[sdiy] High Frequency ripple in PSU - no clue..

[Oscar Salas]

Hi Mike 


A 24VCT transformer for a +15 -15 power supply is not correct. Go for a 36VCT transformer.
The faulty that you describe on the negative rail without load could be because negative regulator is unstable. Negative regulator LM337 need the output capacitor. Try something like 10uF . Take care that it is correctly soldered.
A schematic with the values that you are using would be useful.



----- Original Message -----
From: Mike Beauchamp <mikebeauchamp at gmail.com>
To: Oren Leavitt <obl64 at ix.netcom.com>
Cc: synth-diy at dropmix.xs4all.nl
Sent: Wednesday, September 14, 2011 3:18 AM
Subject: Re: [sdiy] High Frequency ripple in PSU - no clue..

Hi Oren,
I'm not at the workshop right now, but I remember measuring about
18VDC before the regulators. FWIW, I'm using a 24VCT transformer and
1N4002's for rectifying.

I'll be sure to get an accurate measurement tomorrow, I'm guessing
that -18VDC going into the LM337 set to output -15V is cutting it a
bit close, although the datasheet I have doesn't even give a minimum
differential!

Mike


On Tue, Sep 13, 2011 at 7:56 PM, Oren Leavitt <obl64 at ix.netcom.com> wrote:
>
>
> On 9/13/2011 5:56 PM, Mike Beauchamp wrote:
>>
>> Hey all,
>> I'm trying to debug a build of a VCO. As I sweep the frequency,
>> there's noticeable interference the higher the frequency gets. As the
>> oscillator gets closer to past my hearing range, the output starts to
>> sound sort of like an AM radio tuning in dead-air.
>>
>> Looking at the waveform on my scope, the output has jagged edges that
>> have a period of about 6 uS (166KHz).
>>
>> Looking at the power supply (+/-15V LM317/337), the positive output
>> shows a rising sawtooth wave with the same period @ .1V p-p. However,
>> the negative rail shows this high frequency sawtooth at nearly 1V p-p!
>>
>> If I disconnect the VCO from the powersupply and measure the negative
>> rail, the same 1V p-p sawtooth is there, only the frequency has
>> changed to have a period of about 28 uS (35KHz)
>>
>> I'm pretty much out of ideas to be honest. At first I thought it was
>> the oscillator itself, but seeing that 35KHz sawtooth on the negative
>> rail of the power supply with nothing attached has led me back to the
>> power supply. Different dummy loads on the power supply will affect
>> the frequency of this.
>>
>> The power supply is basically an exact duplicate of the LM317/337's
>> datasheet, so basically this
>>
>> (http://www.musicfromouterspace.com/analogsynth_new/POWERSUPPLY2009/POWERSUPPLY2009.php)
>> but set up for +/-15V. I can post a screenshot of the actual schematic
>> and I have some scope shots as well if that will help.
>>
>>
>
> Mike,
>
> What is the unregulated input voltage at the regulators?
> - Oren
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-- 
[mike]
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