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Old 29th September 2016, 04:07 PM   #1
nlgbo1 is offline nlgbo1  United Kingdom
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LM3875 Kit, too much voltage
Default LM3875 Kit, too much voltage

Just started building audiosector lm3875 standard kit.
I'm reusing a transformer, which I've measured at 25 volts on secondaries
However, the DC output after the rectifier is +49V on V+ and PG+, and -54V on V- and PG- !
This was a lot more than I was expecting! I would've thought 25 * 1.41 = 35V DC ?
I've checked soldering and connections and can't spot anything wrong, does anyone have any ideas?

Cheers,
geoff.
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Old 29th September 2016, 04:17 PM   #2
AndrewT is offline AndrewT  Scotland
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measure the open circuit output voltages. Measure (safely) the input voltage.
That will give you enough information to work out the maximum AC voltage when mains is at 253Vac, the maximum on a UK supply.

Using a 230:25Vac 7% regulation transformer expect the maximum output voltage to be
Vout = Vin/Vrated * Voutrated * (1+regulation)
Vin = 240 nominal
Voutrated =25
Vout = 240/230*25*(1+0.07) = 27.9Vac
Max Vout = 253/230*25*1.07 = 29.4Vac
Max DC is 29.4*sqrt(2) -0.5Vf ~41.1Vdc
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Old 29th September 2016, 07:00 PM   #3
nlgbo1 is offline nlgbo1  United Kingdom
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LM3875 Kit, too much voltage
Default 250VAC mains

Thanks Andrew, I did not realize the mains voltage went that high, but just measured it at ~250vac. secondaries are just over ~25vac, and dc voltage after the rectifier is ~50vdc
I assume something is wrong with the rectifier, just not sure how to diagnose it further. All the diodes seem to be ok according to my dmm.

Thanks,
geoff.
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Old 29th September 2016, 07:15 PM   #4
JMFahey is offline JMFahey  Argentina
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Please draw what you actually built, do not just reproduce the original schematic but what you see in the flesh, show voltages and in each case where did you connect black and red probes.
A picture or two can help.
Thanks.
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Old 29th September 2016, 07:31 PM   #5
nlgbo1 is offline nlgbo1  United Kingdom
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LM3875 Kit, too much voltage
Default Solved? grounding issue?

I had originally connected PG+ and PG- DC output grounds to common / star ground. If I remove this, I get +- 24vdc output.

geoff.
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Old 30th September 2016, 08:18 AM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by nlgbo1 View Post
I had originally connected PG+ and PG- DC output grounds to common / star ground. If I remove this, I get +- 24vdc output.

geoff.
24Vdc from a 25-0-25Vac transformer is still not right.
Post info as requested by JMF.

Did you use the Mains Bulb Tester?
Is the bulb lit bright, or glowing?
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Old 30th September 2016, 08:57 AM   #7
nlgbo1 is offline nlgbo1  United Kingdom
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LM3875 Kit, too much voltage
Default Diagrams

Ok, excuse the sloppy ms paint work, attached is the original diagram from the kit, and the two tests I have performed, the first with ground connected getting ~+- 50VDC, the second with ground removed at around +-25VDC

I did use a mains bulb tester, but did not see the bulb light up at all.

Cheers,
geoff.
Attached Images
File Type: png lm3875 ps kit.png (299.1 KB, 155 views)
File Type: png lm3875 test 1 - grounded.png (205.5 KB, 150 views)
File Type: png lm3875 test 2 ground removed.png (199.5 KB, 148 views)
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Old 30th September 2016, 09:04 AM   #8
AndrewT is offline AndrewT  Scotland
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You are not measuring DC when attaching probes to the rectifier. You are measuring the average voltage of the rectified sinewave.

You need to add the smoothing capacitor to be able to store the DC voltage and then attach your probes.
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Old 30th September 2016, 09:07 AM   #9
AndrewT is offline AndrewT  Scotland
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The middle diagram shows 250Vac input and 25Vac output.
This seems impossible for a 230:25Vac transformer !
As stated in post2, expect ~29Vac from a 230:25Vac 7% regulation transformer connected to 250Vac
Quote:
Max Vout = 253/230*25*1.07 = 29.4Vac
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Old 30th September 2016, 09:42 AM   #10
nlgbo1 is offline nlgbo1  United Kingdom
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LM3875 Kit, too much voltage
Not sure exactly what the transformer is rated at, as I'm reusing an old b&o amplifier chassis and transformer. The 25VAC is what I measured on the secondaries using a multimeter.

The service manual is still online here:

http://www.vintageshifi.com/repertoi...ice-manual.pdf

They seem to be showing 33VDC after rectification.

Cheers,
geoff.
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