Overload protection circuit. Does it work correctly?

[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.

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

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

Thanks alot
//Fritzell

[darkfenriz]

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]

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 Maybe it's better to put a more powerful transistor there..

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

[CBS240]

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]

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]

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.

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

Is this really correct? Why can this happen? Is it because the current is amplified by the VAS? 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!
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]

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.

[Eva]

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]

Originally posted by powerbecker


It is not correct,...

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

[jan.didden]

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.

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

Is this really correct? Why can this happen? Is it because the current is amplified by the VAS? 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