[time-nuts] Switching regulator replacement for 7805

Bob Camp kb8tq at n1k.org
Mon Dec 5 07:54:08 EST 2016


The BLM18 gives you 300 ohms at zero current. It’s impedance at 
zero current per the data sheet. If you look at other parts of similar
size, the impedance drops to nearly zero at the “rated current”. Great
if you have a signal lead and no current. Not so great if you have an 
OCXO that is pulling 600 ma in a cold garage lab. A bead for close 
to 1A use with 100 ohms at the 100 MHz would be much larger. At
the 10 MHz frequency of interest, anything close to 100 ohms  would 
be even larger still.


> On Dec 5, 2016, at 7:24 AM, Attila Kinali <attila at kinali.ch> wrote:
> On Sun, 4 Dec 2016 17:07:53 -0800
> jimlux <jimlux at earthlink.net> wrote:
>>> Wouldn't it be better to use a ferrite bead instead, for this application?
>>> The much lower series capacitance and thus higher self-resonance frequency
>>> should help damping the spurs.
>> sure, if you can get enough L.  The other thing is that SMD inductors 
>> can be placed by machine, which isn't necessarily the case with ferrite 
>> beads (depending on how you do them).
> A BLM18 gets you into the order of 300Ω @ 100MHz, which is about
> the equivalent of about 480nH. Using a lower current version gets
> you into the 1kΩ @ 100MHz range (~1.6µH). Shouldn't that be enough
> inductance?
> Combined with some 10µF of capacitance and you already get ~15dB @ 1MHz
> for a simple LC-lowpass. For additional filtering, add something like an
> NFM18PS105 and you  get good filtering up to ~100MHz.
> 			Attila Kinali
> -- 
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