>> Be sure to measure Vin on the circuit side of any Iin ammeter.
>> Be aware that high frequency power supply noise on power wiring (in
>> and out) can play havoc with some meters). Largish cap (100 uF?)
>> across meter terminals can help.

> Since the noise most likely is in the 100s of MHz range, a smallish cap p=
erhaps
> would be better. A ferrite clamp can also help.

As the smps oscillator typically runs in the 100's of kHz region,
that's where the noise is.

The capacitor value is based on my practical experience of such
situations. In extremis I have been known to parallel eg 100 NF with
an ecap and even use a series inductor or small resistor between meter
and measured circuit, BUT usually an ecap alone works well. Such
filtering relies on forming an RC filter and as the series resistance
involved is in the milliohms range (track to board edge, short leads)
a 10's of uF value tends not to go astray.  My practical experience is
that some meters can be badly affected by such noise and that a cap
fixes it to the extent that using any of a wide range of meters then
gives similar results.

Without such a cap, efficiencies of in excess of 100% can readily be
obtained :-).


         Russell



>
> /Ruben
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=3D=3D=3D=3D=3D=3D
> Ruben J=F6nsson
> AB Liros Electronic
> Box 9124, 200 39 Malm=F6, Sweden
> TEL INT +46 40142078
> FAX INT +46 40947388
> ruben@pp.sbbs.se
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