[sdiy] PSU grounding problem

Dave Manley dlmanley at sonic.net
Mon Aug 6 02:20:47 CEST 2012


I'm guessing you see the same sinus on the +/-15 volts.  If so you're measuring some form of leakage/coupling between the primary and secondary side of the transformer.

In the schematic the output side of the transformer is floating with respect to earth ground.  

One solution is to provide a connection between the secondary side center tap and earth ground so that the output side isn't floating.  I don't know what the safety standards are in the UK.  There are diferent techniques.

-Dave


-------- Original Message --------
 From: Dave Manley <dlmanley at sonic.net>
 Sent: Sun, Aug 5, 2012 04:20 PM
 To: ganesha <goaqihai at yahoo.co.uk>
 CC: Synth-diy at synth-diy.org
 Subject: Re: [sdiy] PSU grounding problem

>What is the ground reference for the measurement? 
>
>-Dave
>
>-------- Original Message --------
> From: ganesha <goaqihai at yahoo.co.uk>
> Sent: Sun, Aug 5, 2012 03:45 PM
> To: Oren Leavitt <obl64 at ix.netcom.com>
> CC: Synth-diy at synth-diy.org
> Subject: Re: [sdiy] PSU grounding problem
>
>>Thanks for your reply.
>>I tried that but still, If I check the center tap with my scope, I see a large sinus. Next, I disconnected the transfo from the pcb and tried every possible connection for the center tap.
>>Yellow - grey
>>blue - yellow
>>red - blue
>>red - grey
>>
>>Nomatter what I try, the center tap always has a sinus. My scope is set to 5V/div and it goes 2 divisions up and 2 divisions down.
>>
>>Am I missing something? If I connect my scope to the 0V of any other synth I own, I always see a flat line...
>>
>>
>>
>>
>>----- Original Message -----
>>From: Oren Leavitt <obl64 at ix.netcom.com>
>>To: ganesha <goaqihai at yahoo.co.uk>
>>Cc: 
>>Sent: Sunday, 5 August 2012, 23:32
>>Subject: Re: [sdiy] PSU grounding problem
>>
>>
>>
>>On 8/5/2012 4:19 PM, ganesha wrote:
>>> Hi,
>>>
>>> I'm making my own psu for the first time. I used this schematic from Jürgen Haible:
>>> https://dl.dropbox.com/u/28535891/jh_matrix_fx_power_supply_sch.pdf
>>> If I measure the voltages with my DC voltmeter, everything seems to be ok. I have +15v and -15V.
>>> However, I noticed that if I check the 0V/ground with my oscilloscope, I see a sinus.
>>> Could it be that I connected the transfo incorrect?
>>> I use this transfo : http://www.velleman.eu/products/view/?country=nl&lang=en&id=2516
>>> It has 2 secondary windings:
>>> red yellow
>>> blue grey
>>> It says that blue and red are both the start of the secondary windings.
>>> I connected yellow and grey together (both ends from the secondary windings) for the 0V. Is this perhaps the problem? should I have connected both starts (blue-red) together or perhaps the end of the first secondary winding with the start of the second secondary winding?
>>>
>>> Thanks for the help!
>>
>>Sounds like you have one of the secondaries reversed. Try yellow-blue 
>>for the center tap.
>>
>>- Oren
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