Amplifier protection flyback diodes

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KCHARROIS

Joined Jun 29, 2012
302
Hello

I've designed another amplifier and now im in need of a way to limit current when theres a short. The picture below uses flyback diodes which I've seen in motors for protection but I dont know hpw they apply in this circuit.

Thanks
 

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panic mode

Joined Oct 10, 2011
1,868
it looks like bias circuit (represented by block) maintains 2.6V drop. diodes D1 and D2 are normally reverse biased and don't affect circuit. but when output is shorted to -Vrail, D2 is forward biased and pulls down base of Q1 which reduces current through Q1 and Q3.

when output is shorted to +Vrail, same thing happens D1 becomes forward biased, Voltage or base of Q2 increases and current through Q2 and Q4 decreases.
 
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timescope

Joined Dec 14, 2011
298
Amplifier short circuits usually occur between the output and ground for example when the speaker wires are shorted. In this case, the diodes will have no effect. The attached circuit is common (shown for the NPN transistors, change the transistor to PNP for the other half). The added transistor starts to conduct and limit the base drive current when the voltage across the emitter resistor is 0.6(1+R2/R1). The diode prevents the transistor collector-base junction from becoming forward biased when the drive is negative. The small capacitor maintains the transient response of the amplifier.

Timescope
 

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MrChips

Joined Oct 2, 2009
22,868
Power amplifiers coupled directly to the speaker present a number of challenges when:

  1. the output stage is not perfectly balanced
  2. the output is accidentally shorted
  3. the output overdrives the speaker for an extended length of time

Here are some solutions that are used:

  1. AC couple the output through a large value non-polarized capacitor (not an ideal solution)
  2. insert a thermal resettable fuse in series with the speaker
  3. insert an incandescent light bulb in series with the speaker
 
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