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-   -   Overload protection circuit. Does it work correctly? (http://www.diyaudio.com/forums/solid-state/64268-overload-protection-circuit-does-work-correctly.html)

Fritzell 14th September 2005 01:18 PM

Overload protection circuit. Does it work correctly?
 
Ok here's a circuit of an amplifier with a overload protection circuit by monitoring the Re of the output transistors.

http://sound.westhost.com/p27b-f2.gif

Im thinking of adding one of these to my amp but in simulation it shows strange results. :confused:

If i put a DC signal on the output and short-circuit the output, the current in the upper output transistors is limited, and the current through the diode and the current-limiting transistor is about 18mA (the VAS current). This is correct.

But when i simulate it with a negative voltage on the output, the curent IS limited in the output transistor BUT the current through the diode and protection-transistor inreases to 450mA :eek:

Is this really correct? Why can this happen? Is it because the current is amplified by the VAS? :apathic: Shouldn't it have the same current as the positive transistor?

Thanks alot
//Fritzell

darkfenriz 14th September 2005 01:25 PM

I think it is correct.
It is because single ended VAS together with feedback.
Feedback is doing its best to force output to zero Volts.

Fritzell 14th September 2005 02:52 PM

Ah yes i think i understand. The feedback tells the LTP that the VAS needs some more base current to establish the voltage. But if the BC556 is going to sink 400mA it would be hot :hot: Maybe it's better to put a more powerful transistor there..

Thanks for your reply :)
other people please fill in if you wish!

CBS240 15th September 2005 04:17 AM

I have noticed that this type of DC protection circuit is used when using paralelled output transistors. My question is, is it prudent to use this circuit when using just a single pair of outputs?

amplifierguru 15th September 2005 05:11 AM

Hi Fritzell,

The Vas transistor needs to be clamped under overdrive - it needs a 39R in the emitter leg and the LTP 560R -> 820. Then put two series diodes pointing down across this 820R. Sim that and it should limit symmetrically.

Cheers,
greg

powerbecker 15th September 2005 06:40 AM

Re: Overload protection circuit. Does it work correctly?
 
Quote:

Originally posted by Fritzell
Ok here's a circuit of an amplifier with a overload protection circuit by monitoring the Re of the output transistors.

http://sound.westhost.com/p27b-f2.gif

Im thinking of adding one of these to my amp but in simulation it shows strange results. :confused:

If i put a DC signal on the output and short-circuit the output, the current in the upper output transistors is limited, and the current through the diode and the current-limiting transistor is about 18mA (the VAS current). This is correct.

But when i simulate it with a negative voltage on the output, the curent IS limited in the output transistor BUT the current through the diode and protection-transistor inreases to 450mA :eek:

Is this really correct? Why can this happen? Is it because the current is amplified by the VAS? :apathic: Shouldn't it have the same current as the positive transistor?

Thanks alot
//Fritzell

It is not correct, current through the BD139 (and so also through the BC556) must be limited!:hot:
Put a small resistor between the emitter and -35V and two diodes in series over the 560E.
Even than the current through the BC556 is greater than in the
Bc546 but you don`t kill both!

Fritzell 15th September 2005 12:11 PM

Hi all thank you very much for your replies.

Im using a current-mirror in my simulation instead of the resistor. But adding a 39 ohm resistor on emitter of VAS didn't really make a difference. If i increase it to something like 390 ohm things start happening and current goes down to 70mA but i will loose too much output swing with this.

:xeye:

Eva 15th September 2005 05:33 PM

As other people has already mentioned, that circuit is not reliable in overcurrent conditions because the current limiting protection is incomplete.

In order to fix that I prefer to use a emitter resistor, and a current limiting transistor, connected in such a way that it shunts the base drive of the BD139 when the voltage drop across the emitter resistor reaches the Vbe thresold of the limiter transistor.

darkfenriz 15th September 2005 05:52 PM

Re: Re: Overload protection circuit. Does it work correctly?
 
Quote:

Originally posted by powerbecker


It is not correct,...

Yes, the protection circut works incorrect. I meant simulator shows correct (life-like) situation.

jan.didden 15th September 2005 05:54 PM

Re: Overload protection circuit. Does it work correctly?
 
Quote:

Originally posted by Fritzell
Ok here's a circuit of an amplifier with a overload protection circuit by monitoring the Re of the output transistors.

http://sound.westhost.com/p27b-f2.gif

Im thinking of adding one of these to my amp but in simulation it shows strange results. :confused:

If i put a DC signal on the output and short-circuit the output, the current in the upper output transistors is limited, and the current through the diode and the current-limiting transistor is about 18mA (the VAS current). This is correct.

But when i simulate it with a negative voltage on the output, the curent IS limited in the output transistor BUT the current through the diode and protection-transistor inreases to 450mA :eek:

Is this really correct? Why can this happen? Is it because the current is amplified by the VAS? :apathic: Shouldn't it have the same current as the positive transistor?

Thanks alot
//Fritzell


Fritz,

Sorry for coming in so late, but are you putting a DC on the output and then shorting it, or are you putting a DC on the input that gives the DC out and then shorting it in the sim?

Jan Didden


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