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Old 1st July 2010, 01:53 PM   #151
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Most of the limiters seen here on DIY are peak current only no matter the output. These I call "the always fail blow up the amplifier" circuit and are truly worse than nothing at all. The Leech amp limits current to a small value when the output voltage is small and increases current capacity as output voltage becomes larger up to the peak current. This loosely tracks the SOA of the transistors, well enough to protect output from ever blowing from over current. So there is the minimum current which is always allowed, the slope with the relationship between output voltage and output current, and then there is the peak current limit which the output voltage is above a certain value.

So possibly I got the name wrong but above is how it works. Please give it a proper name if you like.
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Old 1st July 2010, 02:09 PM   #152
AndrewT is offline AndrewT  Scotland
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Leach discusses his limiter in his Lo Tim paper.
The graph clearly shows a horizontal slope and an inclined slope.
Both of these slopes are controlled by the voltage drop across the emitter resistor.

The resistor to turn this current limiter into a VI limiter must enable monitoring of Vce. One extra resistor gives a VI limiter.
Jens incorporated the extra resistor in his Leach clone PCBs for a true VI limiting action.

I equate SOA limiting to VI limiting in this respect. This is due to the very important Vce dependance for any of the current/power limits implied by the SOA.
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