series, to protect them against the reverse voltage generated, when the opposite transistor turns on, connecting the positive side of the capacitor to 0V.

I'd recommend using a higher impedance speaker, a transformer with the 8Ω speaker, or a piezo transducer, but it can work with an 8Ω speaker, with relatively poor efficiency.

[hamster_nz]

Hi i tryed to calculate and draw (with paint) the circuit. Did i need a resistor on the place with the question symbol?

Well, I used S8050 NPN Transistors, 330 Ohm and 20k Ohm resistors, 22uF caps, and a 20W speaker, but your design is sound (and makes sound too!)

Works as drawn.

https://youtu.be/TidlGBrLYxk

(sorry for suck a crappy video)

[Zero999]

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.

[hamster_nz]

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.

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]

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.

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.

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]

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.

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]

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.^^

9V / 8 = 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.

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.^^

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.

[hamster_nz]

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.^^

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

[james_s]

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

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

[Zero999]

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.

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.

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]

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

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.