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Old 30th September 2016, 10:51 AM   #11
nlgbo1 is offline nlgbo1  United Kingdom
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LM3875 Kit, too much voltage
Quote:
Originally Posted by AndrewT View Post
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.
Thanks Andrew, that makes sense! For some reason the kit doesn't include the smoothing capacitors. This was mentioned in the manual:

Code:
If we measure approx. +/- 20V (positive will be lower because of LED) we are ready to connect amp boards.
Please note that voltages are much lower than expected beacuse of lack of smoothing caps.
If you have 10uF caps installed on rectifiers board both voltages would exceed 30V DC
Thanks for all your help on this, much appreciated!

geoff.
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Old 30th September 2016, 10:52 AM   #12
AndrewT is offline AndrewT  Scotland
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It's not a 25Vac transformer. It may be ~22Vac

33Vdc is a bit low, but may be due to mains voltage being different between this latest measurement and your first measurement.
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Old 30th September 2016, 01:09 PM   #13
Mark Whitney is online now Mark Whitney  Netherlands
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Hi nlglo1, did you remove the connection between the two secondaries? In the schematic the secondaries are also connected to ground. I am guessing you removed them too.

Photo's will make helping a lot easier.
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Old 30th September 2016, 02:23 PM   #14
nlgbo1 is offline nlgbo1  United Kingdom
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LM3875 Kit, too much voltage
Hi Mark,

There used to be a connection via a 220k resistor from the secondary the ground, but removing this seemed to make no difference to the DC voltages after the rectifier.

Attached is a photo with the grounds still attached.
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Old 30th September 2016, 03:05 PM   #15
JMFahey is offline JMFahey  Argentina
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Quote:
They seem to be showing 33VDC after rectification.
Correct that to: "after rectification and smoothing"

You are missing C1 and C2 filter caps, which will charge to peak value of 25V minus diode loss, so some:
(25*1.4142)- 1.4 .
Do the Math yourself
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Old 1st October 2016, 09:50 AM   #16
Mark Whitney is online now Mark Whitney  Netherlands
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Quote:
Originally Posted by nlgbo1 View Post
Hi Mark,

There used to be a connection via a 220k resistor from the secondary the ground, but removing this seemed to make no difference to the DC voltages after the rectifier.

Attached is a photo with the grounds still attached.
The schematic (on page 14-5) shows connections between 10->14 and 11->16 on the transformer. You have connected 10 to one PSU input and 14 to the other input. Best to check that the secondaries are not connected as shown in the schematic.
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Old 3rd October 2016, 04:04 PM   #17
nlgbo1 is offline nlgbo1  United Kingdom
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LM3875 Kit, too much voltage
Ordered some smoothing capacitors, as I do not have any of the right voltage.

If I connect 10->14, I think that would essentially make it more like a centre tapped transformer? In which case, should I be removing the diodes as per this post:

Center tap torroid wiring to peter daniels kit???

Cheers,
geoff.
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Old 4th October 2016, 06:49 AM   #18
AndrewT is offline AndrewT  Scotland
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Yes.
Retain 4 diodes to mimic the bridge rectifier.
You must not connect a dual bridge rectifier to a centre tapped transformer.
Use a Mains Bulb Tester to power ON every restart until after you have completed your last modification. It will prevent you blowing up your project if you make a wiring mistake.
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Old 4th October 2016, 03:22 PM   #19
nlgbo1 is offline nlgbo1  United Kingdom
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LM3875 Kit, too much voltage
Ok, re-wired the bridge to use 4 diodes, now I get the following voltages

V+ to V- 46 volts
V+ to ground(black wire from transfomer) 23 volts
V- to ground(black wire from transfomer) -23 volts

Which I think is expected without the smoothing capacitors in place.

Cheers,
geoff.
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Old 10th October 2016, 04:29 PM   #20
nlgbo1 is offline nlgbo1  United Kingdom
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LM3875 Kit, too much voltage
Default Finished!

Amps are now all built and working
Many thanks to AndrewT, Mark Whitney and JMFahey for all of your help and guidance, much appreciated
FWIW, the rail voltages ended up at +/- 34VDC at the amp board.
Cheers,
geoff.
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