A old-style PSUs has 50/60/100/120 Hz hum.
A switched-ode PSU has hum in line with the
switching frequency, from 10's of KHz and up.

My bet is that something else is not right...

Jan-Erik.


Jacopo Monegato wrote 2011-09-27 18:02:
>
> Ok, but... Let's pretend for a moment that i come from a technical
> institute in high school, i studied Electrotechnics (i hope i spelled it
> right), but thanks to our public school minister, if i was born five
> years ago i wouldn't have this problem. I have NOT studied  power supply
> system, so I don't know WHY this happens. I know that a lot of hum comes
> from switching power supplies (and since i come from italy, i think it
> has 220V hum ;) )... Because it works basically like a pwm module.
>
> Now what i can't understand is WHY only the pwm module doesn't work....
> If it's so obvious that it's not written anywhere, i am sorry ;)
>
>> From: bobblick@ftml.net To: piclist@mit.edu Subject: RE:
>> [PIC]PIC18F2550A,	Pwm doesn't work when using on external power
>> supply Date: Tue, 27 Sep 2011 08:48:19 -0700
>>
>>
>>
>> On Tuesday, September 27, 2011 5:16 PM, "Jacopo Monegato"  wrote:
>>>
>>> Yes, i was thinking about that. the 12 VDC is from a switching
>>> laptop power supply. Using my sound card as an "oscilloscope" i see
>>> that there is a LOT of hum in the ground signal, but i don't know
>>> how to remove it..
>>
>> Laptop power supplies can have 100 VAC common mode hum. Yes, 100
>> volts.
>>
>> Cheerful regards,
>>
>> Bob
>>
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