[sdiy] PSU grounding problem

Ove Ridé nitro2k01 at gmail.com
Mon Aug 6 03:12:46 CEST 2012

Which model of 'scope is this?
You need a ground reference to be able to take measurements. In some
circuits, mains earth is connected internally which means that in
principle (though it might be bad for signal integrity for precise
measurements) yo don't need to connect your own ground reference.
Since the job of the transformer is to isolate the two side and you
(presumably) don't have a ground connection between the secondary
side's ground, and mains earth or the 'scope's ground, the 'scope
probe is basically picking up interference. This is what happens when
an input is what's called "floating", i.e. has no ground reference.

A scope probe typically comes with an alligator clip that you can
attach on the side, to establish a ground reference. See this image:


Your 'scope should also have a banana plug sized whole on the front
panel with the ground symbol, which you can use to connect a ground

I recommend you watch this video, which explains how grounding works.


On 6 August 2012 01:21, ganesha <goaqihai at yahoo.co.uk> wrote:
> Thanks for your reply.
> I have a very simple / cheap scope with only one connection for the probe called Y input. I also have a switch with 3 positions wich is the reference DC - ground -AC.
> This switch is set to DC and I connect the Y-input probe to the center tap.
> This is how I always use the scope. F.e. If I  plug in a jack cable into a vco saw out. I set the switch to dc and connect the probe to the tip of the jackplug and than I see the saw waveform.
> If I do the same but connect the scope probe to the 0V of a synth, I see a flat line.
> Now I connect the probe the same way to the center tap /0V of my transfo but I see the sinus. This is confusing me.
> ________________________________
> From: bbob <fluxmonk at gmail.com>
> To: ganesha <goaqihai at yahoo.co.uk>
> Cc: "Synth-diy at synth-diy.org" <Synth-diy at synth-diy.org>
> Sent: Monday, 6 August 2012, 1:03
> Subject: Re: [sdiy] PSU grounding problem
> what is your measurement reference?  ie, what 2 points are you connecting your scope to: the transformer center tap, and what else?
> On Sun, Aug 5, 2012 at 6:45 PM, ganesha <goaqihai at yahoo.co.uk> wrote:
> 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>
>>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
>>Synth-diy mailing list
>>Synth-diy at synth-diy.org
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