Page 52 - Troubleshooting Analog Circuits

References 39

Consider the Effects of Magnetic Fields

One problem recently illustrated the foibles of inductor design: Our applications
engineers had designed several DC/DC converters to run off 5 V and to put out var-
ious voltages. such as +15 V and -15 V DC. One engineer built his converter using
the least expensive components, including a 16-cent, 300 FH inductor wound on a
ferrite rod. Another engineer built the same basic circuit but used a toroidal inductor
that cost almost a dollar. Each engineer did a full evaluation of his converter: both
designs worked well. Then the engineers swapped breadboards with each other. The
data on the toroid-equipped converter was quite repeatable. But. they couldn't obtain
repeatable measurements on the cheaper version. After several hours of poking and
fiddling, the engineers realized that the rod-shaped inductor radiated so much flux
into the adjacent area that all measurements of AC voltage and current were affected.
With the toroid. the flux was nicely contained inside the core. and there were no
problems making measurements. The engineers concluded that they could tell you
how to build the cheapest possible converter, but any nearby circuit would be sub.ject
to such large magnetic fields that the converter might be useless.
When 1 am building a complicated precision test box, I don't even try to build the
power supply in the main box because I know that the magnetic fields from even the
best power transformer will preclude low-noise measurements, and the heat from the
transformer and regulators will degrade the instrument's accuracy. Instead. I build a
separate power-supply box on the end of a 3-ft cable: the heat and magnetic t1u.c are
properly banished far away from my precision circuits.

Reference

I . Passi\.e Componenrs-A User's Guide. Ian Sinclair, Heinemann Newnes. Halley Court.
London, England. 1990. p. 125. Order from Buttenvorth-Heinemann. 80 Montvale Avenue.
Stoneham. Mass. 02 180.