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Old 23rd July 2008, 02:30 PM   #1
copilu is offline copilu  Romania
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Default short protection for AV400

any ideea? what do you think abaut this? can someone give me the values?

http://users.swing.be/edwinpaij/imag...rotection1.gifClick the image to open in full size.
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Old 24th July 2008, 01:00 PM   #2
copilu is offline copilu  Romania
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come on, nobody?
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Old 24th July 2008, 02:10 PM   #3
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Check out National semi. LM143 application notes.

art
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Old 24th July 2008, 03:27 PM   #4
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Default Re: short protection for AV400

Quote:
Originally posted by copilu
any ideea? what do you think abaut this? can someone give me the values?

http://users.swing.be/edwinpaij/imag...rotection1.gifClick the image to open in full size.

R1/2/3/4/5 1K at a guess.

Re 0R22 10 watt depending on amp ouput power.

I would also have included 1k's from the other output pair in case one goes short cct.
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Old 24th July 2008, 03:33 PM   #5
jkraxi is offline jkraxi  Romania
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Your answers are here http://users.swing.be/edwinpaij/shor...protection.htm
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Old 24th July 2008, 03:40 PM   #6
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Quote:
Originally posted by jkraxi
Your answers are here http://users.swing.be/edwinpaij/shor...protection.htm

One very obvious omission is the flywheel diodes for when the output transistors switch off quickly.
Without these you are likely to blow up the output transistors when a short cct occurrs.
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Old 24th July 2008, 03:46 PM   #7
AndrewT is online now AndrewT  Scotland
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r4a=r4b=r5a=r5b=200r to allow monitoring of all 4output currents.
add those diodes or else.
add Zeners across all FET gate to source.
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Old 24th July 2008, 08:38 PM   #8
copilu is offline copilu  Romania
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Quote:
Check out National semi. LM143 application notes.
yes, i saw that but that amp`s are with bipolar transistors

yes i know, but i think that values are not proprier for AV400. one of the reason is the transistors, BC`s... this transistors should "beat" the prefinals witch are MJE340-350.

Quote:
r4a=r4b=r5a=r5b=200r to allow monitoring of all 4output currents.
can you tell me what is the curernt for each final transistor is case of short? and what transistors should i use for the protection circuit?

that questions are for nigelwright7557 too.
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Old 24th July 2008, 09:30 PM   #9
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Quote:
Originally posted by copilu


yes, i saw that but that amp`s are with bipolar transistors



yes i know, but i think that values are not proprier for AV400. one of the reason is the transistors, BC`s... this transistors should "beat" the prefinals witch are MJE340-350.


can you tell me what is the curernt for each final transistor is case of short? and what transistors should i use for the protection circuit?

that questions are for nigelwright7557 too.

The resistors values should be OK for MOSFET's as its the voltage across the output resistors that needs to be checked not the transistors.


If the MJE350 and the short protect transistors both switch on you will probably blow up the transistors. The harder the short protect transistors swith on the harder the MJE350 will fight it.

I used a constant current source in the VAS cct and also in the VAS transistor itself to limit the current taken by the short cct protect transistors.

If the output resistors are 0R22 then the short cct protect current will about 3 amps.

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Old 25th July 2008, 12:08 AM   #10
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"add those diodes or else."
Output clamping diodes? Don't you believe the MOSFET body diodes will do that satisfactorily?

Are the output transistors laterals or HEXFETS or similar?

I don't know what sort of input stage is used but there needs to be a limit on the VAS current or the protection transistors and/or VAS might blow up when the protection circuit tries to work. Depending on the type of input circuitry there might be one already.

edit: The schematics I can find shows the AV400 as a LTP VAS circuit. Is this correct? In that case the VAS current will be limited. It also shows vertical fets used. Is this correct? You schematic doesn't show a Vbe/Vgs multiplier - is this just beacause you just didn't draw it or is it because you are using lateral mosfets?
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