less hum: switching power supply?

[Ingo Debus]

jhaible wrote:
> I don't know if this is the same phenomenon I have noticed
> or not, but what I have seen is that 78xx regulators produce
> a higher output voltage when two conditions are valid at the same
> time:
> 1) Input voltage way more than necessary (but still withing the
> specs)
> 2) Output current too low (no minimum load).

I have a 7905 regulated power supply right here on my bench, and yes,
without load the output is 9.6 volts. Input voltage is about 18 volts.
For the correct output voltage it requires a load of 2.2 kOhms or lower.
The 7905 is made by ST.
(...there's a 7805 too in this circuit, but it's difficult to disconnect
the load...)

Harry Bissell wrote:
> USE a ceramic cap at the output as recommended in the data sheet...
> and locate them AT the regulator pins.

I remember once having problems when there was only a 0.1uF ceramic cap
at the output of a 79xx regulator... capacitance was too low. However, I
use a 1uF tantalum here in my supply, this works fine. A normal
electrolytic should be ok too.

Ingo