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Old 9th February 2009, 07:36 PM   #1
stoc005 is offline stoc005  United States
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Default Current limiting behavior??

Just for fun I inputted the entire Leach amp into LTSpice and wanted to see how the current limiter worked. With a 1 ohm resistive load, the current limiter does work but it is oscillating. When it goes into overcurrent, the drive is "removed" which makes the current in the outputs decrease. Once the current decreases, the drive is allowed to increase the output current, which makes the current limiter reduce the drive again. So, I get around a 10KHz oscillation when the amp is driven into current limiting by a 1KHz signal. Anybody seen this before? Does the real amp do this? I'd hope not. But it makes sense since there is no hysteresis on the current limiter transistors. To me, this is just not a well behaved current limiter and lots of amps use this technique. Comments??
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Old 9th February 2009, 08:10 PM   #2
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That's normal behaviour.
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Old 9th February 2009, 08:28 PM   #3
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Why worry about short circuit stability
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Old 9th February 2009, 08:41 PM   #4
stoc005 is offline stoc005  United States
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Default current limiter stability

Because I do not want a large amount of high frequency "hash" going into my tweeters.
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Old 9th February 2009, 09:00 PM   #5
AndrewT is offline AndrewT  Scotland
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but you have just shorted your amp with a total of 1r0.
What voltage is left to drive your speakers?
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Old 9th February 2009, 09:33 PM   #6
JPV is offline JPV  Belgium
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Quote:
Originally posted by AndrewT
but you have just shorted your amp with a total of 1r0.
What voltage is left to drive your speakers?
It is still a problem because it is not necessary a continuous short that is happening but it can be some kind of overload that will make the amplifier oscillating in a strange way.
This is inherent in a limiting technique that feeds back the control to the input.
It should be better that the circuit should create a led warning if the condition is transient and if the condition remains switch off ones and for all the amplifier.

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Old 9th February 2009, 10:47 PM   #7
wg_ski is offline wg_ski  United States
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The magnitude of the oscillation is generally a LOT less than full signal voltage. A few volts p-p won't hurt anything. If you're worried about it, hang a capacitor from base to emitter and set the time constant with the resistor that feeds it to 100 milliseconds or so. That will slow the feedback down to the point of being stable.
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Old 10th February 2009, 07:47 AM   #8
AndrewT is offline AndrewT  Scotland
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and that capacitor is needed to allow valid transient signals to pass even though the resistor setting for the trigger points is based on DC limitations.
I would expect that a carefully set up limiter would pass approximately twice the transient current that triggers in the DC condition.
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Old 10th February 2009, 08:44 AM   #9
Hurtig is offline Hurtig  Denmark
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Short circuit protection sdhould NOT be made as a current limiter... You should use a circuit that shouts down the amp when current is to high, and then wait ex 3 seconds before output is enabled again.
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Old 10th February 2009, 02:44 PM   #10
stoc005 is offline stoc005  United States
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Default short circuit protection vs current limiting

If you change the caps to around 10uf instead of .1 uf, I see the oscillation go away and the behavior to be a bit more "civilized". The output just flat tops and won't let the current/voltage output get any higher. The amp behaves normally if the current stays below 9.5 amps per output device. They are rated at 15 amps. If it were me, I'd prefer this over the original way it worked.
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