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Old 23rd November 2011, 04:48 PM   #1
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Question BD911 got blown in my power supply

Hi Guys

I am building power supply for my PA100 (3886 X 2, 10W) amplifier. Its based on the circuit:

http://bazylewicz.pl/lm3886_pa100/sc...wer_supply.pdf


After assembling everything I checked the DC voltage and my digital multimeter is showing around 28 volts, then I switched the multimeter mode to check current (AMP)... switched on mains.. boom... my power supply got blown off. When checked closely found that the BD911 at output was broken.

My question is.. why did this happen ? I am not sure if the polarity of multimeter leads were wrong when I connected it for testing current... but will that matter ?
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Old 23rd November 2011, 05:03 PM   #2
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Quote:
Originally Posted by arunraj.in View Post
I switched the multimeter mode to check current (AMP)... switched on mains.. boom... my power supply got blown off. When checked closely found that the BD911 at output was broken.
That's exactly what happens when you short the output of a unprotected supply, which it is.

Anyway, you don't need to fix that supply, your amp would be better without it: a regulated supply needs to be of an exceptional standard not to degrade the performances of a decent amplifier, and this one with a 317 patched with a transistor to increase the current certainly doesn't qualify.
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Old 23rd November 2011, 05:09 PM   #3
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Thanks for the reply..

So what you recommend is remove both transistors and use the output from 317 directly ?

And can you refer some good quality power-supply which I can use instead of this ?

Last edited by arunraj.in; 23rd November 2011 at 05:14 PM.
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Old 23rd November 2011, 05:31 PM   #4
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Quote:
Originally Posted by arunraj.in View Post
Thanks for the reply..

So what you recommend is remove both transistors and use the output from 317 directly ?

And can you refer some good quality power-supply which I can use instead of this ?
No, you can completely eliminate the regulator and connect the amplifier directly to the filter cap, provided of course the voltage is not too high for the amplifier.
Use large, good quality filter caps, that will be far superior to any bog standard regulator.
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Old 23rd November 2011, 06:08 PM   #5
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What will the maximum output voltage from your rectified supply be in the worst case conditions?
If it stays under +-42V, you will be fine without the regulators...
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Old 23rd November 2011, 10:19 PM   #6
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Quote:
Originally Posted by arunraj.in View Post
Hi Guys

My question is.. why did this happen ? I am not sure if the polarity of multimeter leads were wrong when I connected it for testing current... but will that matter ?
The multimeter is a virtual short circuit when measuring current so it needs to be in series with the load.
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Old 23rd November 2011, 10:39 PM   #7
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Originally Posted by nigelwright7557 View Post
The multimeter is a virtual short circuit when measuring current so it needs to be in series with the load.
Then there was the electrician who learned this lesson on a 660 volt line. The meter exploded. Surprisingly he was not hurt.

So blowing up a few small parts is a low price to pay for a very important lesson.

Current meters always go in series. If you make a mistake with a voltmeter in series things just don't work. Voltmeters work in parallel.
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Old 25th November 2011, 06:07 AM   #8
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Thank you all for the info ..... I didn't knew that it should be in series..
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Old 25th November 2011, 06:09 AM   #9
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Originally Posted by palstanturhin View Post
What will the maximum output voltage from your rectified supply be in the worst case conditions?
If it stays under +-42V, you will be fine without the regulators...
Ya it stays under +-42V.... Thankz
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