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Old 8th July 2003, 10:33 PM   #21
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Quote:
Originally posted by millwood
the datasheet for the opa541 has very detailed instructions on how to calculate the current sensing resistor and its wattage requirement. check it out.

why add a 10 watt resistor and effect the output, with a value of 0.01 ohms, when you can short it?
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Old 9th July 2003, 01:16 AM   #22
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Quote:
Originally posted by Matttcattt



why add a 10 watt resistor and effect the output, with a value of 0.01 ohms, when you can short it?

so you can set the current limit to protect your amp?
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Old 9th July 2003, 08:20 AM   #23
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Quote:
Originally posted by millwood



so you can set the current limit to protect your amp?

but in my case, i just wanted te max limit (0.01 ohms) this seems pointless to me.
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Old 9th July 2003, 12:41 PM   #24
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Quote:
Originally posted by Matttcattt



but in my case, i just wanted te max limit (0.01 ohms) this seems pointless to me.

I assume you wanted to be able to reach the 10A peak current output limit. The datasheet isn't clear as to if there is any protection if you short the current sensing resistor. My guess is no, as the datasheet mentioned nothing about thermal shutdown.

If that is true, shorting the current sensing resistor may lead to permanent damage to the chip.
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Old 9th July 2003, 02:01 PM   #25
miguel2 is offline miguel2  Portugal
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have you compared these chips with the LM3875?

Miguel
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Old 9th July 2003, 03:11 PM   #26
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how can 0.01 ohms not cause damage, but 0 ohms can?
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Old 9th July 2003, 03:18 PM   #27
Mad_K is offline Mad_K  Norway
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Quote:
Originally posted by Matttcattt
how can 0.01 ohms not cause damage, but 0 ohms can?

if you check the datasheet's SOA chart you'll see that 0,01 ohm with any decent rail voltage can cause some smoke (especially with the AP version)s


edit: or calculate it
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Old 9th July 2003, 03:22 PM   #28
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Quote:
Originally posted by Matttcattt
how can 0.01 ohms not cause damage, but 0 ohms can?

the trigger seems to be a Vbe junction, per the TI schematics. that is confirmed by the formula used to calculate the current sensing resistor:

Rcl=0.8/Icl-0.06 (or 0.02, depending which side of the cycle you are on).

so a 0.01ohm resistor will limit the output current to 11A - 26A; while a 0phm resistor will limit the output current to 13A - 40A.

as you can tell, if the current is large enough, it will make a significant voltage drop on a smaller resistor. more detailed infor can be found in a TI AN as well.

the datasheet for 549 seems to indicate that it uses a different protection mechanism.
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