Re: Better buy a new multimeter then a new battery!

On 2008-12-24, Jan Panteltje <pNaonStpealmtje@xxxxxxxxx> wrote:

Yes, those chips need an isolated supply.
I am facing the same thing just now with a thing I am designing.
This little voltage converter circuit, that uses a very small potcore,
has saved me several times in the past:
ftp://panteltje.com/pub/voltage_converter.jpg

a blocking oscillator driving a forwards converter.
efficiency will be low because the magnetising energy
is not utilised.

That image it heard to read particularly the component values.

So I tried the ring core solution, took some ring core with about 50 turns from a choke
in some old cellphone charger (switch mode).
Cut the wire in the middle of the ringcore, so that left me with 2 separate
isolated turns.
Connected it as in above referenced diagram, but used an NPN this time:
ftp://panteltje.com/pub/DC_DC_converter.jpg
The grey wire through the ring is the base feedback, if it does not oscillate connect it
the other way around.

The circuit pictured differs from the one in your diagram by not
having the flyback derived "-ve supply" part attached to the primary.

when the transistor tries to turn off Vce will increase until the
transistor lets the current through

a choke core will have low density ferrite "distributed gap",
and so will store more energy before saturating, it's saturation
that commutates a blocking oscilator, in your circuit this stored
energy is wasted.

For a forwards converter you want a "transformer core" with the
marnetic particles packed tightly together. for efficiency

it should possibly have a "reset winding" too which returns energy to
the input when the transistor turns off.

.

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