Tweet
Hi guys,
I've been reading the debate about bifilar windings with some interest.
I don't think it is worth it for the following reasons. Let's compare two
sensors; one with a single winding and the second with two bifilar windings
each of which is half the numbr of turns of the single winding. Then, let's
connect them in parallel for polarization and then in series to extract the
signal.
For each bifilar winding, the resistance is half that of a the single
winding so, using the same battery to polarize it, the current is double
that of the single winding. The polarizing field is proportional to the
number of turns times the current so each bifilar winding gives exactly the
same polarizing field as the single winding - but, because there are two
them in parallel, the total polarizing field is doubled.
When they are connected together in series, they will give a total
signal equal to what the single winding would give for the SAME polarizing
field.Therefore, since the polarizing field is twice as big, you get a total
output of twice what you would get from a single winding.
However, the total polarizing power needed is four times what the single
winding needs because there are two windings in parallel each taking twice
the current of a single winding.
Summary: you get twice the output signal from a switched bifilar winding
but you use four times the power in polarizing the thing. You therefore use
up your battery four times as fast - probably faster because battery
lifetime depends slightly on current - the greater the current, the smaller
then number of amp-hours the battery will supply. In addition, you have the
problem of the additional complexity. However, if you're not worried about
power (i.e., if you're building an instrument for use from a boat or a
vehicle) or complexity (more things to go wrong - and relays are the weak
link in these sort of systems because they are electro-mechanical devices),
you DO get twice the signal for the same number of turns of wire overall.
In general, the signal is proportional to polarizing current times the
square of the number of turns. If you want a bigger signal, it is easiest
to get it by just increasing the number of turns. For a given battery
voltage and wire size, if you double the number of turns, you double the
resistance and so halve the current. You get, overall, a doubling of the
signal. As well, because you've reduced the polarizing current, you
increase battery life. That is, by simply doubling the number of turns, you
get twice the signal and use only half the power.
Jim
I've been reading the debate about bifilar windings with some interest.
I don't think it is worth it for the following reasons. Let's compare two
sensors; one with a single winding and the second with two bifilar windings
each of which is half the numbr of turns of the single winding. Then, let's
connect them in parallel for polarization and then in series to extract the
signal.
For each bifilar winding, the resistance is half that of a the single
winding so, using the same battery to polarize it, the current is double
that of the single winding. The polarizing field is proportional to the
number of turns times the current so each bifilar winding gives exactly the
same polarizing field as the single winding - but, because there are two
them in parallel, the total polarizing field is doubled.
When they are connected together in series, they will give a total
signal equal to what the single winding would give for the SAME polarizing
field.Therefore, since the polarizing field is twice as big, you get a total
output of twice what you would get from a single winding.
However, the total polarizing power needed is four times what the single
winding needs because there are two windings in parallel each taking twice
the current of a single winding.
Summary: you get twice the output signal from a switched bifilar winding
but you use four times the power in polarizing the thing. You therefore use
up your battery four times as fast - probably faster because battery
lifetime depends slightly on current - the greater the current, the smaller
then number of amp-hours the battery will supply. In addition, you have the
problem of the additional complexity. However, if you're not worried about
power (i.e., if you're building an instrument for use from a boat or a
vehicle) or complexity (more things to go wrong - and relays are the weak
link in these sort of systems because they are electro-mechanical devices),
you DO get twice the signal for the same number of turns of wire overall.
In general, the signal is proportional to polarizing current times the
square of the number of turns. If you want a bigger signal, it is easiest
to get it by just increasing the number of turns. For a given battery
voltage and wire size, if you double the number of turns, you double the
resistance and so halve the current. You get, overall, a doubling of the
signal. As well, because you've reduced the polarizing current, you
increase battery life. That is, by simply doubling the number of turns, you
get twice the signal and use only half the power.
Jim