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Old 24th October 2006, 09:32 AM   #1
Qsilver is offline Qsilver  Singapore
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Default Help... Heat issue, is this normal?

Hi there...

I'm in the process of building up a LM3875 chip amp , input rails are at + / - 37 Vdc

The amp is giving out alot of heat when connected to a set of 8ohm test speakers.. Is this normal? Within like 1 min and the amp heats up tremedously, afterwhich, the SpiKe protection kicks in.

I'm running it without a heat sink , for testing purposes. Also, there's no input signal going into the amp. So , its just a little bit of humming and within 1 min or so, the amp heats up badly..

is this normal??
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Old 24th October 2006, 09:34 AM   #2
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No, it's probably oscillating. What schematic are you using?
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Old 24th October 2006, 09:45 AM   #3
sangram is offline sangram  India
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Default Re: Help... Heat issue, is this normal?

Quote:
Originally posted by Qsilver

I'm running it without a heat sink , for testing purposes. is this normal??

Maybe that's why?
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Old 24th October 2006, 09:59 AM   #4
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Without an input signal, the chip should get barely warm if it is working correctly. The heatsink is not relevant.
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Old 24th October 2006, 11:04 AM   #5
sangram is offline sangram  India
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It seems that at 50 mA of quiscent current and 74 volts rail, the chip would be dissipating about 3.7 watts even without a load, let alone the input signal.

The datasheet shows a Junction-Ambient of 43 C/W, so that would push the temp to about 160 degrees, the threshold for Spike protection.

Then if you add a load, the dissipation would increase even more.
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Old 24th October 2006, 05:08 PM   #6
Qsilver is offline Qsilver  Singapore
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Hi guys,

thanks for the info.

Both of u are right.!!

Pinkmouse,

yeah i think the Amp was oscillating and I found the cause of it.

I used a Series coupling cap, 4.7uF to the non inverting input pin. Somehow, this caused the amp to oscillate. Not sure why though, still tryin to fig it out...

The schematic's my own but then again, its just a normal non-inverted op amp .


Sangram,


yup, this lm3875 does put up a significant amount of heat at my biasing voltage and need a heat Sink to dissappate the heat before Spike Kicks in

I think it was due to the inital oscillation that caused the IC to Heat up super fast, less then a min and the Amp's burning hot!



Anyweis, After removing the Coupling cap and putting the source thru a Log pot to the non-inverting Input . The Lm3875's working fine now.. Phew =)

Thanks guys!
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Old 24th October 2006, 10:59 PM   #7
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Default HEAT SINK

Put a heat sink on it you cant even run it for the shortest time without one.
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Old 25th October 2006, 06:48 AM   #8
Nordic is offline Nordic  South Africa
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I don't want to sound disagreeable.. but I have never been able to run one of those chips without at least some source of heatsinke... even if it was too small...
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Old 25th October 2006, 07:51 AM   #9
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Of course, I wouldn't run it into a load without a heatsink...
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Old 26th October 2006, 09:45 AM   #10
Qsilver is offline Qsilver  Singapore
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Running the Lm3875's with Heat sink now. Yeah , without the heatsinks, the chips wouldn't last very long before spike kicks in!
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