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Old 30th May 2007, 08:40 AM   #1
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Default Amplifier Protection

i wonder if any come across a crowbar protection circuit the ones that operate with a triac

it seems to me that the principal of operation is correct i.e. since your amp has dc in the out meaning that its doomed any way so what harm more can make a short circuit in the output ????
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Old 30th May 2007, 10:03 AM   #2
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Default Select from your junk box a small transformer...something that has an already know

power....... let's say that it can produce 1 ampere when holding the voltage at the terminal at a reasonable level.

Now, after rectification, make a short circuit with an amperimeter that can read 20 Amperes continuous and able to hold 3 times more for half a second (time needed to read digital ones).

You will see that your supply, during short circuit, will be able to produce more than 10 amperes during short circuit....for sure the voltage there will be very small...but 10 amperes will be circulating.

Now let's imagine your amplifier....let's say 50 volts and 10 amperes..... imagine how many amperes it can produce during a short.....maybe 100 or more.

This is the problem with short circuits, accidental shorts, speaker shorts or shorts caused by some types of protections.... for a small period of time 100 amperes or more will circulate.

And they will circulate inside the output transistor or output of Field Effect transistor that is unable to hold that current.

If you have nine transistors...and 10 amperes maximum capacity each one, for instance, 90 amperes will be held for sometime....and this will happens only if you have emitter resistances to equalize current or analog parts to field effect and others.

Having not equalizer resistance, the unit that has bigger gain will hold bigger current than the others...so.... those 90 amperes will not be divided equally between all transistors....one of them may hold 13 amperes and burn (if limit was 10 amperes).

This is the problem you may have during short circuit..... and not having enougth output transistors (twice the maximum short circuit current available by your supply)...and not having equalizer resistances.

regards,

Carlos
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Old 30th May 2007, 10:23 AM   #3
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Default To be more clear Sakis... even suspecting that you already know that


...But certainly there are folks that do not know those basic things.

Imagine, please, that you have two transistors in some kind of circuit..... the supply is huge enougth to hold 40 volts in both colectors when enormous current is crossing....more than 25 amperes in total are crossing.

Having emitter resistances...when current crossed those ones will develop voltage into those resistance's extremes.... some voltage will be there.

As power, measured over some transistor (or FET transistor) is the result of multiplication of the voltage measure from the colector to the emitter multiplied by the current that is crossing them, you can see the results in the image attached.

Those currents were different because the rigth transistor has much more gain that the left one...so...more current will cross the one that has more gain.... the resistance, beeing there will reduce the the voltage at the emitter....that could be zero (having in that worst sittuation 40 volts from colector to emitter into the left one).

In the left transistor as a result of the resistance beeing there...the emitter voltage raises to 10 volts (ohms law).... colector voltage is 40.... emitter voltage is 10..... over the transistor, measured from colector to emitter..the voltage is 30... and this will be multiplied by the current to know the power over this selected transistor.... will result 300 watts.

But remove those resistances and you will have the red values...very different...much more different and really more dangerous to the higher gain unit.

Because of that, output transistors, beeing mounted in parallel..will need equalizing resistances.

regards,

Carlos
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Old 30th May 2007, 10:23 AM   #4
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Default i dont get it

sorry carlos ...i dont get it ......as far as i know this is only to protect the speakers on running on rail voltage .....

meaning that if something goes wrong with amp and any of transistors is shorted then dc comes in the out and then triac short circuits the out to ground and rail fuses goes of ..... no mater if 7 amps or 100 amps makes them go off ...

the point is that if a circuit like that is quick enough to make shure that works a lot faster than dc will take to go to your speaker ....

i dont think or is expecteted from a thing like that mosfets or bjt to be able to stand a short circuit of as you said 100 amps may be and still working after that ...

it seems to me that this protection is only active if we take as a fact that the amp is allready doomed and one or all of trs is gone to mars ...

correct ????
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Old 30th May 2007, 10:29 AM   #5
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Default Sorry... i tried my best...there's a problem on me related English language


Well...one problem is not really true...i have "zilions" of problems with language.

Also you have to translate things..this turns things even worst.

Portuguese to English and than English to Greek.

Also i am not clear enougth i suppose...let's wait someone that can explain this more clearly to you.

Maybe fótios, as he can send you explanations directly in Greek...or..at least someone native in English language that can remove one step into the language versions made...using brain translation or software to translations.

I use my old brain.... has a very slow chemical microprocessor..something alike Zilog Z80.... and the astounding auxiliary memory of 2Kilobytes to speak and write in english..for sure a big mess will result.

Not correct Sakis..... an amplifier with many transistors in the output...many pairs...each pair able to hold 10 amps...may hold shorts of 100 or 200 amperes if your supply and condensers could supply such current.

This was the base of advertizing, the main point into Harman Kardon published texts into electronics magazines...the possibility, because supply and big condensers, to hold, short time short circuits providing 60 to 150 amperes.

The idea was not to provide energy to face real shorts..but to face strange speaker impedances that would need enormous current to face the deep of impedance in some frequencies...this intended to show us that those amplifier were able to hold "difficult loads"...strange speakers that has enormous capacitance and inductances into their internal crossovers.

regards,

Carlos
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Old 30th May 2007, 10:49 AM   #6
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Yes sakis, your question is simple and you are correct in your assumption. The danger I think Carlos points out is that if just one transistor goes short/faulty in a bank of several, you may damage all of them when the crowbar comes into operation.

There is also the danger that somebody uses an incorrect fuse/nail(!) at some time and the crowbar comes into force and sets the amp on fire.
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Old 30th May 2007, 04:53 PM   #7
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Hi,
Richie has hit it on the head.
Quote:
uses an incorrect fuse/nail(!) at some time and the crowbar comes into force
one MUST use a fuse on each supply to allow the crowbar to be effective. I have found that F3.1A rail fuses allow 6Apk to flow to the load without melting. This allows 150W into 8ohm speakers.
F4A allows about 250W into 8ohm.

The crowbar can be linked from output to ground or one can fit twin crowbars one from each supply to ground. They can even work differentially. If the output goes +ve then the +ve rail crowbar fires and vice versa for the -ve output offset.
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Old 31st May 2007, 07:27 AM   #8
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Default thanks andrew t

this what i had in mind on the first place ......

will see any body has a circuit for about 180w of power ????
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Old 31st May 2007, 07:40 AM   #9
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The output power is irrelevant.
The time it takes the fuse to blow at X*current rating is the part the triac (or other switch) has to survive.
This X can be ten times or a hundred times or even higher.

I just looked back at this post http://www.diyaudio.com/forums/showt...34#post1164234
to see that 35Vpk into 4r resulting in 2.8times the fuse rating did not blow the rail fuse. On a short time scale fuses can pass large currents.
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Old 31st May 2007, 08:35 AM   #10
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Default Here we have difference between practice and theories.


Fuse blows slowly.... triac, beeing solid state reacts faster...it produces short term current near the limit of the supply... output transistors can goes shorted before the fuse blows.

Fuse is something that serves for "half of nothing"...it blows when the transistor is already dead....it will just avoid the short to continue to burn the board and wires melt.

I suggest, strongly, to avoid triac and shorts into the output.

To protect you have to put a lot of transistors in parallel..better other sensors and relays to switch the output off.

Do not trust in fuses Sakis.... they will do a very bad job for you.

During audio reproduction you have bursts of high current...and fuse do not blows....it blows only after the transistor already gone.

regards,

Carlos
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